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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Song Liu <songliubraving@fb.com>
Cc: Peter Zijlstra <peterz@infradead.org>, bpf <bpf@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"acme@kernel.org" <acme@kernel.org>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"kjain@linux.ibm.com" <kjain@linux.ibm.com>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v5 bpf-next 1/3] perf: enable branch record for software events
Date: Tue, 7 Sep 2021 13:50:07 -0700	[thread overview]
Message-ID: <CAEf4Bzacgy=u+aTmTGbWo6UhHUWk6uA-zsKdsNCk5g6oPNycog@mail.gmail.com> (raw)
In-Reply-To: <B3CCDCB4-1D06-4331-A3C7-B1D413A4ABA5@fb.com>

On Tue, Sep 7, 2021 at 12:02 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Sep 3, 2021, at 9:50 AM, Song Liu <songliubraving@fb.com> wrote:
> >
> >
> >
> >> On Sep 3, 2021, at 1:02 AM, Peter Zijlstra <peterz@infradead.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
>

  reply	other threads:[~2021-09-07 20:50 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-02 16:57 [PATCH v5 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Song Liu
2021-09-02 16:57 ` [PATCH v5 bpf-next 1/3] perf: enable branch record for software events Song Liu
2021-09-02 20:49   ` John Fastabend
2021-09-03  8:02   ` Peter Zijlstra
2021-09-03 16:50     ` Song Liu
2021-09-07 18:59       ` Song Liu
2021-09-07 20:50         ` Andrii Nakryiko [this message]
2021-09-03  8:27   ` Peter Zijlstra
2021-09-03 16:45     ` Song Liu
2021-09-04 10:18       ` Peter Zijlstra
2021-09-02 16:57 ` [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot Song Liu
2021-09-02 20:56   ` John Fastabend
2021-09-02 22:04     ` Song Liu
2021-09-02 22:53   ` Andrii Nakryiko
2021-09-02 23:03     ` Song Liu
2021-09-02 23:05       ` Andrii Nakryiko
2021-09-03  1:03   ` Alexei Starovoitov
2021-09-03  8:28   ` Peter Zijlstra
2021-09-03 16:58     ` Song Liu
2021-09-03  8:47   ` Peter Zijlstra
2021-09-03 17:06     ` Song Liu
2021-09-03 17:10     ` Andrii Nakryiko
2021-09-04 10:24       ` Peter Zijlstra
2021-09-04 10:55         ` Peter Zijlstra
2021-09-02 16:57 ` [PATCH v5 bpf-next 3/3] selftests/bpf: add test for bpf_get_branch_snapshot Song Liu
2021-09-02 21:05   ` John Fastabend
2021-09-02 22:54 ` [PATCH v5 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Andrii Nakryiko

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