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

[Puranjay Mohan <puranjay@kernel.org>]

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/20260616155716.2631508-1-puranjay@kernel.org/

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(-)

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