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From: Will Deacon <email@example.com>
To: Leo Yan <firstname.lastname@example.org>
Cc: Peter Zijlstra <email@example.com>,
Marc Zyngier <firstname.lastname@example.org>,
Mark Rutland <email@example.com>,
Ingo Molnar <firstname.lastname@example.org>,
Arnaldo Carvalho de Melo <email@example.com>,
Alexander Shishkin <firstname.lastname@example.org>,
Jiri Olsa <email@example.com>, Namhyung Kim <firstname.lastname@example.org>,
Catalin Marinas <email@example.com>,
Mathieu Poirier <firstname.lastname@example.org>,
Mike Leach <email@example.com>, Al Grant <Al.Grant@arm.com>,
James Clark <James.Clark@arm.com>,
Subject: Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer
Date: Fri, 1 May 2020 18:13:53 +0100 [thread overview]
Message-ID: <20200501171353.GB19048@willie-the-truck> (raw)
On Sat, May 02, 2020 at 12:10:50AM +0800, Leo Yan wrote:
> On Fri, May 01, 2020 at 04:26:09PM +0100, Will Deacon wrote:
> > > > > 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's understanding is exactly what I want to resolve in this patch.
> > >
> > > The background info is for the ARM SPE  decoding with perf tool, if
> > > the timestamp is enabled, it uses the generic timer's counter as
> > > timestamp source. SPE trace data only contains the raw counter cycles,
> > > as Will mentioned, the perf tool needs to convert it to a coordinate
> > > value with sched_clock. This is why this patch tries to calculate the
> > > offset between the raw counter's ns value and sched_clock, eventually
> > > this offset value will be used by SPE's decoding code in Perf tool to
> > > calibrate a 'correct' timestamp.
> > >
> > > Based on your suggestions, I will use __sched_clock_offset to resolve
> > > the accuracy issue in patch v2. (I noticed Peter suggested to use a
> > > new API for wrapping clock_data structure, IIUC, __sched_clock_offset
> > > is more straightforward for this case).
> > >
> > > Please correct if I miss anything. Thank you for reviewing and
> > > suggestions!
> > I don't think you can use __sched_clock_offset without selecting
> > HAVE_UNSTABLE_SCHED_CLOCK, and we really don't want to do that just
> > for this. So Peter's idea about exposing what we need is better, although
> > you'll probably need to take care with the switch-over from jiffies.
> > It needs some thought, but one possibility would be to introduce a new
> > variant of sthe ched_clock_register() function that returns the cycle
> > offset, and then we could fish that out of the timer driver.
> Thanks a lot for you and Marc for correction.
> > If we're
> > crossing all the 'i's and dotting all the 't's then we'd want to disable the
> > perf userpage if sched_clock changes clocksource too (a bit like we do for
> > the vDSO).
> To be honest, one thing is not clear for me is how the perf tool to
> update the arch timer's parameters in the middle of tracing after
> disable and re-enable per userpage. I will note for this and look
> into detailed implementation for this part.
I don't fully understand the concern but, generally, the seqlock should
take care of any inconsistencies in the data page.
next prev parent reply other threads:[~2020-05-01 17:14 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
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 [this message]
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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