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From: Will Deacon <will@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Marc Zyngier <maz@kernel.org>, Leo Yan <leo.yan@linaro.org>,
Mark Rutland <mark.rutland@arm.com>,
Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org,
Mathieu Poirier <mathieu.poirier@linaro.org>,
Mike Leach <mike.leach@linaro.org>, Al Grant <Al.Grant@arm.com>,
James Clark <James.Clark@arm.com>,
tglx@linutronix.de
Subject: Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer
Date: Thu, 30 Apr 2020 17:18:15 +0100 [thread overview]
Message-ID: <20200430161815.GE25258@willie-the-truck> (raw)
In-Reply-To: <20200430160436.GC13575@hirez.programming.kicks-ass.net>
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
next prev parent reply other threads:[~2020-04-30 16:18 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-20 9:35 Leo Yan
2020-04-01 1:24 ` Leo Yan
2020-04-30 14:58 ` Will Deacon
2020-04-30 15:29 ` Marc Zyngier
2020-04-30 16:04 ` Peter Zijlstra
2020-04-30 16:18 ` Will Deacon [this message]
2020-04-30 17:33 ` Marc Zyngier
2020-05-01 15:14 ` Leo Yan
2020-05-01 15:26 ` Will Deacon
2020-05-01 16:10 ` Leo Yan
2020-05-01 17:13 ` Will Deacon
2020-05-01 15:29 ` Marc Zyngier
2020-04-30 16:27 ` Peter Zijlstra
2020-04-30 16:45 ` Will Deacon
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