Re: [PATCH] drivers/perf: Enable PID_IN_CONTEXTIDR with SPE

[Will Deacon]

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?

> > 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.

Will