Re: [RFC v2] perf: Rewrite core context handling

Ravi Bangoria
2022-08-23T17:36:15Z

>> 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