linux-kernel - Re: High wake up latencies with FAIR_USER

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Date:	Mon, 28 Jan 2008 08:01:29 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	"Guillaume Chazarain" <guichaz@...oo.fr>
Cc:	"Ingo Molnar" <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	a.p.zijlstra@...llo.nl, dhaval@...ux.vnet.ibm.com
Subject: Re: High wake up latencies with FAIR_USER_SCHED

On Sun, Jan 27, 2008 at 09:01:15PM +0100, Guillaume Chazarain wrote:
> I noticed some strangely high wake up latencies with FAIR_USER_SCHED
> using this script:

<snip>

> We have two busy loops with UID=1.
> And UID=2 maintains the running median of its wake up latency.
> I get these latencies:
> 
> # ./sched.py
> 4.300022 ms
> 4.801178 ms
> 4.604006 ms

Given that sysctl_sched_wakeup_granularity is set to 10ms by default,
this doesn't sound abnormal.

<snip>

> Disabling FAIR_USER_SCHED restores wake up latencies in the noise:
> 
> # ./sched.py
> -0.156975 ms
> -0.067091 ms
> -0.022984 ms

The reason why we are getting better wakeup latencies for !FAIR_USER_SCHED is 
because of this snippet of code in place_entity():

	if (!initial) {
		/* sleeps upto a single latency don't count. */
		if (sched_feat(NEW_FAIR_SLEEPERS) && entity_is_task(se))
						     ^^^^^^^^^^^^^^^^^^
			vruntime -= sysctl_sched_latency;

		/* ensure we never gain time by being placed backwards. */
		vruntime = max_vruntime(se->vruntime, vruntime);
	}

NEW_FAIR_SLEEPERS feature gives credit for sleeping only to tasks and
not group-level entities. With the patch attached, I could see that wakeup 
latencies with FAIR_USER_SCHED are restored to the same level as 
!FAIR_USER_SCHED. 

However I am not sure whether that is the way to go. We want to let one group of
tasks running as much as possible until the fairness/wakeup-latency threshold is
exceeded. If someone does want better wakeup latencies between groups too, they 
can always tune sysctl_sched_wakeup_granularity.

<snip>

> Strangely enough, another way to restore normal latencies is to change
> setuid(2) to setuid(1), that is, putting the latency measurement in
> the same group as the two busy loops.

-- 
Regards,
vatsa

View attachment "fix.patch" of type "text/plain" (549 bytes)