linux-kernel - Re: [PATCH bpf] bpf: Add LBR data to BPF_PROG_TYPE_PERF

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Date:   Fri, 6 Dec 2019 06:55:05 +0000
From:   Martin Lau <kafai@...com>
To:     Daniel Xu <dxu@...uu.xyz>
CC:     "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>, Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andriin@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf] bpf: Add LBR data to BPF_PROG_TYPE_PERF_EVENT prog
 context

On Thu, Dec 05, 2019 at 04:12:26PM -0800, Daniel Xu wrote:
> Last-branch-record is an intel CPU feature that can be configured to
> record certain branches that are taken during code execution. This data
> is particularly interesting for profile guided optimizations. perf has
> had LBR support for a while but the data collection can be a bit coarse
> grained.
> 
> We (Facebook) have recently run a lot of experiments with feeding
> filtered LBR data to various PGO pipelines. We've seen really good
> results (+2.5% throughput with lower cpu util and lower latency) by
> feeding high request latency LBR branches to the compiler on a
> request-oriented service. We used bpf to read a special request context
> ID (which is how we associate branches with latency) from a fixed
> userspace address. Reading from the fixed address is why bpf support is
> useful.
> 
> Aside from this particular use case, having LBR data available to bpf
> progs can be useful to get stack traces out of userspace applications
> that omit frame pointers.
> 
> This patch adds support for LBR data to bpf perf progs.
Please re-submit when bpf-next opens.