Re: [PATCH v7 0/3] perf/core: sched_task() dispatch and branch entry fixes

[Ian Rogers]

On Thu, Sep 3, 2026 at 7:45 PM wuyifan <wuyifan50@xxxxxxxxxx> wrote:
>
> Hi Puranjay,
>
> On 8/10/2026 9:35 PM, Puranjay Mohan wrote:
> > These three fixes were found while adding BRBE support for
> > bpf_get_branch_snapshot() on arm64 and were carried in that series until
> > v6 [1]. They do not depend on it, so they go on their own; the version
> > number continues from that series to avoid two numbering schemes for the
> > same patches.
> perf record -C <cpu> -b [<command>] triggers a NULL pointer dereference
> in armv8pmu_sched_task().
>
> Commit fa9d27773873 ("perf: arm_pmu: Kill last use of per-CPU cpu_armpmu
> pointer") changed armv8pmu_sched_task() from reading the per-CPU
> cpu_armpmu variable to dereferencing pmu_ctx->pmu via
> to_arm_pmu(pmu_ctx->pmu). Meanwhile, commit bd2756811766 ("perf:
> Rewrite core context handling") made __perf_pmu_sched_task() pass
> cpc->task_epc to pmu->sched_task(). cpc->task_epc is only non-NULL
> while a task context is scheduled in; CPU-bound events do not schedule a
> task context, so cpc->task_epc stays NULL. The NULL argument is dereferenced
> and crashes.
>
> With the patches applied, perf record -C <cpu> -b no longer crashes,
> and system-wide perf record -b -e <event> -a alongside a task-bound event
> on a different CPU runs without any WARN_ON.
>
> Tested-by: Yifan Wu <wuyifan50@xxxxxxxxxx>

Hi,

Is there anything that needs doing to land this series? Avoiding a
perf crash seems worthwhile. I believe Peter and Ingo are best placed
to review the changes to kernel/events/core.c.

Thanks,
Ian