linux-kernel - Re: [.32-rc3] scheduler: iwlagn consistently high in "waiting for CPU"

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Date:	Thu, 08 Oct 2009 20:23:37 +0200
From:	Mike Galbraith <efault@....de>
To:	Frans Pop <elendil@...net.nl>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-wireless@...r.kernel.org
Subject: Re: [.32-rc3] scheduler: iwlagn consistently high in "waiting for
 CPU"

On Thu, 2009-10-08 at 16:55 +0200, Frans Pop wrote:
> On Thursday 08 October 2009, Arjan van de Ven wrote:
> > From: Arjan van de Ven <arjan@...ux.intel.com>
> > Date: Thu, 24 Sep 2009 13:24:16 +0200
> > Subject: [PATCH] x86, timers: check for pending timers after (device)
> > interrupts
> >
> > Now that range timers and deferred timers are common, I found a
> > problem with these using the "perf timechart" tool.
> >
> > It turns out that on x86, these two 'opportunistic' timers only
> > get checked when another "real" timer happens.
> > These opportunistic timers have the objective to save power by
> > hitchhiking on other wakeups, as to avoid CPU wakeups by themselves
> > as much as possible.
> 
> This patch makes quite a difference for me. iwlagn and phy0 now 
> consistently show at ~10 ms or lower.
> 
> I do still get occasional high latencies, but those are for things like
> "[rpc_wait_bit_killable]" or "Writing a page to disk", where I guess you'd 
> expect them. Those high latencies are mostly only listed for "Global" and 
> don't translate to individual processes.

I still see very high latencies coming out of idle (last noted was >
300ms, NO_HZ) with this patch, and wonder if the hunk below makes any
difference whatsoever for you.  Here, it definitely does. (shouldn't)

Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -495,8 +495,10 @@ static void update_curr(struct cfs_rq *c
 	u64 now = rq_of(cfs_rq)->clock;
 	unsigned long delta_exec;
 
-	if (unlikely(!curr))
+	if (unlikely(!curr)) {
+		update_rq_clock(rq_of(cfs_rq));
 		return;
+	}
 
 	/*
 	 * Get the amount of time the current task was running


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