[PATCH] drivers/perf: Enable PID_IN_CONTEXTIDR with SPE

[Leo Yan]

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 at kernel.org>
> > Cc: Mark Rutland <mark.rutland at arm.com>
> > Cc: Al Grant <al.grant at arm.com>
> > Cc: Leo Yan <leo.yan at linaro.org>
> > Cc: John Garry <john.garry at huawei.com>
> > Cc: Suzuki K Poulose <suzuki.poulose at arm.com>
> > Signed-off-by: James Clark <james.clark at arm.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