linux-kernel - Re: [PATCH v3 1/2] perf/core: Share an event with multiple cgroups

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Message-ID: <YHmTl4bcjx6UA9Ko@hirez.programming.kicks-ass.net>
Date:   Fri, 16 Apr 2021 15:39:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Song Liu <songliubraving@...com>, Tejun Heo <tj@...nel.org>,
        kernel test robot <lkp@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/2] perf/core: Share an event with multiple cgroups

On Fri, Apr 16, 2021 at 09:19:08PM +0900, Namhyung Kim wrote:
> > Are you actually using mmap() to read? I had a proposal for FORMAT_GROUP
> > like thing for mmap(), but I never implemented that (didn't get the
> > enthousiatic response I thought it would). But yeah, there's nowhere
> > near enough space in there for PERCPU.
> 
> Recently there's a patch to do it with rdpmc which needs to mmap first.
> 
> https://lore.kernel.org/lkml/20210414155412.3697605-1-robh@kernel.org/

Yeah, I'm not sure about that, I've not looked at it recently. The thing
is though; for RDPMC to work, you *NEED* to be on the same CPU as the
counter.

The typical RDPMC use-case is self monitoring.