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

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Date:   Thu, 30 Sep 2021 11:47:56 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        James Clark <james.clark@....com>,
        Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [RFC] perf arm-spe: Track task context switch for cpu-mode events

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