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

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Message-ID: <20161110125804.GT3117@twins.programming.kicks-ass.net>
Date:   Thu, 10 Nov 2016 13:58:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     kan.liang@...el.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, acme@...nel.org, tglx@...utronix.de,
        alexander.shishkin@...ux.intel.com, vince@...ter.net,
        eranian@...gle.com, andi@...stfloor.org
Subject: Re: [PATCH] perf/core: introduce context per CPU event list

On Thu, Nov 10, 2016 at 12:26:18PM +0000, Mark Rutland wrote:
> On Thu, Nov 10, 2016 at 01:12:53PM +0100, Peter Zijlstra wrote:

> > Ah, so the tree would in fact only contain 'INACTIVE' events :-)
> 
> Ah. :)
> 
> That explains some of the magic, but...
> 
> > That is, when no events are on the hardware, all events (if there are
> > any) are INACTIVE.
> > 
> > Then on sched-in, we find the relevant subtree, and linearly try and
> > program all events from that subtree onto the PMU. Once adding an event
> > fails programming, we stop (like we do now).
> > 
> > These programmed events transition from INACTIVE to ACTIVE, and we take
> > them out of the tree.
> > 
> > Then on sched-out, we remove all events from the hardware, increase the
> > events their runtime value by however long they were ACTIVE, flip them
> > to INACTIVE and stuff them back in the tree.
> 
> ... per the above, won't the tree also contain 'OFF' events (and
> 'ERROR', etc)?
> 
> ... or do we keep them somewhere else (another list or sub-tree)?

I don't think those need be tracked at all, they're immaterial for
actual scheduling. Once we ioctl() them back to life we can insert them
into the tree.

> If not, we still have to walk all of those in perf_iterate_ctx().
> 
> > (I'm can't quite recall if we can easily find ACTIVE events from a PMU,
> > but if not, we can easily track those on a separate list).
> 
> I think we just iterate the perf_event_context::event list and look at
> the state. Regardless, adding lists is fairly simple.
> 
> Thanks,
> Mark.