linux-kernel - Re: [PATCH v5 bpf-next 1/3] perf: enable branch record for software events

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Date:   Tue, 7 Sep 2021 13:50:07 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Peter Zijlstra <peterz@...radead.org>, bpf <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "kjain@...ux.ibm.com" <kjain@...ux.ibm.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v5 bpf-next 1/3] perf: enable branch record for software events

On Tue, Sep 7, 2021 at 12:02 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Sep 3, 2021, at 9:50 AM, Song Liu <songliubraving@...com> wrote:
> >
> >
> >
> >> On Sep 3, 2021, at 1:02 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >>
> >> On Thu, Sep 02, 2021 at 09:57:04AM -0700, Song Liu wrote:
> >>> +static int
> >>> +intel_pmu_snapshot_branch_stack(struct perf_branch_entry *entries, unsigned int cnt)
> >>> +{
> >>> +   struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> >>> +
> >>> +   intel_pmu_disable_all();
> >>> +   intel_pmu_lbr_read();
> >>> +   cnt = min_t(unsigned int, cnt, x86_pmu.lbr_nr);
> >>> +
> >>> +   memcpy(entries, cpuc->lbr_entries, sizeof(struct perf_branch_entry) * cnt);
> >>> +   intel_pmu_enable_all(0);
> >>> +   return cnt;
> >>> +}
> >>
> >> Would something like the below help get rid of that memcpy() ?
> >>
> >> (compile tested only)
> >
> > We can get rid of the memcpy. But we will need an extra "size" or "num_entries"
> > parameter for intel_pmu_lbr_read. I can add this change in the next version.
> >
>
> This is trickier than I thought. As current lbr_read() function works with
> perf_branch_stack, while the BPF helper side uses array of perf_branch_entry.
> And the array is passed into the helper by the BPF program. Therefore, to
> really get rid of the memcpy, we need to refactor the lbr driver code more.
> How about we keep the memcpy for now, and add the optimization later (if we
> think it is necessary)?
>

Sounds good to me!

> Thanks,
> Song
>