linux-kernel - Re: [RFC v2] perf: Rewrite core context handling

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Message-ID: <b39d0bd3-aa6f-c044-c668-2108878fd004@amd.com>
Date:   Tue, 23 Aug 2022 20:44:51 +0530
From:   Ravi Bangoria <ravi.bangoria@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, songliubraving@...com,
        eranian@...gle.com, alexey.budankov@...ux.intel.com,
        ak@...ux.intel.com, mark.rutland@....com, megha.dey@...el.com,
        frederic@...nel.org, maddy@...ux.ibm.com, irogers@...gle.com,
        kim.phillips@....com, linux-kernel@...r.kernel.org,
        santosh.shukla@....com, ravi.bangoria@....com
Subject: Re: [RFC v2] perf: Rewrite core context handling


>> Also, this hunk is under if (is_active ^ EVENT_TIME), which effectively is
>> (is_active != EVENT_TIME). I'm assuming it should be (is_active & EVENT_TIME)?
> 
> So that code is identical to what it currently is upstream; but yes that
> looks somewhat dodgy.
> 
> So the code itself (does as the comment says) starts time.

Got it.

> This should only be done if EVENT_TIME is not set.

Does that mean context time should be started only when context is getting
scheduled I.e. ctx->is_active is 0 ?

> That is, I'm thinking it should be something like:
> 
> 	!(is_active & EVENT_TIME)
> 
> which happens to be the same as:
> 
> 	is_active ^ EVENT_TIME
> 
> under the assumption is_active contains no other bits -- which I don't
> think is a valid assumption.

Correct, we can't assume that. There are cases where we call
ctx_sched_out(EVENT_TIME) followed by ctx_sched_in(EVENT_TIME) when PINNED /
FLEXIBLE are also set in ctx->is_active. For ex, perf_event_enable_on_exec().
In such cases, we will not advance ctx->time. Example:

  child()
  {
      ...
      execv();
  }

  main()
  {
      pid = fork();

      attr.enable_on_exec = 0;
      fd0 = perf_event_open(&attr, pid, -1, -1, 0);
      ...
      wait(NULL);
  }

Here execv() will cause call to ctx_sched_in() --> __update_context_time()
with adv=false. I think that's fine. Sometime later we will anyway advance
ctx->time.

Sorry, I've not spend enough time with this time keeping code. Please let
me know if I'm talking nonsense.

Thanks,
Ravi