[PATCH] drivers/perf: Enable PID_IN_CONTEXTIDR with SPE

[Leo Yan]

Looping in Mathieu at this time.

On Thu, Dec 03, 2020 at 02:39:41PM +0800, Leo Yan wrote:
> Hi Will,
> 
> [ + Mathieu ]
> 
> On Tue, Dec 01, 2020 at 11:09:36PM +0000, Will Deacon wrote:
> > On Tue, Dec 01, 2020 at 12:10:40PM +0800, Leo Yan wrote:
> > > 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.
> > 
> > Is there no way to correlate them with something like CNTVCT?
> 
> Good point.  Yes, we can convert CNTVCT to system time; I read the
> code in the perf's intel-pt.c and found the timestamp is used to
> correlate the auxtrace heap.  I think it's better to dig more for
> detailed implementation.
> 
> > > > 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.
> > 
> > My point was that the pid value written to CONTEXTIDR is a global pid
> > and does not take namespacing into account. If perf is run inside a pid
> > namespace, it will therefore not work.
> 
> Understand now.
> 
> The perf events PERF_RECORD_ITRACE_START/PERF_RECORD_SWITCH/
> PERF_RECORD_SWITCH_CPU_WIDE can be used to set pid/tid in perf.  So this
> would be a safe way for perf tool running in pid namespace.
> 
> Loop in Mathieu, this is a common issue for both Arm SPE and CoreSight
> (IIRC, though CoreSight's timestamp is not strictly attaching to Arm arch
> timer counter, the trend is to unify this for using arch timer
> counter).
> 
> I think James could continue to upstream a new patch by following your
> suggestion for enabling PID_IN_CONTEXTIDR, eventually, it's a feature
> for Arm SPE to record CONTEXTIDR in its packet.
> 
> Your questions inspired me, thanks!
> 
> Leo