linux-kernel - Re: [git] CFS-devel, latest code

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Message-ID: <20070925101044.GA923@elte.hu>
Date:	Tue, 25 Sep 2007 12:10:44 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Cc:	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [git] CFS-devel, latest code


* Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com> wrote:
> 
> > On Tue, Sep 25, 2007 at 11:13:31AM +0200, Ingo Molnar wrote:
> > > ok, i'm too seeing some sort of latency weirdness with 
> > > CONFIG_FAIR_GROUP_SCHED enabled, _if_ there's Xorg involved which runs 
> > > under root uid on my box - and hence gets 50% of all CPU time.
> > > 
> > > Srivatsa, any ideas? It could either be an accounting buglet (less 
> > > likely, seems like the group scheduling bits stick to the 50% splitup 
> > > nicely), or a preemption buglet. One potential preemption buglet would 
> > > be for the group scheduler to not properly preempt a running task when a 
> > > task from another uid is woken?
> > 
> > Yep, I noticed that too.
> > 
> > check_preempt_wakeup()
> > {
> > 	...
> > 
> > 	if (is_same_group(curr, p)) {
> > 	    ^^^^^^^^^^^^^
> > 
> > 		resched_task();
> > 	}
> > 
> > }
> > 
> > Will try a fix to check for preemption at higher levels ..
> 
> i bet fixing this will increase precision of group scheduling as well. 
> Those long latencies can be thought of as noise as well, and the 
> fair-scheduling "engine" might not be capable to offset all sources of 
> noise. So generally, while we allow a certain amount of lag in 
> preemption decisions (wakeup-granularity, etc.), with which the 
> fairness engine will cope just fine, we do not want to allow unlimited 
> lag.

hm, i tried the naive patch. In theory the vruntime of all scheduling 
entities should be 'compatible' and comparable (that's the point behind 
using vruntime - the fairness engine drives each vruntime forward and 
tries to balance them).

So the patch below just removes the is_same_group() condition. But i can 
still see bad (and obvious) latencies with Mike's 2-hogs test:

 taskset 01 perl -e 'while (1) {}' &
 nice -19 taskset 02 perl -e 'while (1) {}' &

So something's amiss.

	Ingo

------------------->
Subject: sched: group scheduler wakeup latency fix
From: Ingo Molnar <mingo@...e.hu>

group scheduler wakeup latency fix.

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 kernel/sched_fair.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

Index: linux/kernel/sched_fair.c
===================================================================
--- linux.orig/kernel/sched_fair.c
+++ linux/kernel/sched_fair.c
@@ -785,6 +785,7 @@ static void check_preempt_wakeup(struct 
 {
 	struct task_struct *curr = rq->curr;
 	struct cfs_rq *cfs_rq = task_cfs_rq(curr);
+	s64 delta;
 
 	if (unlikely(rt_prio(p->prio))) {
 		update_rq_clock(rq);
@@ -792,12 +793,10 @@ static void check_preempt_wakeup(struct 
 		resched_task(curr);
 		return;
 	}
-	if (is_same_group(curr, p)) {
-		s64 delta = curr->se.vruntime - p->se.vruntime;
+	delta = curr->se.vruntime - p->se.vruntime;
 
-		if (delta > (s64)sysctl_sched_wakeup_granularity)
-			resched_task(curr);
-	}
+	if (delta > (s64)sysctl_sched_wakeup_granularity)
+		resched_task(curr);
 }
 
 static struct task_struct *pick_next_task_fair(struct rq *rq)
-
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