From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
nicolas.pitre@linaro.org, daniel.lezcano@linaro.org,
Mike Galbraith <umgwanakikbuti@gmail.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH 0/8] sched,idle: need resched polling rework
Date: Fri, 11 Apr 2014 17:14:09 +0200 [thread overview]
Message-ID: <20140411151409.GW11096@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CALCETrWJZ1-T+qOY9v=KBOSJkck7YYe7khw_dYPc9Ar8WFgMKw@mail.gmail.com>
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.
next prev parent reply other threads:[~2014-04-11 15:14 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-11 13:42 [RFC][PATCH 0/8] sched,idle: need resched polling rework Peter Zijlstra
2014-04-11 13:42 ` [RFC][PATCH 1/8] sched,idle,alpha: Switch from TS_POLLING to TIF_POLLING_NRFLAG Peter Zijlstra
2014-04-11 14:38 ` Richard Henderson
2014-04-11 13:42 ` [RFC][PATCH 2/8] sched,idle,tile: " Peter Zijlstra
2014-04-11 15:15 ` Chris Metcalf
2014-04-11 15:30 ` Peter Zijlstra
2014-04-11 13:42 ` [RFC][PATCH 3/8] sched,idle,ia64: " Peter Zijlstra
2014-04-11 13:42 ` [RFC][PATCH 4/8] sched,idle,x86: " Peter Zijlstra
2014-04-11 13:42 ` [RFC][PATCH 5/8] sched,idle: Remove TS_POLLING support Peter Zijlstra
2014-04-11 13:42 ` [RFC][PATCH 6/8] sched,idle: Avoid spurious wakeup IPIs Peter Zijlstra
2014-04-13 21:41 ` Nicolas Pitre
2014-05-09 13:37 ` James Hogan
2014-05-09 14:15 ` Peter Zijlstra
2014-05-09 14:40 ` Catalin Marinas
2014-05-09 14:50 ` Peter Zijlstra
2014-05-09 14:57 ` Catalin Marinas
2014-05-09 17:02 ` Peter Zijlstra
2014-05-09 17:06 ` Peter Zijlstra
2014-05-09 17:09 ` Catalin Marinas
2014-05-09 17:20 ` Peter Zijlstra
2014-05-19 12:54 ` [tip:sched/arch] arm64: Remove TIF_POLLING_NRFLAG tip-bot for Peter Zijlstra
2014-05-22 12:26 ` [tip:sched/core] " tip-bot for Peter Zijlstra
2014-05-09 14:51 ` [RFC][PATCH 6/8] sched,idle: Avoid spurious wakeup IPIs James Hogan
2014-05-15 9:17 ` James Hogan
2014-05-19 12:54 ` [tip:sched/arch] metag: Remove TIF_POLLING_NRFLAG tip-bot for James Hogan
2014-05-22 12:26 ` [tip:sched/core] " tip-bot for James Hogan
2014-04-11 13:42 ` [RFC][PATCH 7/8] sched,idle: Delay clearing the polling bit Peter Zijlstra
2014-04-13 21:51 ` Nicolas Pitre
2014-04-11 13:42 ` [RFC][PATCH 8/8] sched,idle: Reflow cpuidle_idle_call() Peter Zijlstra
2014-04-13 21:36 ` Nicolas Pitre
2014-04-14 8:59 ` Peter Zijlstra
2014-04-14 9:25 ` Peter Zijlstra
2014-04-14 13:55 ` Nicolas Pitre
2014-04-11 15:00 ` [RFC][PATCH 0/8] sched,idle: need resched polling rework Andy Lutomirski
2014-04-11 15:14 ` Peter Zijlstra [this message]
2014-05-22 12:58 ` Peter Zijlstra
2014-05-22 13:09 ` Peter Zijlstra
2014-05-29 0:01 ` Andy Lutomirski
2014-05-29 6:48 ` Peter Zijlstra
2014-06-03 6:40 ` Andy Lutomirski
2014-06-03 6:51 ` Peter Zijlstra
2014-06-03 10:43 ` Peter Zijlstra
2014-06-03 14:02 ` Peter Zijlstra
2014-06-03 16:05 ` Andy Lutomirski
2014-06-03 16:19 ` Peter Zijlstra
2014-06-03 16:52 ` Andy Lutomirski
2014-06-03 17:00 ` Peter Zijlstra
2014-06-03 18:28 ` Peter Zijlstra
2014-06-03 18:44 ` Andy Lutomirski
2014-06-03 20:07 ` Andy Lutomirski
2014-04-12 8:35 ` Mike Galbraith
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