linux-kernel - Re: [RFC][PATCH 6/8] perf/x86: Clear ->event

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Date:   Wed, 20 Mar 2019 13:25:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, tonyj@...e.com,
        nelson.dsouza@...el.com
Subject: Re: [RFC][PATCH 6/8] perf/x86: Clear ->event_constraint[] on put

On Tue, Mar 19, 2019 at 02:50:21PM -0700, Stephane Eranian wrote:
> On Thu, Mar 14, 2019 at 6:11 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > The current code unconditionally clears cpuc->event_constraint[i]
> > before calling get_event_constraints(.idx=i). The only site that cares
> > is intel_get_event_constraints() where the c1 load will always be
> > NULL.
> >
> > However, always calling get_event_constraints() on all events is
> > wastefull, most times it will return the exact same result. Therefore
> > retain the logic in intel_get_event_constraints() and change the
> > generic code to only clear the constraint on put.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> I thought there was caching of the constraint in case it was static
> (or unconstrained)

Yeah, I was under the same impression :-/

> to avoid walking the constraint table each time between invocations
> on the same group_sched_in() call. But the way the c1 vs. c2 logic
> is written I don't see it. In which case, this could be another opportunity.

Right, the next patch attempts this.