linux-kernel - Re: [PATCH] perf/core: introduce context per CPU event list

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Date:   Sun, 1 Jan 2017 13:18:27 -0800
From:   David Carrillo-Cisneros <davidcc@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        vince@...ter.net, Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <andi@...stfloor.org>,
        Kan Liang <kan.liang@...el.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>
Subject: Re: [PATCH] perf/core: introduce context per CPU event list

On Sun, Jan 1, 2017 at 12:20 PM, David Carrillo-Cisneros
<davidcc@...gle.com> wrote:
> From: Mark Rutland <mark.rutland@....com>
>
> On Thu, Nov 10, 2016 at 05:26:32PM +0100, Peter Zijlstra wrote:
>> On Thu, Nov 10, 2016 at 02:10:37PM +0000, Mark Rutland wrote:
>>
>> > Sure, that sounds fine for scheduling (including big.LITTLE).
>> >
>> > I might still be misunderstanding something, but I don't think that
>> > helps Kan's case: since INACTIVE events which will fail their filters
>> > (including the CPU check) will still be in the tree, they will still
>> > have to be iterated over.
>> >
>> > That is, unless we also sort the tree by event->cpu, or if in those
>> > cases we only care about ACTIVE events and can use an active list.
>>
>> A few emails back up I wrote:
>>
>> >> If we stick all events in an RB-tree sorted on: {pmu,cpu,runtime} we
>
> Ah, sorry. Clearly I wouldn't pass a reading comprehension test today.
>
>> Looking at the code there's also cgroup muck, not entirely sure where in
>> the sort order that should go if at all.
>>
>> But having pmu and cpu in there would cure the big-little and
>> per-task-per-cpu event issues.
>
> Yup, that all makes sense to me now (modulo the cgroup stuff I also
> haven't considered yet).

cgroup events are stored in each pmu's cpuctx, so they wouldn't benefit
from a pmu,cpu sort order. Yet the RB-tree would help if it could use cgroup
as key for cpu contexts.

Is there a reason to have runtime as part of the RB-tree?
Couldn't a FIFO list work just fine? A node could have an ACTIVE and
an INACTIVE FIFO list and just move the events in out the tree in ioctl and
to/from ACTIVE from/to INACTIVE on sched in/out.
This would speed up both sched in and sched out.

The node would be something like this:

struct ctx_rbnode {
        struct rb_node node;
        struct list_head active_events;
        struct list_head inactive_events;
};

And the insertion order would be {pmu, cpu} for task contexts (cpu == -1
for events without fixed cpu) and {cgroup} for cpuctxs (CPU events would
have NULL cgrp).

Am I interested on getting this to work as part of the cgroup context switch
optimization that CQM/CMT needs. See discussion in:

https://patchwork.kernel.org/patch/9478617/

Is anyone actively working on it?


Thanks,
David