linux-kernel - Re: [PATCH] arm64: perf_event: Fix time

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Date:   Thu, 30 Apr 2020 17:18:15 +0100
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marc Zyngier <maz@...nel.org>, Leo Yan <leo.yan@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Mike Leach <mike.leach@...aro.org>,
        Al Grant <Al.Grant@....com>, James Clark <James.Clark@....com>,
        tglx@...utronix.de
Subject: Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer

On Thu, Apr 30, 2020 at 06:04:36PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 30, 2020 at 04:29:23PM +0100, Marc Zyngier wrote:
> 
> > I wonder if we could/should make __sched_clock_offset available even when
> > CONFIG_HAVE_UNSTABLE_SCHED_CLOCK isn't defined. It feels like it would
> > help with this particular can or worm...
> 
> Errrgh. __sched_clock_offset is only needed on x86 because we transition
> from one clock device to another on boot. It really shouldn't exist on
> anything sane.

I think we still transition from jiffies on arm64, because we don't register
with sched_clock until the timer driver probes. Marc, is that right?

> Let me try and understand your particular problem better.

I think the long and short of it is that userspace needs a way to convert
the raw counter cycles into a ns value that can be compared against values
coming out of sched_clock. To do this accurately, I think it needs the
cycles value at the point when sched_clock was initialised.

Will