Re: [PATCH v6 0/3] perf/core: sched_task() dispatch and branch entry fixes

On 06/08/2026 14:52, Puranjay Mohan wrote:
> These three fixes were found while adding BRBE support for
> bpf_get_branch_snapshot() on arm64 and have been carried in that series
> since v1 [1]. They do not depend on it, so they go on their own from
> here; the version number continues from that series to avoid two
> numbering schemes for the same patches.
> 
> Patch 1 stops __perf_pmu_sched_task() passing a NULL pmu_ctx to
> pmu->sched_task(). armv8pmu_sched_task() is the only implementation that
> dereferences the argument, so the oops needs BRBE.
> 
> Patch 2 makes perf_pmu_sched_task() visit PMUs whose events are all
> CPU-wide. They are skipped today whenever the scheduled task has a perf
> event of its own, so branch records leak across task boundaries with
> perf record -b -a. intel_pmu_lbr_add() calls perf_sched_cb_inc()
> unconditionally, so x86 LBR is affected the same way.
> 
> Dropping the early return alone leaves both dispatch paths running for
> one case, so perf_ctx_sched_task_cb() gains a matching gate. The two
> could instead be collapsed into perf_pmu_sched_task() alone, since
> __perf_pmu_sched_task() already passes the same epc, but that would move
> the callback out of the perf_ctx_disable() window for every PMU rather
> than just that one case, which seemed like too much for a fix tagged for
> stable.
> 
> Patch 3 clears struct perf_branch_entry with a single struct assignment.
> perf_clear_branch_entry_bitfields() had drifted from the struct: new_type
> and priv were never cleared, and arm_pmuv3.c allocates the per-CPU branch
> stack with kmalloc().
> 
> Tested on a 128 CPU arm64 machine with BRBE. A WARN_ON_ONCE() at the gate
> patch 2 adds to perf_ctx_sched_task_cb() fires within seconds of running
> perf record -b -a alongside a task-bound event pinned to a different CPU.
> 
> Changes in v6:
> - Split the sched_task() fix into patches 1 and 2; the NULL dereference
>    and the missed dispatch are separate bugs with different reachability.
> - Gate perf_ctx_sched_task_cb() on cpc->task_epc. v5 removed the early
>    return in perf_pmu_sched_task() without it, so both paths ran for a
>    task whose event for that PMU is pinned to another CPU. Caught by the
>    WARN_ON_ONCE() described above.
> - Tag patches 1 and 2 for stable.
> - Send separately from the BRBE series, rebased onto tip perf/core.
> 
> [1] https://lore.kernel.org/all/[email protected]/
> 
> Based on tip perf/core (f4dfab174244).
> 
> Puranjay Mohan (3):
>    perf/core: Fix NULL pmu_ctx passed to pmu->sched_task()
>    perf/core: Run sched_task() for PMUs with only CPU-wide events
>    perf/core: Clear the whole branch entry in perf_clear_branch_entry()
> 
>   arch/x86/events/amd/brs.c   |  2 +-
>   arch/x86/events/amd/lbr.c   |  2 +-
>   arch/x86/events/intel/lbr.c |  6 +++---
>   drivers/perf/arm_brbe.c     |  2 +-
>   include/linux/perf_event.h  | 16 ++--------------
>   kernel/events/core.c        | 15 +++++++++++----
>   6 files changed, 19 insertions(+), 24 deletions(-)
> 

Reviewed-by: James Clark <[email protected]>