linux-kernel - Re: [PATCH] perf/x86: fix wrong assumption that LBR is only useful for sampling events

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Message-ID: <CAEf4Bza9H=nH4+=dDNm55X5LZp4MVSkKyBcnuNq3+8cP6qt=uQ@mail.gmail.com>
Date: Thu, 5 Sep 2024 13:22:37 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: "Liang, Kan" <kan.liang@...ux.intel.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-perf-users@...r.kernel.org, 
	peterz@...radead.org, x86@...nel.org, mingo@...hat.com, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, acme@...nel.org, 
	kernel-team@...a.com, stable@...r.kernel.org
Subject: Re: [PATCH] perf/x86: fix wrong assumption that LBR is only useful
 for sampling events

On Thu, Sep 5, 2024 at 12:21 PM Liang, Kan <kan.liang@...ux.intel.com> wrote:
>
>
>
> On 2024-09-05 2:00 p.m., Andrii Nakryiko wrote:
> > It's incorrect to assume that LBR can/should only be used with sampling
> > events. BPF subsystem provides bpf_get_branch_snapshot() BPF helper,
> > which expects a properly setup and activated perf event which allows
> > kernel to capture LBR data.
> >
> > For instance, retsnoop tool ([0]) makes an extensive use of this
> > functionality and sets up perf event as follows:
> >
> >       struct perf_event_attr attr;
> >
> >       memset(&attr, 0, sizeof(attr));
> >       attr.size = sizeof(attr);
> >       attr.type = PERF_TYPE_HARDWARE;
> >       attr.config = PERF_COUNT_HW_CPU_CYCLES;
> >       attr.sample_type = PERF_SAMPLE_BRANCH_STACK;
> >       attr.branch_sample_type = PERF_SAMPLE_BRANCH_KERNEL;
> >
> > Commit referenced in Fixes tag broke this setup by making invalid assumption
> > that LBR is useful only for sampling events. Remove that assumption.
> >
> > Note, earlier we removed a similar assumption on AMD side of LBR support,
> > see [1] for details.
> >
> >   [0] https://github.com/anakryiko/retsnoop
> >   [1] 9794563d4d05 ("perf/x86/amd: Don't reject non-sampling events with configured LBR")
> >
> > Cc: stable@...r.kernel.org # 6.8+
> > Fixes: 85846b27072d ("perf/x86: Add PERF_X86_EVENT_NEEDS_BRANCH_STACK flag")
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> > ---
> >  arch/x86/events/intel/core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > index 9e519d8a810a..f82a342b8852 100644
> > --- a/arch/x86/events/intel/core.c
> > +++ b/arch/x86/events/intel/core.c
> > @@ -3972,7 +3972,7 @@ static int intel_pmu_hw_config(struct perf_event *event)
> >                       x86_pmu.pebs_aliases(event);
> >       }
> >
> > -     if (needs_branch_stack(event) && is_sampling_event(event))
> > +     if (needs_branch_stack(event))
> >               event->hw.flags  |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
>
> To limit the LBR for a sampling event is to avoid unnecessary branch
> stack setup for a counting event in the sample read. The above change
> should break the sample read case.
>
> How about the below patch (not test)? Is it good enough for the BPF usage?
>
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 0c9c2706d4ec..8d67cbda916b 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -3972,8 +3972,12 @@ static int intel_pmu_hw_config(struct perf_event
> *event)
>                 x86_pmu.pebs_aliases(event);
>         }
>
> -       if (needs_branch_stack(event) && is_sampling_event(event))
> -               event->hw.flags  |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
> +       if (needs_branch_stack(event)) {
> +               /* Avoid branch stack setup for counting events in SAMPLE READ */
> +               if (is_sampling_event(event) ||
> +                   !(event->attr.sample_type & PERF_SAMPLE_READ))
> +                       event->hw.flags  |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
> +       }
>

I'm sure it will be fine for my use case, as I set only
PERF_SAMPLE_BRANCH_STACK.

But I'll leave it up to perf subsystem experts to decide if this
condition makes sense, because looking at what PERF_SAMPLE_READ is:

          PERF_SAMPLE_READ
                 Record counter values for all events in a group,
                 not just the group leader.

It's not clear why this would disable LBR, if specified.

>         if (branch_sample_counters(event)) {
>                 struct perf_event *leader, *sibling;
>
>
> Thanks,
> Kan
> >
> >       if (branch_sample_counters(event)) {