Messages in this thread | | | From | Stephane Eranian <> | Date | Thu, 30 Sep 2021 11:47:56 -0700 | Subject | Re: [RFC] perf arm-spe: Track task context switch for cpu-mode events |
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On Thu, Sep 23, 2021 at 9:02 AM Namhyung Kim <namhyung@kernel.org> wrote: > > 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. > > I'd like to understand why it was believed that having SPE record to PID could be too expensive vs. what I am seeing with all the tracking of context switches and the volume of data this generates.
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