linux-kernel - Re: [RFC][PATCH 0/8] sched,idle: need resched polling rework

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Date:	Fri, 11 Apr 2014 17:14:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, nicolas.pitre@...aro.org,
	daniel.lezcano@...aro.org,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 0/8] sched,idle: need resched polling rework

On Fri, Apr 11, 2014 at 08:00:23AM -0700, Andy Lutomirski wrote:
> That being said, I think that this addresses once one of the two major
> issues.  While the race you're fixing is more interesting, I think its
> impact is dwarfed by the fact that ttwu_queue_remote completely
> ignores polling.  (NB: I haven't actually tested this patch set, but I
> did try to instrument this stuff awhile ago.)
> 
> To fix this, presumably the wake-from-idle path needs a
> sched_ttwu_pending call, and ttwu_queue_remote could use resched_task.
>  sched_ttwu_pending could benefit from a straightforward optimization:
> it doesn't need rq->lock if llist is empty.
> 
> If you're not planning on trying to fix that, I can try to write up a
> patch in the next day or two.

Right; I forgot to write about that; I was going to look at both ttwu
and arch_send_call_function_single_ipi() after this got sorted.

While you didn't complain about the remote function call IPI, Venki
(while @google) did and this was their reason to look at this.

> Even with all of this fixed, what happens when ttwu_queue_remote is
> called with a task that has lower priority than whatever is currently
> running on the targeted cpu?  I think the result is an IPI that serves
> very little purpose other than avoiding taking a spinlock in the
> waking thread.  This may be a bad tradeoff.  I doubt that this matters
> for my particular workload, though.

Today Mike also noted that on very high freq the IPI is actually a lot
slower than doing the remote accesses for some weird reason --
previously I've seen the remote wakeups queue a lot of wakeups and have
the IPI take too long.

So there's definitely something to prod at there.
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