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Message-ID: <20201201041040.GC28939@leoy-ThinkPad-X240s>
Date: Tue, 1 Dec 2020 12:10:40 +0800
From: Leo Yan <leo.yan@...aro.org>
To: Will Deacon <will@...nel.org>
Cc: James Clark <james.clark@....com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-perf-users@...r.kernel.org,
Mark Rutland <mark.rutland@....com>,
Al Grant <al.grant@....com>,
John Garry <john.garry@...wei.com>,
Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [PATCH] drivers/perf: Enable PID_IN_CONTEXTIDR with SPE
Hi Will,
On Mon, Nov 30, 2020 at 04:46:51PM +0000, Will Deacon wrote:
> On Mon, Nov 30, 2020 at 06:24:54PM +0200, James Clark wrote:
> > Enable PID_IN_CONTEXTIDR by default when Arm SPE is enabled.
> > This flag is required to get PID data in the SPE trace. Without
> > it the perf tool will report 0 for PID which isn't very useful,
> > especially when doing system wide profiling or profiling
> > applications that fork.
>
> Can perf not figure out the pid some other way? (e.g. by tracing context
> switches and correlating that with the SPE data?).
For perf 'per-thread' mode, we can use context switch trace event as
assisted info to select thread context. But for "system wide" mode and
"snapshot" mode in perf tool, since the trace data is continuous, I
think we cannot use context switch trace event to correlate the SPE
trace data.
> Also, how does this work with pid namespaces?
Here we are studying the implemetation of Intel-PT and Arm CoreSight.
The context ID is stored into the hardware trace data when record;
afterwards when perf tool decodes the trace data and detects the
packet for context ID, it will select the machine's thread context in
perf [1]. Since the perf tool gathers all the threads infomation in
perf data file, based on the context ID, it can find the corresponding
thread pointer with function machine__find_thread() [2].
Since your question is for "pid namespace", to be honest, I don't know
how perf tool to handle any confliction for differrent processes share
the same PID, and I am not sure if you are asking CGroup related stuff
or not. If this cannot answer your question, please let me know.
Thanks,
Leo
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c#n510
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/util/cs-etm.c#n1076
>
> > There is a small performance overhead when enabling
> > PID_IN_CONTEXTIDR, but SPE itself is optional and not enabled by
> > default so the impact is minimised.
> >
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Mark Rutland <mark.rutland@....com>
> > Cc: Al Grant <al.grant@....com>
> > Cc: Leo Yan <leo.yan@...aro.org>
> > Cc: John Garry <john.garry@...wei.com>
> > Cc: Suzuki K Poulose <suzuki.poulose@....com>
> > Signed-off-by: James Clark <james.clark@....com>
> > ---
> > drivers/perf/Kconfig | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/perf/Kconfig b/drivers/perf/Kconfig
> > index 130327ff0b0e..47ede46c3d57 100644
> > --- a/drivers/perf/Kconfig
> > +++ b/drivers/perf/Kconfig
> > @@ -125,6 +125,7 @@ config XGENE_PMU
> > config ARM_SPE_PMU
> > tristate "Enable support for the ARMv8.2 Statistical Profiling Extension"
> > depends on ARM64
> > + select PID_IN_CONTEXTIDR
>
> Probably better to make PID_IN_CONTEXTIDR 'default y' if SPE is enabled,
> rather than selecting it directly. That way, at least people can turn it
> off if they don't want it.
>
> Will
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