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

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Message-ID: <26D16201-2CEE-4D4A-B2FD-8F472B48A273@fb.com>
Date:   Wed, 17 Oct 2018 18:57:25 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Stephane Eranian <eranian@...gle.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "megha.dey@...el.com" <megha.dey@...el.com>,
        "frederic@...nel.org" <frederic@...nel.org>
Subject: Re: [RFC][PATCH] perf: Rewrite core context handling

> On Oct 17, 2018, at 11:33 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Wed, Oct 17, 2018 at 07:19:55PM +0200, Peter Zijlstra wrote:
>> On Wed, Oct 17, 2018 at 04:43:27PM +0000, Song Liu wrote:
>> 
>>>> That makes task and cpu contexts wildly different, which will complicate
>>>> matters I feel.
>>>> 
>>> 
>>> I think we only need different logic when adding events to the task/cpu 
>>> contexts. The ctx_sched_in() and ctx_sched_out() will need some extra
>>> logic to filter out events that are not being scheduled (don't schedule
>>> events on PMU-a when rotating PMU-b). This logic will be the same for 
>>> task and cpu context. The difference is, the CPU context will not have
>>> such events, because we never added such event to CPU context. 
>>> 
>>> Does this make sense? I could try draft a RFC to see how difficult it is. 
>> 
>> I'm not sure it saves much, if we have multiple per-cpu contexts we get
>> to re-introduce the active_ctx_list and loose the simplification for the
>> online status.
>> 
>> Plus that fundamental assymetry -- which would bother my OCD forever
>> more :-)
> 
> Worse, the whole syscall that installs the events will come apart. The
> locking for the two cases is different :/

I agree... I didn't get into details of locking. I just consider these all
as part of "adding event to context". 

I believe this patch should give close to the optimal performance. However, 
I do feel it makes the logic more complicate. Before this patch, perf_cpu_context
and perf_event_context don't need to know much about multiple PMUs. With
this patch, the two extra *_pmu_context are necessary for performance (and
maybe also for correctness). 

If we take first a baby step, how about adding more perf_event_ctx to 
task_struct->perf_event_ctxp? We need one sw perf_event_ctx and a few hw 
perf_event_ctx (one for each hw PMU). (I haven't checked whether it is OK
to allocate these when attaching events). (And I guess you don't really 
like this..)

On the other hand, this patch makes it possible to create groups of events
from different hw PMUs. I guess that will be useful. 

Thanks,
Song