Re: [RFC] perf arm-spe: Track task context switch for cpu-mode events

[Stephane Eranian]

On Thu, Sep 23, 2021 at 9:02 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> Hi Leo,
>
> On Thu, Sep 23, 2021 at 7:23 AM Leo Yan <leo.yan@xxxxxxxxxx> 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.