linux-kernel - Re: [PATCH V5 4/8] perf/x86/intel/uncore: add new data structures for free running counters

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Message-ID: <20180119213355.GE2269@hirez.programming.kicks-ass.net>
Date:   Fri, 19 Jan 2018 22:33:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     "Liang, Kan" <kan.liang@...el.com>,
        Stephane Eranian <eranian@...gle.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>
Subject: Re: [PATCH V5 4/8] perf/x86/intel/uncore: add new data structures
 for free running counters

On Fri, Jan 19, 2018 at 12:24:17PM -0800, Andi Kleen wrote:
> > Oh, think a bit more.
> > I think we cannot do the same thing as we did for CPU PMU's fixed counters.
> > 
> > The counters here are free running counters. They cannot be start/stop.
> 
> Yes free running counter have completely different semantics. They
> need a separate event code.

The only thing that matters is if they count the same thing or not.

The not start/stop thing is not important. See arch/x86/events/msr.c on
how to deal with that. The short story is that you simply ignore stop
and update the prev_count on start. Then any next update will increment
with the correct delta.

(if the counter is short you also need to run a timer to deal with
wraps).