| 34423456 | 29-Oct-2025 |
Tejun Heo <tj@kernel.org> |
sched_ext/tools: Restore backward compat with v6.12 kernels
Commit 111a79800aed ("tools/sched_ext: Strip compatibility macros for
cgroup and dispatch APIs") removed the compat layer for v6.12-v6.13
sched_ext/tools: Restore backward compat with v6.12 kernels
Commit 111a79800aed ("tools/sched_ext: Strip compatibility macros for
cgroup and dispatch APIs") removed the compat layer for v6.12-v6.13 kfunc
renaming, but v6.12 is the current LTS kernel and will remain supported
through 2026. Restore backward compatibility so schedulers built with v6.19+
headers can run on v6.12 kernels.
The restored compat differs from the original in two ways:
1. Uses ___new/___old suffixes instead of ___compat for clarity. The new
macros check for v6.13+ names (scx_bpf_dsq_move*), fall back to v6.12
names (scx_bpf_dispatch_from_dsq*, scx_bpf_consume), then return safe
no-ops for missing symbols.
2. Integrates with the args-struct-packing changes added in c0d630ba347c
("sched_ext: Wrap kfunc args in struct to prepare for aux__prog").
scx_bpf_dsq_insert_vtime() now tries __scx_bpf_dsq_insert_vtime (args
struct), then scx_bpf_dsq_insert_vtime___compat (v6.13-v6.18), then
scx_bpf_dispatch_vtime___compat (pre-v6.13).
Forward compatibility is not restored - binaries built against v6.13 or
earlier headers won't run on v6.19+ kernels, as the old kfunc names are not
exported. This is acceptable since the priority is new binaries running on
older kernels.
Also add missing compat checks for ops.cgroup_set_bandwidth() (added v6.17)
and ops.cgroup_set_idle() (added v6.19). These need to be NULLed out in
userspace on older kernels.
Reported-by: Andrea Righi <arighi@nvidia.com>
Acked-and-tested-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| a3f5d482 | 27-Oct-2025 |
Tejun Heo <tj@kernel.org> |
sched_ext: Allow scx_bpf_reenqueue_local() to be called from anywhere
The ops.cpu_acquire/release() callbacks miss events under multiple conditions.
There are two distinct task dispatch gaps that ca
sched_ext: Allow scx_bpf_reenqueue_local() to be called from anywhere
The ops.cpu_acquire/release() callbacks miss events under multiple conditions.
There are two distinct task dispatch gaps that can cause cpu_released flag
desynchronization:
1. balance-to-pick_task gap: This is what was originally reported. balance_scx()
can enqueue a task, but during consume_remote_task() when the rq lock is
released, a higher priority task can be enqueued and ultimately picked while
cpu_released remains false. This gap is closeable via RETRY_TASK handling.
2. ttwu-to-pick_task gap: ttwu() can directly dispatch a task to a CPU's local
DSQ. By the time the sched path runs on the target CPU, higher class tasks may
already be queued. In such cases, nothing on sched_ext side will be invoked,
and the only solution would be a hook invoked regardless of sched class, which
isn't desirable.
Rather than adding invasive core hooks, BPF schedulers can use generic BPF
mechanisms like tracepoints. From SCX scheduler's perspective, this is congruent
with other mechanisms it already uses and doesn't add further friction.
The main use case for cpu_release() was calling scx_bpf_reenqueue_local() when
a CPU gets preempted by a higher priority scheduling class. However, the old
scx_bpf_reenqueue_local() could only be called from cpu_release() context.
Add a new version of scx_bpf_reenqueue_local() that can be called from any
context by deferring the actual re-enqueue operation. This eliminates the need
for cpu_acquire/release() ops entirely. Schedulers can now use standard BPF
mechanisms like the sched_switch tracepoint to detect and handle CPU preemption.
Update scx_qmap to demonstrate the new approach using sched_switch instead of
cpu_release, with compat support for older kernels. Mark cpu_acquire/release()
as deprecated. The old scx_bpf_reenqueue_local() variant will be removed in
v6.23.
Reported-by: Wen-Fang Liu <liuwenfang@honor.com>
Link: https://lore.kernel.org/all/8d64c74118c6440f81bcf5a4ac6b9f00@honor.com/
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| dcb938c4 | 25-Oct-2025 |
Tejun Heo <tj@kernel.org> |
sched_ext: Add ___compat suffix to scx_bpf_dsq_insert___v2 in compat.bpf.h
2dbbdeda77a6 ("sched_ext: Fix scx_bpf_dsq_insert() backward binary
compatibility") renamed the new bool-returning variant t
sched_ext: Add ___compat suffix to scx_bpf_dsq_insert___v2 in compat.bpf.h
2dbbdeda77a6 ("sched_ext: Fix scx_bpf_dsq_insert() backward binary
compatibility") renamed the new bool-returning variant to scx_bpf_dsq_insert___v2
in the kernel. However, libbpf currently only strips ___SUFFIX on the BPF side,
not on kernel symbols, so the compat wrapper couldn't match the kernel kfunc and
would always fall back to the old variant even when the new one was available.
Add an extra ___compat suffix as a workaround - libbpf strips one suffix on the
BPF side leaving ___v2, which then matches the kernel kfunc directly. In the
future when libbpf strips all suffixes on both sides, all suffixes can be
dropped.
Fixes: 2dbbdeda77a6 ("sched_ext: Fix scx_bpf_dsq_insert() backward binary compatibility")
Cc: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 2dbbdeda | 21-Oct-2025 |
Tejun Heo <tj@kernel.org> |
sched_ext: Fix scx_bpf_dsq_insert() backward binary compatibility
cded46d97159 ("sched_ext: Make scx_bpf_dsq_insert*() return bool")
introduced a new bool-returning scx_bpf_dsq_insert() and renamed
sched_ext: Fix scx_bpf_dsq_insert() backward binary compatibility
cded46d97159 ("sched_ext: Make scx_bpf_dsq_insert*() return bool")
introduced a new bool-returning scx_bpf_dsq_insert() and renamed the old
void-returning version to scx_bpf_dsq_insert___compat, with the expectation
that libbpf would match old binaries to the ___compat variant, maintaining
backward binary compatibility. However, while libbpf ignores ___suffix on
the BPF side when matching symbols, it doesn't do so for kernel-side symbols.
Old binaries compiled with the original scx_bpf_dsq_insert() could no longer
resolve the symbol.
Fix by reversing the naming: Keep scx_bpf_dsq_insert() as the old
void-returning interface and add ___v2 to the new bool-returning version.
This allows old binaries to continue working while new code can use the
___v2 variant. Once libbpf is updated to ignore kernel-side ___SUFFIX, the
___v2 suffix can be dropped when the compat interface is removed.
v2: Use ___v2 instead of ___new.
Fixes: cded46d97159 ("sched_ext: Make scx_bpf_dsq_insert*() return bool")
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 44f5c8ec | 15-Oct-2025 |
Ryan Newton <newton@meta.com> |
sched_ext: Add lockless peek operation for DSQs
The builtin DSQ queue data structures are meant to be used by a wide
range of different sched_ext schedulers with different demands on these
data stru
sched_ext: Add lockless peek operation for DSQs
The builtin DSQ queue data structures are meant to be used by a wide
range of different sched_ext schedulers with different demands on these
data structures. They might be per-cpu with low-contention, or
high-contention shared queues. Unfortunately, DSQs have a coarse-grained
lock around the whole data structure. Without going all the way to a
lock-free, more scalable implementation, a small step we can take to
reduce lock contention is to allow a lockless, small-fixed-cost peek at
the head of the queue.
This change allows certain custom SCX schedulers to cheaply peek at
queues, e.g. during load balancing, before locking them. But it
represents a few extra memory operations to update the pointer each
time the DSQ is modified, including a memory barrier on ARM so the write
appears correctly ordered.
This commit adds a first_task pointer field which is updated
atomically when the DSQ is modified, and allows any thread to peek at
the head of the queue without holding the lock.
Signed-off-by: Ryan Newton <newton@meta.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| bd7143e7 | 07-Oct-2025 |
Tejun Heo <tj@kernel.org> |
sched_ext/tools: Add compat wrapper for scx_bpf_task_set_slice/dsq_vtime()
for sub-scheduler authority checks. Add compat wrappers which fall back to
direct p->scx field writes on older kernels.
Su
sched_ext/tools: Add compat wrapper for scx_bpf_task_set_slice/dsq_vtime()
for sub-scheduler authority checks. Add compat wrappers which fall back to
direct p->scx field writes on older kernels.
Suggested-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| cded46d9 | 07-Oct-2025 |
Tejun Heo <tj@kernel.org> |
sched_ext: Make scx_bpf_dsq_insert*() return bool
In preparation for hierarchical schedulers, change scx_bpf_dsq_insert() and
scx_bpf_dsq_insert_vtime() to return bool instead of void. With
sub-sche
sched_ext: Make scx_bpf_dsq_insert*() return bool
In preparation for hierarchical schedulers, change scx_bpf_dsq_insert() and
scx_bpf_dsq_insert_vtime() to return bool instead of void. With
sub-schedulers, there will be no reliable way to guarantee a task is still
owned by the sub-scheduler at insertion time (e.g., the task may have been
migrated to another scheduler). The bool return value will enable
sub-schedulers to detect and gracefully handle insertion failures.
For the root scheduler, insertion failures will continue to trigger scheduler
abort via scx_error(), so existing code doesn't need to check the return
value. Backward compatibility is maintained through compat wrappers.
Also update scx_bpf_dsq_move() documentation to clarify that it can return
false for sub-schedulers when @dsq_id points to a disallowed local DSQ.
Reviewed-by: Changwoo Min <changwoo@igalia.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| c0d630ba | 07-Oct-2025 |
Tejun Heo <tj@kernel.org> |
sched_ext: Wrap kfunc args in struct to prepare for aux__prog
scx_bpf_dsq_insert_vtime() and scx_bpf_select_cpu_and() currently have 5
parameters. An upcoming change will add aux__prog parameter whi
sched_ext: Wrap kfunc args in struct to prepare for aux__prog
scx_bpf_dsq_insert_vtime() and scx_bpf_select_cpu_and() currently have 5
parameters. An upcoming change will add aux__prog parameter which will exceed
BPF's 5 argument limit.
Prepare by adding new kfuncs __scx_bpf_dsq_insert_vtime() and
__scx_bpf_select_cpu_and() that take args structs. The existing kfuncs are
kept as compatibility wrappers. BPF programs use inline wrappers that detect
kernel API version via bpf_core_type_exists() and use the new struct-based
kfuncs when available, falling back to compat kfuncs otherwise. This allows
BPF programs to work with both old and new kernels.
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 3035addf | 07-Oct-2025 |
Tejun Heo <tj@kernel.org> |
sched_ext: Add scx_bpf_task_set_slice() and scx_bpf_task_set_dsq_vtime()
With the planned hierarchical scheduler support, sub-schedulers will need to
be verified for authority before being allowed t
sched_ext: Add scx_bpf_task_set_slice() and scx_bpf_task_set_dsq_vtime()
With the planned hierarchical scheduler support, sub-schedulers will need to
be verified for authority before being allowed to modify task->scx.slice and
task->scx.dsq_vtime. Add scx_bpf_task_set_slice() and
scx_bpf_task_set_dsq_vtime() which will perform the necessary permission
checks.
Root schedulers can still directly write to these fields, so this doesn't
affect existing schedulers.
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| a08b4dca | 04-Sep-2025 |
Andrea Righi <arighi@nvidia.com> |
tools/sched_ext: Add compat helper for scx_bpf_cpu_curr()
Introduce a compatibility helper that allows BPF schedulers to use
scx_bpf_cpu_curr() on older kernels.
Fixes: 20b158094a1ad ("sched_ext: I
tools/sched_ext: Add compat helper for scx_bpf_cpu_curr()
Introduce a compatibility helper that allows BPF schedulers to use
scx_bpf_cpu_curr() on older kernels.
Fixes: 20b158094a1ad ("sched_ext: Introduce scx_bpf_cpu_curr()")
Cc: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 20b15809 | 03-Sep-2025 |
Christian Loehle <christian.loehle@arm.com> |
sched_ext: Introduce scx_bpf_cpu_curr()
Provide scx_bpf_cpu_curr() as a way for scx schedulers to check the curr
task of a remote rq without assuming its lock is held.
Many scx schedulers make use
sched_ext: Introduce scx_bpf_cpu_curr()
Provide scx_bpf_cpu_curr() as a way for scx schedulers to check the curr
task of a remote rq without assuming its lock is held.
Many scx schedulers make use of scx_bpf_cpu_rq() to check a remote curr
(e.g. to see if it should be preempted). This is problematic because
scx_bpf_cpu_rq() provides access to all fields of struct rq, most of
which aren't safe to use without holding the associated rq lock.
Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| e0ca1696 | 03-Sep-2025 |
Christian Loehle <christian.loehle@arm.com> |
sched_ext: Introduce scx_bpf_locked_rq()
Most fields in scx_bpf_cpu_rq() assume that its rq_lock is held.
Furthermore they become meaningless without rq lock, too.
Make a safer version of scx_bpf_cp
sched_ext: Introduce scx_bpf_locked_rq()
Most fields in scx_bpf_cpu_rq() assume that its rq_lock is held.
Furthermore they become meaningless without rq lock, too.
Make a safer version of scx_bpf_cpu_rq() that only returns a rq
if we hold rq lock of that rq.
Also mark the new scx_bpf_locked_rq() as returning NULL as
scx_bpf_cpu_rq() should've been too.
Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| b214b04d | 27-Feb-2025 |
Andrea Righi <arighi@nvidia.com> |
tools/sched_ext: Provide a compatible helper for scx_bpf_events()
Introduce __COMPAT_scx_bpf_events() to use scx_bpf_events() in a
compatible way also with kernels that don't provide this kfunc.
Th
tools/sched_ext: Provide a compatible helper for scx_bpf_events()
Introduce __COMPAT_scx_bpf_events() to use scx_bpf_events() in a
compatible way also with kernels that don't provide this kfunc.
This also fixes the following error with scx_qmap when running on a
kernel that does not provide scx_bpf_events():
; scx_bpf_events(&events, sizeof(events)); @ scx_qmap.bpf.c:777
318: (b7) r2 = 72 ; R2_w=72 async_cb
319: <invalid kfunc call>
kfunc 'scx_bpf_events' is referenced but wasn't resolved
Fixes: 9865f31d852a4 ("sched_ext: Add scx_bpf_events() and scx_read_event() for BPF schedulers")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 5e3b6424 | 24-Feb-2025 |
Andrea Righi <arighi@nvidia.com> |
tools/sched_ext: Provide consistent access to scx flags
Make all the SCX_OPS_* and SCX_PICK_IDLE_* flags available to the
user-space part of the schedulers via the compat interface.
This allows sch
tools/sched_ext: Provide consistent access to scx flags
Make all the SCX_OPS_* and SCX_PICK_IDLE_* flags available to the
user-space part of the schedulers via the compat interface.
This allows schedulers / selftests to set all the ops flags in
user-space, rather than having them split between BPF and user-space.
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Changwoo Min <changwoo@igalia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 0e9b4c10 | 24-Feb-2025 |
Andrea Righi <arighi@nvidia.com> |
sched_ext: idle: Introduce scx_bpf_nr_node_ids()
Similarly to scx_bpf_nr_cpu_ids(), introduce a new kfunc
scx_bpf_nr_node_ids() to expose the maximum number of NUMA nodes in the
system.
BPF schedul
sched_ext: idle: Introduce scx_bpf_nr_node_ids()
Similarly to scx_bpf_nr_cpu_ids(), introduce a new kfunc
scx_bpf_nr_node_ids() to expose the maximum number of NUMA nodes in the
system.
BPF schedulers can use this information together with the new node-aware
kfuncs, for example to create per-node DSQs, validate node IDs, etc.
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 01059219 | 18-Feb-2025 |
Andrea Righi <arighi@nvidia.com> |
sched_ext: idle: Introduce node-aware idle cpu kfunc helpers
Introduce a new kfunc to retrieve the node associated to a CPU:
int scx_bpf_cpu_node(s32 cpu)
Add the following kfuncs to provide BPF
sched_ext: idle: Introduce node-aware idle cpu kfunc helpers
Introduce a new kfunc to retrieve the node associated to a CPU:
int scx_bpf_cpu_node(s32 cpu)
Add the following kfuncs to provide BPF schedulers direct access to
per-node idle cpumasks information:
const struct cpumask *scx_bpf_get_idle_cpumask_node(int node)
const struct cpumask *scx_bpf_get_idle_smtmask_node(int node)
s32 scx_bpf_pick_idle_cpu_node(const cpumask_t *cpus_allowed,
int node, u64 flags)
s32 scx_bpf_pick_any_cpu_node(const cpumask_t *cpus_allowed,
int node, u64 flags)
Moreover, trigger an scx error when any of the non-node aware idle CPU
kfuncs are used when SCX_OPS_BUILTIN_IDLE_PER_NODE is enabled.
Cc: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 48849271 | 14-Feb-2025 |
Andrea Righi <arighi@nvidia.com> |
sched_ext: idle: Per-node idle cpumasks
Using a single global idle mask can lead to inefficiencies and a lot of
stress on the cache coherency protocol on large systems with multiple
NUMA nodes, sinc
sched_ext: idle: Per-node idle cpumasks
Using a single global idle mask can lead to inefficiencies and a lot of
stress on the cache coherency protocol on large systems with multiple
NUMA nodes, since all the CPUs can create a really intense read/write
activity on the single global cpumask.
Therefore, split the global cpumask into multiple per-NUMA node cpumasks
to improve scalability and performance on large systems.
The concept is that each cpumask will track only the idle CPUs within
its corresponding NUMA node, treating CPUs in other NUMA nodes as busy.
In this way concurrent access to the idle cpumask will be restricted
within each NUMA node.
The split of multiple per-node idle cpumasks can be controlled using the
SCX_OPS_BUILTIN_IDLE_PER_NODE flag.
By default SCX_OPS_BUILTIN_IDLE_PER_NODE is not enabled and a global
host-wide idle cpumask is used, maintaining the previous behavior.
NOTE: if a scheduler explicitly enables the per-node idle cpumasks (via
SCX_OPS_BUILTIN_IDLE_PER_NODE), scx_bpf_get_idle_cpu/smtmask() will
trigger an scx error, since there are no system-wide cpumasks.
= Test =
Hardware:
- System: DGX B200
- CPUs: 224 SMT threads (112 physical cores)
- Processor: INTEL(R) XEON(R) PLATINUM 8570
- 2 NUMA nodes
Scheduler:
- scx_simple [1] (so that we can focus at the built-in idle selection
policy and not at the scheduling policy itself)
Test:
- Run a parallel kernel build `make -j $(nproc)` and measure the average
elapsed time over 10 runs:
avg time | stdev
---------+------
before: 52.431s | 2.895
after: 50.342s | 2.895
= Conclusion =
Splitting the global cpumask into multiple per-NUMA cpumasks helped to
achieve a speedup of approximately +4% with this particular architecture
and test case.
The same test on a DGX-1 (40 physical cores, Intel Xeon E5-2698 v4 @
2.20GHz, 2 NUMA nodes) shows a speedup of around 1.5-3%.
On smaller systems, I haven't noticed any measurable regressions or
improvements with the same test (parallel kernel build) and scheduler
(scx_simple).
Moreover, with a modified scx_bpfland that uses the new NUMA-aware APIs
I observed an additional +2-2.5% performance improvement with the same
test.
[1] https://github.com/sched-ext/scx/blob/main/scheds/c/scx_simple.bpf.c
Cc: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 0aaaf89d | 14-Feb-2025 |
Andrea Righi <arighi@nvidia.com> |
sched_ext: idle: Introduce SCX_OPS_BUILTIN_IDLE_PER_NODE
Add the new scheduler flag SCX_OPS_BUILTIN_IDLE_PER_NODE, which allows
BPF schedulers to select between using a global flat idle cpumask or
m
sched_ext: idle: Introduce SCX_OPS_BUILTIN_IDLE_PER_NODE
Add the new scheduler flag SCX_OPS_BUILTIN_IDLE_PER_NODE, which allows
BPF schedulers to select between using a global flat idle cpumask or
multiple per-node cpumasks.
This only introduces the flag and the mechanism to enable/disable this
feature without affecting any scheduling behavior.
Cc: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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| 2e2006c9 | 12-Feb-2025 |
Chuyi Zhou <zhouchuyi@bytedance.com> |
sched_ext: Fix the incorrect bpf_list kfunc API in common.bpf.h.
Now BPF only supports bpf_list_push_{front,back}_impl kfunc, not bpf_list_
push_{front,back}.
This patch fix this issue. Without thi
sched_ext: Fix the incorrect bpf_list kfunc API in common.bpf.h.
Now BPF only supports bpf_list_push_{front,back}_impl kfunc, not bpf_list_
push_{front,back}.
This patch fix this issue. Without this patch, if we use bpf_list kfunc
in scx, the BPF verifier would complain:
libbpf: extern (func ksym) 'bpf_list_push_back': not found in kernel or
module BTFs
libbpf: failed to load object 'scx_foo'
libbpf: failed to load BPF skeleton 'scx_foo': -EINVAL
With this patch, the bpf list kfunc will work as expected.
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Fixes: 2a52ca7c98960 ("sched_ext: Add scx_simple and scx_example_qmap example schedulers")
Signed-off-by: Tejun Heo <tj@kernel.org>
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