Messages in this thread | | | From | Namhyung Kim <> | Date | Thu, 23 Sep 2021 09:01:49 -0700 | Subject | Re: [RFC] perf arm-spe: Track task context switch for cpu-mode events |
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Hi Leo,
On Thu, Sep 23, 2021 at 7:23 AM Leo Yan <leo.yan@linaro.org> wrote: > > Hi Namhyung, > > On Thu, Sep 16, 2021 at 02:01:21PM -0700, Namhyung Kim wrote: > > [...] > > > > Before we had discussion for enabling PID/TID for SPE samples; in the patch > > > set [1], patches 07, 08 set sample's pid/tid based on the Arm SPE context > > > packets. To enable hardware tracing context ID, you also needs to enable > > > kernel config CONFIG_PID_IN_CONTEXTIDR. > > > > Thanks for sharing this. > > > > Yeah I also look at the context info but having a dependency on a kconfig > > looks limiting its functionality. Also the kconfig says it has some overhead > > in the critical path (even if perf is not running, right?) - but not sure how > > much it can add. > > Yes, after enabled config PID_IN_CONTEXTIDR, the kernel will always > write PID into the system register CONTEXTIDR during process context > switching. Please see the flow: > > __switch_to() (arch/arm64/kernel/process.c) > `-> contextidr_thread_switch(next)
Thanks for the info. I assume it's a light-weight operation.
> > > We also considered to use PERF_RECORD_SWITCH_CPU_WIDE event for setting > > > pid/tid, the Intel PT implementation uses two things to set sample's > > > pid/tid: one is PERF_RECORD_SWITCH_CPU_WIDE event and another is to detect > > > the branch instruction is the symbol "__switch_to". Since the trace > > > event PERF_RECORD_SWITCH_CPU_WIDE is coarse, so it only uses the new > > > pid/tid after the branch instruction for "__switch_to". Arm SPE is > > > 'statistical', thus it cannot promise the trace data must contain the > > > branch instruction for "__switch_to", please see details [2]. > > > > I can see the need in the Intel PT as it needs to trace all (branch) > > instructions, but is it really needed for ARM SPE too? > > Maybe I am missing something, but it seems enough to have a > > coarse-grained context switch for sampling events.. > > The issue is that the coarse-grained context switch if introduces any > inaccuracy in the reported result. If we can run some workloads and > prove the coarse-grained context switch doesn't cause significant bias, > it will be great and can give us the confidence for this approach. > > Even enabling PERF_RECORD_SWITCH_CPU_WIDE event, I think it's good to > give priority for hardware PID tracing in Arm SPE trace data, if detects > the hardware PID tracing is enabled, then we can rollback to use > context packets from hardware trace data to set sample's PID. > > How about you think for this?
I think it's good as long as it has a fallback when the context info is not available.
Thanks, Namhyung
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