linux-kernel - Re: [PATCH] perf-stat: introduce bperf, share hardware PMCs with BPF

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Date:   Fri, 12 Mar 2021 13:12:48 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, acme@...nel.org,
        acme@...hat.com, namhyung@...nel.org, jolsa@...nel.org
Subject: Re: [PATCH] perf-stat: introduce bperf, share hardware PMCs with BPF

On Thu, Mar 11, 2021 at 06:02:57PM -0800, Song Liu wrote:
> perf uses performance monitoring counters (PMCs) to monitor system
> performance. The PMCs are limited hardware resources. For example,
> Intel CPUs have 3x fixed PMCs and 4x programmable PMCs per cpu.
> 
> Modern data center systems use these PMCs in many different ways:
> system level monitoring, (maybe nested) container level monitoring, per
> process monitoring, profiling (in sample mode), etc. In some cases,
> there are more active perf_events than available hardware PMCs. To allow
> all perf_events to have a chance to run, it is necessary to do expensive
> time multiplexing of events.
> 
> On the other hand, many monitoring tools count the common metrics (cycles,
> instructions). It is a waste to have multiple tools create multiple
> perf_events of "cycles" and occupy multiple PMCs.
> 
> bperf tries to reduce such wastes by allowing multiple perf_events of
> "cycles" or "instructions" (at different scopes) to share PMUs. Instead
> of having each perf-stat session to read its own perf_events, bperf uses
> BPF programs to read the perf_events and aggregate readings to BPF maps.
> Then, the perf-stat session(s) reads the values from these BPF maps.
> 
> Please refer to the comment before the definition of bperf_ops for the
> description of bperf architecture.
> 
> bperf is off by default. To enable it, pass --use-bpf option to perf-stat.
> bperf uses a BPF hashmap to share information about BPF programs and maps
> used by bperf. This map is pinned to bpffs. The default address is
> /sys/fs/bpf/bperf_attr_map. The user could change the address with option
> --attr-map.

nice, I recall the presentation about that and was wondering
when this will come up ;-)

> 
> ---
> Known limitations:
> 1. Do not support per cgroup events;
> 2. Do not support monitoring of BPF program (perf-stat -b);
> 3. Do not support event groups.
> 
> The following commands have been tested:
> 
>    perf stat --use-bpf -e cycles -a
>    perf stat --use-bpf -e cycles -C 1,3,4
>    perf stat --use-bpf -e cycles -p 123
>    perf stat --use-bpf -e cycles -t 100,101

I assume the output is same as standard perf?

jirka