Messages in this thread | | | Date | Tue, 1 Dec 2020 23:09:36 +0000 | From | Will Deacon <> | Subject | Re: [PATCH] drivers/perf: Enable PID_IN_CONTEXTIDR with SPE |
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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
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