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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Song Liu <songliubraving@fb.com>
Cc: bpf <bpf@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Peter Ziljstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Kajol Jain <kjain@linux.ibm.com>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot
Date: Thu, 2 Sep 2021 16:05:41 -0700	[thread overview]
Message-ID: <CAEf4BzbpibAeK-PDiu0TEgKXXuU4MbPdfaaBB057e7DS2G35-A@mail.gmail.com> (raw)
In-Reply-To: <F7A1D3BF-357A-46FB-92EA-938DB99D8193@fb.com>

On Thu, Sep 2, 2021 at 4:03 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Sep 2, 2021, at 3:53 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> >
> > On Thu, Sep 2, 2021 at 9:58 AM Song Liu <songliubraving@fb.com> wrote:
> >>
> >> Introduce bpf_get_branch_snapshot(), which allows tracing pogram to get
> >> branch trace from hardware (e.g. Intel LBR). To use the feature, the
> >> user need to create perf_event with proper branch_record filtering
> >> on each cpu, and then calls bpf_get_branch_snapshot in the bpf function.
> >> On Intel CPUs, VLBR event (raw event 0x1b00) can be use for this.
> >>
> >> Signed-off-by: Song Liu <songliubraving@fb.com>
> >> ---
> >> include/uapi/linux/bpf.h       | 22 ++++++++++++++++++++++
> >> kernel/bpf/trampoline.c        |  3 ++-
> >> kernel/trace/bpf_trace.c       | 33 +++++++++++++++++++++++++++++++++
> >> tools/include/uapi/linux/bpf.h | 22 ++++++++++++++++++++++
> >> 4 files changed, 79 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> >> index 791f31dd0abee..c986e6fad5bc0 100644
> >> --- a/include/uapi/linux/bpf.h
> >> +++ b/include/uapi/linux/bpf.h
> >> @@ -4877,6 +4877,27 @@ union bpf_attr {
> >>  *             Get the struct pt_regs associated with **task**.
> >>  *     Return
> >>  *             A pointer to struct pt_regs.
> >> + *
> >> + * long bpf_get_branch_snapshot(void *entries, u32 size, u64 flags)
> >> + *     Description
> >> + *             Get branch trace from hardware engines like Intel LBR. The
> >> + *             branch trace is taken soon after the trigger point of the
> >> + *             BPF program, so it may contain some entries after the
> >
> > This part is a leftover from previous design, so not relevant anymore?
>
> Hmm.. This is still relevant, but not very accurate. I guess we should
> provide more information, like "For more information about branches before
> the trigger point, this should be called early in the BPF program".

I read the part about "taken soon after the trigger point of BPF
program" as a reference to previous implementation. So maybe let's
clarify that because LBR is not frozen, from the time
bpf_get_branch_snapshot() is called to when we actually capture LBRs
we can waste few records due to internal kernel calls, so the user has
to be aware of that.

>
> Song
>
>
> >
> >> + *             trigger point. The user need to filter these entries
> >> + *             accordingly.
> >> + *
> >> + *             The data is stored as struct perf_branch_entry into output
> >> + *             buffer *entries*. *size* is the size of *entries* in bytes.
> >> + *             *flags* is reserved for now and must be zero.
> >> + *
> >> + *     Return
> >> + *             On success, number of bytes written to *buf*. On error, a
> >> + *             negative value.
> >> + *
> >> + *             **-EINVAL** if arguments invalid or **size** not a multiple
> >> + *             of **sizeof**\ (**struct perf_branch_entry**\ ).
> >> + *
> >> + *             **-ENOENT** if architecture does not support branch records.
> >
> > [...]
>

  reply	other threads:[~2021-09-02 23:05 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
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 [this message]
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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