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

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Message-ID: <79ba3f5f9af951b2de52b8eb9e1bc25f@kernel.org>
Date:   Thu, 30 Apr 2020 18:33:48 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.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 2020-04-30 17:18, Will Deacon wrote:
> 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?

Indeed. The clocksource is only available relatively late, as we need to
discover the details of the platform and enable the various workarounds
(because nobody can get a simple 64bit counter right). So it is only at
that stage that we transition to it.

         M.
-- 
Jazz is not dead. It just smells funny...