| #
6ffb9017 |
| 18-Mar-2025 |
Alexei Starovoitov <ast@kernel.org> |
Merge branch 'resilient-queued-spin-lock'
Kumar Kartikeya Dwivedi says:
====================
Resilient Queued Spin Lock
Changelog:
----------
v3 -> v4
v4: https://lore.kernel.org/bpf/2025030315230
Merge branch 'resilient-queued-spin-lock'
Kumar Kartikeya Dwivedi says:
====================
Resilient Queued Spin Lock
Changelog:
----------
v3 -> v4
v4: https://lore.kernel.org/bpf/20250303152305.3195648-1-memxor@gmail.com
* Fix bisectability problem by reordering locktorture commit before
Makefile commit.
* Add EXPORT_SYMBOL_GPL to all used symbols and variables by consumers.
* Skip BPF selftest when nrprocs < 2.
* Fix kdoc to describe return value for res_spin_lock, slowpath.
* Move kernel/locking/rqspinlock.{c,h} to kernel/bpf/rqspinlock.{c,h}.
v2 -> v3
v2: https://lore.kernel.org/bpf/20250206105435.2159977-1-memxor@gmail.com
* Add ifdef's to fallback to Ankur's patch when it gets in, until then
copy-paste the implementation.
* Change the meaning of RES_DEF_TIMEOUT from two critical section
lengths to one for clarity, and use RES_DEF_TIMEOUT * 2 where needed.
* Use NSEC_PER_SEC as timeout for TAS fallback.
* Add Closes: tags for known syzbot reports.
* Change timeout for TAS fallback to 1 second.
* Fix more kernel test robot errors.
* More comments about smp_wmb in release_held_lock_entry interaction.
* Change RES_NR_HELD to 31.
* Address comments from Peter, Eduard, Alexei.
v1 -> v2
v1: https://lore.kernel.org/bpf/20250107140004.2732830-1-memxor@gmail.com
* Address nits from Waiman and Peter
* Fix arm64 WFE bug pointed out by Peter.
* Fix incorrect memory ordering in release_held_lock_entry, and
document subtleties. Explain why release is sufficient in unlock
but not in release_held_lock_entry.
* Remove dependence on CONFIG_QUEUED_SPINLOCKS and introduce a
test-and-set fallback when queued spinlock support is missing on an
architecture.
* Enforce FIFO ordering for BPF program spin unlocks.
* Address comments from Eduard on verifier plumbing.
* Add comments as suggested by Waiman.
* Refactor paravirt TAS lock to use the implemented TAS fallback.
* Use rqspinlock_t as the type throughout so that it can be replaced
with a non-qspinlock type in case of fallback.
* Testing and benchmarking on arm64, added numbers to the cover letter.
* Fix kernel test robot errors.
* Fix a BPF selftest bug leading to spurious failures on arm64.
Introduction
------------
This patch set introduces Resilient Queued Spin Lock (or rqspinlock with
res_spin_lock() and res_spin_unlock() APIs).
This is a qspinlock variant which recovers the kernel from a stalled
state when the lock acquisition path cannot make forward progress. This
can occur when a lock acquisition attempt enters a deadlock situation
(e.g. AA, or ABBA), or more generally, when the owner of the lock (which
we’re trying to acquire) isn’t making forward progress.
The cover letter provides an overview of the motivation, design, and
alternative approaches. We then provide evaluation numbers showcasing
that while rqspinlock incurs overhead, the performance of rqspinlock
approaches that of the normal qspinlock used by the kernel.
The evaluations for rqspinlock were performed by replacing the default
qspinlock implementation with it and booting the kernel to run the
experiments. Support for locktorture is also included with numbers in
this series.
The cover letter's design section provides an overview of the
algorithmic approach. A technical document describing the implementation
in more detail is available here:
https://github.com/kkdwivedi/rqspinlock/blob/main/rqspinlock.pdf
We have a WIP TLA+ proof for liveness and mutual exclusion of rqspinlock
built on top of the qspinlock TLA+ proof from Catalin Marinas [3]. We
will share more details and the links in the near future.
Motivation
----------
In regular kernel code, usage of locks is assumed to be correct, so as
to avoid deadlocks and stalls by construction, however, the same is not
true for BPF programs. Users write normal C code and the in-kernel eBPF
runtime ensures the safety of the kernel by rejecting unsafe programs.
Users can upload programs that use locks in an improper fashion, and may
cause deadlocks when these programs run inside the kernel. The verifier
is responsible for rejecting such programs from being loaded into the
kernel.
Until now, the eBPF verifier ensured deadlock safety by only permitting
one lock acquisition at a time, and by preventing any functions to be
called from within the critical section. Additionally, only a few
restricted program types are allowed to call spin locks. As the usage of
eBPF grows (e.g. with sched_ext) beyond its conventional application in
networking, tracing, and security, the limitations on locking are
becoming a bottleneck for users.
The rqspinlock implementation allows us to permit more flexible locking
patterns in BPF programs, without limiting them to the subset that can
be proven safe statically (which is fairly small, and requires complex
static analysis), while ensuring that the kernel will recover in case we
encounter a locking violation at runtime. We make a tradeoff here by
accepting programs that may potentially have deadlocks, and recover the
kernel quickly at runtime to ensure availability.
Additionally, eBPF programs attached to different parts of the kernel
can introduce new control flow into the kernel, which increases the
likelihood of deadlocks in code not written to handle reentrancy. There
have been multiple syzbot reports surfacing deadlocks in internal kernel
code due to the diverse ways in which eBPF programs can be attached to
different parts of the kernel. By switching the BPF subsystem’s lock
usage to rqspinlock, all of these issues can be mitigated at runtime.
This spin lock implementation allows BPF maps to become safer and remove
mechanisms that have fallen short in assuring safety when nesting
programs in arbitrary ways in the same context or across different
contexts. The red diffs due to patches 16-18 demonstrate this
simplification.
> kernel/bpf/hashtab.c | 102 ++++++++++++++++++++++++++++++++--------------------------...
> kernel/bpf/lpm_trie.c | 25 ++++++++++++++-----------
> kernel/bpf/percpu_freelist.c | 113 +++++++++++++++++++++++++---------------------------------...
> kernel/bpf/percpu_freelist.h | 4 ++--
> 4 files changed, 73 insertions(+), 171 deletions(-)
Design
------
Deadlocks mostly manifest as stalls in the waiting loops of the
qspinlock slow path. Thus, using stalls as a signal for deadlocks avoids
introducing cost to the normal fast path, and ensures bounded
termination of the waiting loop. Our recovery algorithm is focused on
terminating the waiting loops of the qspinlock algorithm when it gets
stuck, and implementing bespoke recovery procedures for each class of
waiter to restore the lock to a usable state. Deadlock detection is the
main mechanism used to provide faster recovery, with the timeout
mechanism acting as a final line of defense.
Deadlock Detection
~~~~~~~~~~~~~~~~~~
We handle two cases of deadlocks: AA deadlocks (attempts to acquire the
same lock again), and ABBA deadlocks (attempts to acquire two locks in
the opposite order from two distinct threads). Variants of ABBA
deadlocks may be encountered with more than two locks being held in the
incorrect order. These are not diagnosed explicitly, as they reduce to
ABBA deadlocks.
Deadlock detection is triggered immediately when beginning the waiting
loop of a lock slow path.
While timeouts ensure that any waiting loops in the locking slow path
terminate and return to the caller, it can be excessively long in some
situations. While the default timeout is short (0.5s), a stall for this
duration inside the kernel can set off alerts for latency-critical
services with strict SLOs. Ideally, the kernel should recover from an
undesired state of the lock as soon as possible.
A multi-step strategy is used to recover the kernel from waiting loops
in the locking algorithm which may fail to terminate in a bounded amount
of time.
* Each CPU maintains a table of held locks. Entries are inserted and
removed upon entry into lock, and exit from unlock, respectively.
* Deadlock detection for AA locks is thus simple: we have an AA
deadlock if we find a held lock entry for the lock we’re attempting
to acquire on the same CPU.
* During deadlock detection for ABBA, we search through the tables of
all other CPUs to find situations where we are holding a lock the
remote CPU is attempting to acquire, and they are holding a lock we
are attempting to acquire. Upon encountering such a condition, we
report an ABBA deadlock.
* We divide the duration between entry time point into the waiting loop
and the timeout time point into intervals of 1 ms, and perform
deadlock detection until timeout happens. Upon entry into the slow
path, and then completion of each 1 ms interval, we perform detection
of both AA and ABBA deadlocks. In the event that deadlock detection
yields a positive result, the recovery happens sooner than the
timeout. Otherwise, it happens as a last resort upon completion of
the timeout.
Timeouts
~~~~~~~~
Timeouts act as final line of defense against stalls for waiting loops.
The ‘ktime_get_mono_fast_ns’ function is used to poll for the current
time, and it is compared to the timestamp indicating the end time in the
waiter loop. Each waiting loop is instrumented to check an extra
condition using a macro. Internally, the macro implementation amortizes
the checking of the timeout to avoid sampling the clock in every
iteration. Precisely, the timeout checks are invoked every 64k
iterations.
Recovery
~~~~~~~~
There is extensive literature in academia on designing locks that
support timeouts [0][1], as timeouts can be used as a proxy for
detecting the presence of deadlocks and recovering from them, without
maintaining explicit metadata to construct a waits-for relationship
between two threads at runtime.
In case of rqspinlock, the key simplification in our algorithm comes
from the fact that upon a timeout, waiters always leave the queue in
FIFO order. As such, the timeout is only enforced by the head of the
wait queue, while other waiters rely on the head to signal them when a
timeout has occurred and when they need to exit. We don’t have to
implement complex algorithms and do not need extra synchronization for
waiters in the middle of the queue timing out before their predecessor
or successor, unlike previous approaches [0][1].
There are three forms of waiters in the original queued spin lock
algorithm. The first is the waiter which acquires the pending bit and
spins on the lock word without forming a wait queue. The second is the
head waiter that is the first waiter heading the wait queue. The third
form is of all the non-head waiters queued behind the head, waiting to
be signalled through their MCS node to overtake the responsibility of
the head.
In rqspinlock's recovery algorithm, we are concerned with the second and
third kind. First, we augment the waiting loop of the head of the wait
queue with a timeout. When this timeout happens, all waiters part of the
wait queue will abort their lock acquisition attempts. This happens in
three steps.
* First, the head breaks out of its loop waiting for pending and locked
bits to turn to 0, and non-head waiters break out of their MCS node
spin (more on that later).
* Next, every waiter (head or non-head) attempts to check whether they
are also the tail waiter, in such a case they attempt to zero out the
tail word and allow a new queue to be built up for this lock. If they
succeed, they have no one to signal next in the queue to stop
spinning.
* Otherwise, they signal the MCS node of the next waiter to break out
of its spin and try resetting the tail word back to 0. This goes on
until the tail waiter is found. In case of races, the new tail will
be responsible for performing the same task, as the old tail will
then fail to reset the tail word and wait for its next pointer to be
updated before it signals the new tail to do the same.
Timeout Bound
~~~~~~~~~~~~~
The timeout is applied by two types of waiters: the pending bit waiter
and the wait queue head waiter. As such, for the pending waiter, only
the lock owner is ahead of it, and for the wait queue head waiter, only
the lock owner and the pending waiter take precedence in executing their
critical sections.
We define the timeout value to span at most 1 critical section length,
and then use the appropriate value (default, or default x 2) depending
on if we are the pending waiter or head of wait queue.
Therefore, the waiting loop wait can span at most 2 critical section
lengths, and thus, it is unaffected by the amount of contention or the
number of CPUs on the host. Non-head waiters simply wait for the wait
queue head to signal them on a timeout.
In Meta's production, we have noticed uncore PMU reads and SMIs
consuming tens of msecs. While these events are rare, a 0.25 second
timeout should absorb such tail events and not raise false alarms for
timeouts. We will continue monitoring this in production and adjust the
timeout if necessary in the future.
More details of the recovery algorithm is described in patch 9 and a
detailed description is available at [2].
Alternatives
------------
Lockdep: We do not rely on the lockdep facility for reporting violations
for primarily two reasons:
* Overhead: The lockdep infrastructure can add significant overhead to
the lock acquisition path, and is not recommended for use in
production due to this reason. While the report is more useful and
exhaustive, the overhead can be prohibitive, especially as BPF
programs run in hot paths of the kernel. Moreover, it also increases
the size of the lock word to store extra metadata, which is not
feasible for BPF spin locks that are 4-bytes in size today (similar to
qspinlock).
* Debug Tool: Lockdep is intended to be used as a debugging facility,
providing extra context to the user about the locking violations
occurring during runtime. It is always turned off on all production
kernels, therefore isn’t available most of the time.
We require a mechanism for detecting common variants of deadlocks that
is always available in production kernels and never turned off. At the
same time, it must not introduce overhead in terms of time (for the slow
path) and memory (for the lock word size).
Evaluation
----------
We run benchmarks that stress locking scalability and perform comparison
against the baseline (qspinlock). For the rqspinlock case, we replace
the default qspinlock with it in the kernel, such that all spin locks in
the kernel use the rqspinlock slow path. As such, benchmarks that stress
kernel spin locks end up exercising rqspinlock.
Evaluation setup
~~~~~~~~~~~~~~~~
We set the CPU governor to performance for all experiments.
Note: Numbers for arm64 have been obtained without the no-WFE fallback
in this series, to perform a fair comparison with the WFE using
qspinlock baseline.
x86_64:
Intel Xeon Platinum 8468 (Sapphire Rapids)
96 cores (48 x 2 sockets)
2 threads per core, 0-95, siblings from 96-191
2 NUMA nodes (every 48 cores), 2 LLCs (every 48 cores), 1 LLC per NUMA node
Hyperthreading enabled
arm64:
Ampere Max Neoverse-N1 256-Core Processor
256 cores (128 cores x 2 sockets)
1 thread per core
2 NUMA nodes (every 128 cores), 1 L2 per core (256 instances), no shared L3
No hyperthreading available
The locktorture experiment is run for 30 seconds.
Average of 25 runs is used for will-it-scale, after an initial warm up.
More information on the locks contended in the will-it-scale experiments
is available in the evaluation section of the CNA paper, in table 1 [4].
Legend:
QL - qspinlock (avg. throughput)
RQL - rqspinlock (avg. throughput)
Results
~~~~~~~
locktorture - x86_64
Threads QL RQL Speedup
-----------------------------------------------
1 46910437 45057327 0.96
2 29871063 25085034 0.84
4 13876024 19242776 1.39
8 14638499 13346847 0.91
16 14380506 14104716 0.98
24 17278144 15293077 0.89
32 19494283 17826675 0.91
40 27760955 21002910 0.76
48 28638897 26432549 0.92
56 29336194 26512029 0.9
64 30040731 27421403 0.91
72 29523599 27010618 0.91
80 28846738 27885141 0.97
88 29277418 25963753 0.89
96 28472339 27423865 0.96
104 28093317 26634895 0.95
112 29914000 27872339 0.93
120 29199580 26682695 0.91
128 27755880 27314662 0.98
136 30349095 27092211 0.89
144 29193933 27805445 0.95
152 28956663 26071497 0.9
160 28950009 28183864 0.97
168 29383520 28135091 0.96
176 28475883 27549601 0.97
184 31958138 28602434 0.89
192 31342633 33394385 1.07
will-it-scale open1_threads - x86_64
Threads QL QL stddev stddev% RQL RQL stddev stddev% Speedup
-----------------------------------------------------------------------------------------------
1 1396323.92 7373.12 0.53 1366616.8 4152.08 0.3 0.98
2 1844403.8 3165.26 0.17 1700301.96 2396.58 0.14 0.92
4 2370590.6 24545.54 1.04 1655872.32 47938.71 2.9 0.7
8 2185227.04 9537.9 0.44 1691205.16 9783.25 0.58 0.77
16 2110672.36 10972.99 0.52 1781696.24 15021.43 0.84 0.84
24 1655042.72 18037.23 1.09 2165125.4 5422.54 0.25 1.31
32 1738928.24 7166.64 0.41 1829468.24 9081.59 0.5 1.05
40 1854430.52 6148.24 0.33 1731062.28 3311.95 0.19 0.93
48 1766529.96 5063.86 0.29 1749375.28 2311.27 0.13 0.99
56 1303016.28 6168.4 0.47 1452656 7695.29 0.53 1.11
64 1169557.96 4353.67 0.37 1287370.56 8477.2 0.66 1.1
72 1036023.4 7116.53 0.69 1135513.92 9542.55 0.84 1.1
80 1097913.64 11356 1.03 1176864.8 6771.41 0.58 1.07
88 1123907.36 12843.13 1.14 1072416.48 7412.25 0.69 0.95
96 1166981.52 9402.71 0.81 1129678.76 9499.14 0.84 0.97
104 1108954.04 8171.46 0.74 1032044.44 7840.17 0.76 0.93
112 1000777.76 8445.7 0.84 1078498.8 6551.47 0.61 1.08
120 1029448.4 6992.29 0.68 1093743 8378.94 0.77 1.06
128 1106670.36 10102.15 0.91 1241438.68 23212.66 1.87 1.12
136 1183776.88 6394.79 0.54 1116799.64 18111.38 1.62 0.94
144 1201122 25917.69 2.16 1301779.96 15792.6 1.21 1.08
152 1099737.08 13567.82 1.23 1053647.2 12704.29 1.21 0.96
160 1031186.32 9048.07 0.88 1069961.4 8293.18 0.78 1.04
168 1068817 16486.06 1.54 1096495.36 14021.93 1.28 1.03
176 966633.96 9623.27 1 1081129.84 9474.81 0.88 1.12
184 1004419.04 12111.11 1.21 1037771.24 12001.66 1.16 1.03
192 1088858.08 16522.93 1.52 1027943.12 14238.57 1.39 0.94
will-it-scale open2_threads - x86_64
Threads QL QL stddev stddev% RQL RQL stddev stddev% Speedup
-----------------------------------------------------------------------------------------------
1 1337797.76 4649.19 0.35 1332609.4 3813.14 0.29 1
2 1598300.2 1059.93 0.07 1771891.36 5667.12 0.32 1.11
4 1736573.76 13025.33 0.75 1396901.2 2682.46 0.19 0.8
8 1794367.84 4879.6 0.27 1917478.56 3751.98 0.2 1.07
16 1990998.44 8332.78 0.42 1864165.56 9648.59 0.52 0.94
24 1868148.56 4248.23 0.23 1710136.68 2760.58 0.16 0.92
32 1955180 6719 0.34 1936149.88 1980.87 0.1 0.99
40 1769646.4 4686.54 0.26 1729653.68 4551.22 0.26 0.98
48 1724861.16 4056.66 0.24 1764900 971.11 0.06 1.02
56 1318568 7758.86 0.59 1385660.84 7039.8 0.51 1.05
64 1143290.28 5351.43 0.47 1316686.6 5597.69 0.43 1.15
72 1196762.68 10655.67 0.89 1230173.24 9858.2 0.8 1.03
80 1126308.24 6901.55 0.61 1085391.16 7444.34 0.69 0.96
88 1035672.96 5452.95 0.53 1035541.52 8095.33 0.78 1
96 1030203.36 6735.71 0.65 1020113.48 8683.13 0.85 0.99
104 1039432.88 6583.59 0.63 1083902.48 5775.72 0.53 1.04
112 1113609.04 4380.62 0.39 1072010.36 8983.14 0.84 0.96
120 1109420.96 7183.5 0.65 1079424.12 10929.97 1.01 0.97
128 1095400.04 4274.6 0.39 1095475.2 12042.02 1.1 1
136 1071605.4 11103.73 1.04 1114757.2 10516.55 0.94 1.04
144 1104147.2 9714.75 0.88 1044954.16 7544.2 0.72 0.95
152 1164280.24 13386.15 1.15 1101213.92 11568.49 1.05 0.95
160 1084892.04 7941.25 0.73 1152273.76 9593.38 0.83 1.06
168 983654.76 11772.85 1.2 1111772.28 9806.83 0.88 1.13
176 1087544.24 11262.35 1.04 1077507.76 9442.02 0.88 0.99
184 1101682.4 24701.68 2.24 1095223.2 16707.29 1.53 0.99
192 983712.08 13453.59 1.37 1051244.2 15662.05 1.49 1.07
will-it-scale lock1_threads - x86_64
Threads QL QL stddev stddev% RQL RQL stddev stddev% Speedup
-----------------------------------------------------------------------------------------------
1 4307484.96 3959.31 0.09 4252908.56 10375.78 0.24 0.99
2 7701844.32 4169.88 0.05 7219233.52 6437.11 0.09 0.94
4 14781878.72 22854.85 0.15 15260565.12 37305.71 0.24 1.03
8 12949698.64 99270.42 0.77 9954660.4 142805.68 1.43 0.77
16 12947690.64 72977.27 0.56 10865245.12 49520.31 0.46 0.84
24 11142990.64 33200.39 0.3 11444391.68 37884.46 0.33 1.03
32 9652335.84 22369.48 0.23 9344086.72 21639.22 0.23 0.97
40 9185931.12 5508.96 0.06 8881506.32 5072.33 0.06 0.97
48 9084385.36 10871.05 0.12 8863579.12 4583.37 0.05 0.98
56 6595540.96 33100.59 0.5 6640389.76 46619.96 0.7 1.01
64 5946726.24 47160.5 0.79 6572155.84 91973.73 1.4 1.11
72 6744894.72 43166.65 0.64 5991363.36 80637.56 1.35 0.89
80 6234502.16 118983.16 1.91 5157894.32 73592.72 1.43 0.83
88 5053879.6 199713.75 3.95 4479758.08 36202.27 0.81 0.89
96 5184302.64 99199.89 1.91 5249210.16 122348.69 2.33 1.01
104 4612391.92 40803.05 0.88 4850209.6 26813.28 0.55 1.05
112 4809209.68 24070.68 0.5 4869477.84 27489.04 0.56 1.01
120 5130746.4 34265.5 0.67 4620047.12 44229.54 0.96 0.9
128 5376465.28 95028.05 1.77 4781179.6 43700.93 0.91 0.89
136 5453742.4 86718.87 1.59 5412457.12 40339.68 0.75 0.99
144 5805040.72 84669.31 1.46 5595382.48 68701.65 1.23 0.96
152 5842897.36 31120.33 0.53 5787587.12 43521.68 0.75 0.99
160 5837665.12 14179.44 0.24 5118808.72 45193.23 0.88 0.88
168 5660332.72 27467.09 0.49 5104959.04 40891.75 0.8 0.9
176 5180312.24 28656.39 0.55 4718407.6 58734.13 1.24 0.91
184 4706824.16 50469.31 1.07 4692962.64 92266.85 1.97 1
192 5126054.56 51082.02 1 4680866.8 58743.51 1.25 0.91
will-it-scale lock2_threads - x86_64
Threads QL QL stddev stddev% RQL RQL stddev stddev% Speedup
-----------------------------------------------------------------------------------------------
1 4316091.2 4933.28 0.11 4293104 30369.71 0.71 0.99
2 3500046.4 19852.62 0.57 4507627.76 23667.66 0.53 1.29
4 3639098.96 26370.65 0.72 3673166.32 30822.71 0.84 1.01
8 3714548.56 49953.44 1.34 4055818.56 71630.41 1.77 1.09
16 4188724.64 105414.49 2.52 4316077.12 68956.15 1.6 1.03
24 3737908.32 47391.46 1.27 3762254.56 55345.7 1.47 1.01
32 3820952.8 45207.66 1.18 3710368.96 52651.92 1.42 0.97
40 3791280.8 28630.55 0.76 3661933.52 37671.27 1.03 0.97
48 3765721.84 59553.83 1.58 3604738.64 50861.36 1.41 0.96
56 3175505.76 64336.17 2.03 2771022.48 66586.99 2.4 0.87
64 2620294.48 71651.34 2.73 2650171.68 44810.83 1.69 1.01
72 2861893.6 86542.61 3.02 2537437.2 84571.75 3.33 0.89
80 2976297.2 83566.43 2.81 2645132.8 85992.34 3.25 0.89
88 2547724.8 102014.36 4 2336852.16 80570.25 3.45 0.92
96 2945310.32 82673.25 2.81 2513316.96 45741.81 1.82 0.85
104 3028818.64 90643.36 2.99 2581787.52 52967.48 2.05 0.85
112 2546264.16 102605.82 4.03 2118812.64 62043.19 2.93 0.83
120 2917334.64 112220.01 3.85 2720418.64 64035.96 2.35 0.93
128 2906621.84 69428.1 2.39 2795310.32 56736.87 2.03 0.96
136 2841833.76 105541.11 3.71 3063404.48 62288.94 2.03 1.08
144 3032822.32 134796.56 4.44 3169985.6 149707.83 4.72 1.05
152 2557694.96 62218.15 2.43 2469887.6 68343.78 2.77 0.97
160 2810214.72 61468.79 2.19 2323768.48 54226.71 2.33 0.83
168 2651146.48 76573.27 2.89 2385936.64 52433.98 2.2 0.9
176 2720616.32 89026.19 3.27 2941400.08 59296.64 2.02 1.08
184 2696086 88541.24 3.28 2598225.2 76365.7 2.94 0.96
192 2908194.48 87023.91 2.99 2377677.68 53299.82 2.24 0.82
locktorture - arm64
Threads QL RQL Speedup
-----------------------------------------------
1 43320464 44718174 1.03
2 21056971 29255448 1.39
4 16040120 11563981 0.72
8 12786398 12838909 1
16 13646408 13436730 0.98
24 13597928 13669457 1.01
32 16456220 14600324 0.89
40 16667726 13883101 0.83
48 14347691 14608641 1.02
56 15624580 15180758 0.97
64 18105114 16009137 0.88
72 16606438 14772256 0.89
80 16550202 14124056 0.85
88 16716082 15930618 0.95
96 16489242 16817657 1.02
104 17915808 17165324 0.96
112 17217482 21343282 1.24
120 20449845 20576123 1.01
128 18700902 20286275 1.08
136 17913378 21142921 1.18
144 18225673 18971921 1.04
152 18374206 19229854 1.05
160 23136514 20129504 0.87
168 21096269 17167777 0.81
176 21376794 21594914 1.01
184 23542989 20638298 0.88
192 22793754 20655980 0.91
200 20933027 19628316 0.94
208 23105684 25572720 1.11
216 24158081 23173848 0.96
224 23388984 22485353 0.96
232 21916401 23899343 1.09
240 22292129 22831784 1.02
248 25812762 22636787 0.88
256 24294738 26127113 1.08
will-it-scale open1_threads - arm64
Threads QL QL stddev stddev% RQL RQL stddev stddev% Speedup
-----------------------------------------------------------------------------------------------
1 844452.32 801 0.09 804936.92 900.25 0.11 0.95
2 1309419.08 9495.78 0.73 1265080.24 3171.13 0.25 0.97
4 2113074.24 5363.19 0.25 2041158.28 7883.65 0.39 0.97
8 1916650.96 15749.86 0.82 2039850.04 7562.87 0.37 1.06
16 1835540.72 12940.45 0.7 1937398.56 11461.15 0.59 1.06
24 1876760.48 12581.67 0.67 1966659.16 10012.69 0.51 1.05
32 1834525.6 5571.08 0.3 1929180.4 6221.96 0.32 1.05
40 1851592.76 7848.18 0.42 1937504.44 5991.55 0.31 1.05
48 1845067 4118.68 0.22 1773331.56 6068.23 0.34 0.96
56 1742709.36 6874.03 0.39 1716184.92 6713.16 0.39 0.98
64 1685339.72 6688.91 0.4 1676046.16 5844.06 0.35 0.99
72 1694838.84 2433.41 0.14 1821189.6 2906.89 0.16 1.07
80 1738778.68 2916.74 0.17 1729212.6 3714.41 0.21 0.99
88 1753131.76 2734.34 0.16 1713294.32 4652.82 0.27 0.98
96 1694112.52 4449.69 0.26 1714438.36 5621.66 0.33 1.01
104 1780279.76 2420.52 0.14 1767679.12 3067.66 0.17 0.99
112 1700284.72 9796.23 0.58 1796674.6 4066.06 0.23 1.06
120 1760466.72 3978.65 0.23 1704706.08 4080.04 0.24 0.97
128 1634067.96 5187.94 0.32 1764115.48 3545.02 0.2 1.08
136 1170303.84 7602.29 0.65 1227188.04 8090.84 0.66 1.05
144 953186.16 7859.02 0.82 964822.08 10536.61 1.09 1.01
152 818893.96 7238.86 0.88 853412.44 5932.25 0.7 1.04
160 707460.48 3868.26 0.55 746985.68 10363.03 1.39 1.06
168 658380.56 4938.77 0.75 672101.12 5442.95 0.81 1.02
176 614692.04 3137.74 0.51 615143.36 6197.19 1.01 1
184 574808.88 4741.61 0.82 592395.08 8840.92 1.49 1.03
192 548142.92 6116.31 1.12 571299.68 8388.56 1.47 1.04
200 511621.96 2182.33 0.43 532144.88 5467.04 1.03 1.04
208 506583.32 6834.39 1.35 521427.08 10318.65 1.98 1.03
216 480438.04 3608.96 0.75 510697.76 8086.47 1.58 1.06
224 470644.96 3451.35 0.73 467433.92 5008.59 1.07 0.99
232 466973.72 6599.97 1.41 444345.92 2144.96 0.48 0.95
240 442927.68 2351.56 0.53 440503.56 4289.01 0.97 0.99
248 432991.16 5829.92 1.35 445462.6 5944.03 1.33 1.03
256 409455.44 1430.5 0.35 422219.4 4007.04 0.95 1.03
will-it-scale open2_threads - arm64
Threads QL QL stddev stddev% RQL RQL stddev stddev% Speedup
-----------------------------------------------------------------------------------------------
1 818645.4 1097.02 0.13 774110.24 1562.45 0.2 0.95
2 1281013.04 2188.78 0.17 1238346.24 2149.97 0.17 0.97
4 2058514.16 13105.36 0.64 1985375 3204.48 0.16 0.96
8 1920414.8 16154.63 0.84 1911667.92 8882.98 0.46 1
16 1943729.68 8714.38 0.45 1978946.72 7465.65 0.38 1.02
24 1915846.88 7749.9 0.4 1914442.72 9841.71 0.51 1
32 1964695.92 8854.83 0.45 1914650.28 9357.82 0.49 0.97
40 1845071.12 5103.26 0.28 1891685.44 4278.34 0.23 1.03
48 1838897.6 5123.61 0.28 1843498.2 5391.94 0.29 1
56 1823768.32 3214.14 0.18 1736477.48 5675.49 0.33 0.95
64 1627162.36 3528.1 0.22 1685727.16 6102.63 0.36 1.04
72 1725320.16 4709.83 0.27 1710174.4 6707.54 0.39 0.99
80 1692288.44 9110.89 0.54 1773676.24 4327.94 0.24 1.05
88 1725496.64 4249.71 0.25 1695173.84 5097.14 0.3 0.98
96 1766093.08 2280.09 0.13 1732782.64 3606.1 0.21 0.98
104 1647753 2926.83 0.18 1710876.4 4416.04 0.26 1.04
112 1763785.52 3838.26 0.22 1803813.76 1859.2 0.1 1.02
120 1684095.16 2385.31 0.14 1766903.08 3258.34 0.18 1.05
128 1733528.56 2800.62 0.16 1677446.32 3201.14 0.19 0.97
136 1179187.84 6804.86 0.58 1241839.52 10698.51 0.86 1.05
144 969456.36 6421.85 0.66 1018441.96 8732.19 0.86 1.05
152 839295.64 10422.66 1.24 817531.92 6778.37 0.83 0.97
160 743010.72 6957.98 0.94 749291.16 9388.47 1.25 1.01
168 666049.88 13159.73 1.98 689408.08 10192.66 1.48 1.04
176 609185.56 5685.18 0.93 653744.24 10847.35 1.66 1.07
184 602232.08 12089.72 2.01 597718.6 13856.45 2.32 0.99
192 563919.32 9870.46 1.75 560080.4 8388.47 1.5 0.99
200 522396.28 4155.61 0.8 539168.64 10456.64 1.94 1.03
208 520328.28 9353.14 1.8 510011.4 6061.19 1.19 0.98
216 479797.72 5824.58 1.21 486955.32 4547.05 0.93 1.01
224 467943.8 4484.86 0.96 473252.76 5608.58 1.19 1.01
232 456914.24 3129.5 0.68 457463.2 7474.83 1.63 1
240 450535 5149.78 1.14 437653.56 4604.92 1.05 0.97
248 435475.2 2350.87 0.54 435589.24 6176.01 1.42 1
256 416737.88 2592.76 0.62 424178.28 3932.2 0.93 1.02
will-it-scale lock1_threads - arm64
Threads QL QL stddev stddev% RQL RQL stddev stddev% Speedup
-----------------------------------------------------------------------------------------------
1 2512077.52 3026.1 0.12 2085365.92 1612.44 0.08 0.83
2 4840180.4 3646.31 0.08 4326922.24 3802.17 0.09 0.89
4 9358779.44 6673.07 0.07 8467588.56 5577.05 0.07 0.9
8 9374436.88 18826.26 0.2 8635110.16 4217.66 0.05 0.92
16 9527184.08 14111.94 0.15 8561174.16 3258.6 0.04 0.9
24 8873099.76 17242.32 0.19 9286778.72 4124.51 0.04 1.05
32 8457640.4 10790.92 0.13 8700401.52 5110 0.06 1.03
40 8478771.76 13250.8 0.16 8746198.16 7606.42 0.09 1.03
48 8329097.76 7958.92 0.1 8774265.36 6082.08 0.07 1.05
56 8330143.04 11586.93 0.14 8472426.48 7402.13 0.09 1.02
64 8334684.08 10478.03 0.13 7979193.52 8436.63 0.11 0.96
72 7941815.52 16031.38 0.2 8016885.52 12640.56 0.16 1.01
80 8042221.68 10219.93 0.13 8072222.88 12479.54 0.15 1
88 8190336.8 10751.38 0.13 8432977.6 11865.67 0.14 1.03
96 8235010.08 7267.8 0.09 8022101.28 11910.63 0.15 0.97
104 8154434.08 7770.8 0.1 7987812 7647.42 0.1 0.98
112 7738464.56 11067.72 0.14 7968483.92 20632.93 0.26 1.03
120 8228919.36 10395.79 0.13 8304329.28 11913.76 0.14 1.01
128 7798646.64 8877.8 0.11 8197938.4 7527.81 0.09 1.05
136 5567293.68 66259.82 1.19 5642017.12 126584.59 2.24 1.01
144 4425655.52 55729.96 1.26 4519874.64 82996.01 1.84 1.02
152 3871300.8 77793.78 2.01 3850025.04 80167.3 2.08 0.99
160 3558041.68 55108.3 1.55 3495924.96 83626.42 2.39 0.98
168 3302042.72 45011.89 1.36 3298002.8 59393.64 1.8 1
176 3066165.2 34896.54 1.14 3063027.44 58219.26 1.9 1
184 2817899.6 43585.27 1.55 2859393.84 45258.03 1.58 1.01
192 2690403.76 42236.77 1.57 2630652.24 35953.13 1.37 0.98
200 2563141.44 28145.43 1.1 2539964.32 38556.52 1.52 0.99
208 2502968.8 27687.81 1.11 2477757.28 28240.81 1.14 0.99
216 2474917.76 24128.71 0.97 2483161.44 32198.37 1.3 1
224 2386874.72 32954.66 1.38 2398068.48 37667.29 1.57 1
232 2379248.24 27413.4 1.15 2327601.68 24565.28 1.06 0.98
240 2302146.64 19914.19 0.87 2236074.64 20968.17 0.94 0.97
248 2241798.32 21542.52 0.96 2173312.24 26498.36 1.22 0.97
256 2198765.12 20832.66 0.95 2136159.52 25027.96 1.17 0.97
will-it-scale lock2_threads - arm64
Threads QL QL stddev stddev% RQL RQL stddev stddev% Speedup
-----------------------------------------------------------------------------------------------
1 2499414.32 1932.27 0.08 2075704.8 24589.71 1.18 0.83
2 3887820 34198.36 0.88 4057432.64 11896.04 0.29 1.04
4 3445307.6 7958.3 0.23 3869960.4 3788.5 0.1 1.12
8 4310597.2 14405.9 0.33 3931319.76 5845.33 0.15 0.91
16 3995159.84 22621.85 0.57 3953339.68 15668.9 0.4 0.99
24 4048456.88 22956.51 0.57 3887812.64 30584.77 0.79 0.96
32 3974808.64 20465.87 0.51 3718778.08 27407.24 0.74 0.94
40 3941154.88 15136.68 0.38 3551464.24 33378.67 0.94 0.9
48 3725436.32 17090.67 0.46 3714356.08 19035.26 0.51 1
56 3558449.44 10123.46 0.28 3449656.08 36476.87 1.06 0.97
64 3514616.08 16470.99 0.47 3493197.04 25639.82 0.73 0.99
72 3461700.88 16780.97 0.48 3376565.04 16930.19 0.5 0.98
80 3797008.64 17599.05 0.46 3505856.16 34320.34 0.98 0.92
88 3737459.44 10774.93 0.29 3631757.68 24231.29 0.67 0.97
96 3612816.16 21865.86 0.61 3545354.56 16391.15 0.46 0.98
104 3765167.36 17763.8 0.47 3466467.12 22235.45 0.64 0.92
112 3713386 15455.21 0.42 3402210 18349.66 0.54 0.92
120 3699986.08 15153.08 0.41 3580303.92 19823.01 0.55 0.97
128 3648694.56 11891.62 0.33 3426445.28 22993.32 0.67 0.94
136 800046.88 6039.73 0.75 784412.16 9062.03 1.16 0.98
144 769483.36 5231.74 0.68 714132.8 8953.57 1.25 0.93
152 821081.52 4249.12 0.52 743694.64 8155.18 1.1 0.91
160 789040.16 9187.4 1.16 834865.44 6159.29 0.74 1.06
168 867742.4 8967.66 1.03 734905.36 15582.75 2.12 0.85
176 838650.32 7949.72 0.95 846939.68 8959.8 1.06 1.01
184 854984.48 19475.51 2.28 794549.92 11924.54 1.5 0.93
192 846262.32 13795.86 1.63 899915.12 8639.82 0.96 1.06
200 942602.16 12665.42 1.34 900385.76 8592.23 0.95 0.96
208 954183.68 12853.22 1.35 1166186.96 13045.03 1.12 1.22
216 929319.76 10157.79 1.09 926773.76 10577.01 1.14 1
224 967896.56 9819.6 1.01 951144.32 12343.83 1.3 0.98
232 990621.12 7771.97 0.78 916361.2 17878.44 1.95 0.93
240 995285.04 20104.22 2.02 972119.6 12856.42 1.32 0.98
248 1029436 20404.97 1.98 965301.28 11102.95 1.15 0.94
256 1038724.8 19201.03 1.85 1029942.08 12563.07 1.22 0.99
Written By
----------
Alexei Starovoitov <ast@kernel.org>
Kumar Kartikeya Dwivedi <memxor@gmail.com>
[0]: https://www.cs.rochester.edu/research/synchronization/pseudocode/timeout.html
[1]: https://dl.acm.org/doi/10.1145/571825.571830
[2]: https://github.com/kkdwivedi/rqspinlock/blob/main/rqspinlock.pdf
[3]: https://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/kernel-tla.git/plain/qspinlock.tla
[4]: https://arxiv.org/pdf/1810.05600
====================
Link: https://patch.msgid.link/20250316040541.108729-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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| #
164c2465 |
| 16-Mar-2025 |
Kumar Kartikeya Dwivedi <memxor@gmail.com> |
rqspinlock: Protect waiters in queue from stalls
Implement the wait queue cleanup algorithm for rqspinlock. There are
three forms of waiters in the original queued spin lock algorithm. The
first is
rqspinlock: Protect waiters in queue from stalls
Implement the wait queue cleanup algorithm for rqspinlock. There are
three forms of waiters in the original queued spin lock algorithm. The
first is the waiter which acquires the pending bit and spins on the lock
word without forming a wait queue. The second is the head waiter that is
the first waiter heading the wait queue. The third form is of all the
non-head waiters queued behind the head, waiting to be signalled through
their MCS node to overtake the responsibility of the head.
In this commit, we are concerned with the second and third kind. First,
we augment the waiting loop of the head of the wait queue with a
timeout. When this timeout happens, all waiters part of the wait queue
will abort their lock acquisition attempts. This happens in three steps.
First, the head breaks out of its loop waiting for pending and locked
bits to turn to 0, and non-head waiters break out of their MCS node spin
(more on that later). Next, every waiter (head or non-head) attempts to
check whether they are also the tail waiter, in such a case they attempt
to zero out the tail word and allow a new queue to be built up for this
lock. If they succeed, they have no one to signal next in the queue to
stop spinning. Otherwise, they signal the MCS node of the next waiter to
break out of its spin and try resetting the tail word back to 0. This
goes on until the tail waiter is found. In case of races, the new tail
will be responsible for performing the same task, as the old tail will
then fail to reset the tail word and wait for its next pointer to be
updated before it signals the new tail to do the same.
We terminate the whole wait queue because of two main reasons. Firstly,
we eschew per-waiter timeouts with one applied at the head of the wait
queue. This allows everyone to break out faster once we've seen the
owner / pending waiter not responding for the timeout duration from the
head. Secondly, it avoids complicated synchronization, because when not
leaving in FIFO order, prev's next pointer needs to be fixed up etc.
Lastly, all of these waiters release the rqnode and return to the
caller. This patch underscores the point that rqspinlock's timeout does
not apply to each waiter individually, and cannot be relied upon as an
upper bound. It is possible for the rqspinlock waiters to return early
from a failed lock acquisition attempt as soon as stalls are detected.
The head waiter cannot directly WRITE_ONCE the tail to zero, as it may
race with a concurrent xchg and a non-head waiter linking its MCS node
to the head's MCS node through 'prev->next' assignment.
One notable thing is that we must use RES_DEF_TIMEOUT * 2 as our maximum
duration for the waiting loop (for the wait queue head), since we may
have both the owner and pending bit waiter ahead of us, and in the worst
case, need to span their maximum permitted critical section lengths.
Reviewed-by: Barret Rhoden <brho@google.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250316040541.108729-11-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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