linux-kernel - Re: [RFC] Sharing PMU counters across compatible events

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Date:   Wed, 13 Dec 2017 11:18:07 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [RFC] Sharing PMU counters across compatible events

On Mon, Dec 11, 2017 at 07:34:49AM -0800, Tejun Heo wrote:
> Hello, Jiri.
> 
> On Wed, Dec 06, 2017 at 12:42:04PM +0100, Jiri Olsa wrote:
> > I see this rather on the hw level, since it concerns HW counters
> > 
> > I think we could detect same (alias) events at the time counters
> > are added/removed on/from the cpu and share their HW part like
> > counter idx, regs and such (struct hw_perf_event_cpu in my changes)
> > 
> > this way it'd be completely transparent for generic code
> 
> I don't quite follow why doing this in arch code is better than
> generic.  Doing this in arch means we'd need to do the same thing
> multiple times for different archs.  Why is that better?

so I can see this to be useful for HW conters only, because
of limited number of regs

as for the higher level on which this could be implemented I
see some pitfals with event rotations as Peter mentioned and
task/cpu contexts scheduling.. while the hw-level implementation
seems pretty straight forward

I'll test the code and let's see ;-)

jirka