Re: scheduler oddity [bug?]

[Mike Galbraith <efault@gmx.de>]

On Sun, 2009-03-08 at 23:03 +0100, Willy Tarreau wrote:
> Hi Balazs,
> 
> On Sun, Mar 08, 2009 at 08:45:24PM +0100, Balazs Scheidler wrote:
> > On Sat, 2009-03-07 at 19:47 +0100, Balazs Scheidler wrote:
> > > On Sat, 2009-03-07 at 18:47 +0100, Balazs Scheidler wrote:
> > > > Hi,
> > > > 
> > > > I've tested this on 3 computers and each showed the same symptoms:
> > > >  * quad core Opteron, running Ubuntu kernel 2.6.27-13.29
> > > >  * Core 2 Duo, running Ubuntu kernel 2.6.27-11.27
> > > >  * Dual Core Opteron, Debian backports.org kernel 2.6.26-13~bpo40+1
> > > > 
> > > > Is this a bug, or a feature?
> > > > 
> > > 
> > > One new interesting information: I've retested with a 2.6.22 based
> > > kernel, and it still works there, setting the CPU affinity does not
> > > change the performance of the test program and mpstat nicely shows that
> > > 2 cores are working, not just one.
> > > 
> > > Maybe this is CFS related? That was merged for 2.6.23 IIRC.
> > > 
> > > Also, I tried changing various scheduler knobs
> > > in /proc/sys/kernel/sched_* but they didn't help. I've tried to change
> > > these:
> > > 
> > >  * sched_migration_cost: changed from the default 500000 to 100000 and
> > > then 10000 but neither helped.
> > >  * sched_nr_migrate: increased it to 64, but again nothing
> > > 
> > > I'm starting to think that this is a regression that may or may not be
> > > related to CFS. 
> > > 
> > > I don't have a box where I could bisect on, but the test program makes
> > > the problem quite obvious.
> > 
> > Some more test results:
> > 
> > Latest tree from Linus seems to work, at least the program runs on both
> > cores as it should. I bisected the patch that changed behaviour, and
> > I've found this:
> > 
> > commit 38736f475071b80b66be28af7b44c854073699cc
> > Author: Gautham R Shenoy <ego@in.ibm.com>
> > Date:   Sat Sep 6 14:50:23 2008 +0530
> > 
> >     sched: fix __load_balance_iterator() for cfq with only one task
> >     
> >     The __load_balance_iterator() returns a NULL when there's only one
> >     sched_entity which is a task. It is caused by the following code-path.
> >     
> >     	/* Skip over entities that are not tasks */
> >     	do {
> >     		se = list_entry(next, struct sched_entity, group_node);
> >     		next = next->next;
> >     	} while (next != &cfs_rq->tasks && !entity_is_task(se));
> >     
> >     	if (next == &cfs_rq->tasks)
> >     		return NULL;
> >     	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >           This will return NULL even when se is a task.
> >     
> >     As a side-effect, there was a regression in sched_mc behavior since 2.6.25,
> >     since iter_move_one_task() when it calls load_balance_start_fair(),
> >     would not get any tasks to move!
> >     
> >     Fix this by checking if the last entity was a task or not.
> >     
> >     Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
> >     Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> >     Signed-off-by: Ingo Molnar <mingo@elte.hu>
> > 
> > 
> > This patch was integrated for 2.6.28. With the above patch, my test program uses 
> > two cores as it should. I could only test this in a virtual machine so I don't 
> > know exact performance metrics, but I'll test 2.6.27 + plus this patch on a real 
> > box tomorrow to see if this was the culprit.
> 
> Just tested right here and I can confirm it is the culprit. I can reliably
> reproduce the issue here on my core2 duo, and this patch fixes it. With your
> memset() loop at 20k iterations, I saw exactly 50% CPU usage, and a final
> sum of 794. With the patch, I see 53% CPU and 909. Changing the loop to 80k
> iterations shows 53% CPU usage and 541 loops without the patch, versus
> 639 loops and 63% CPU usage with the patch.

Interesting.  I'm testing in .git (Q6600), and it's only using one CPU
unless I actively intervene.  Doing whatever to pry the pair apart takes
loops/sec from 70 to 84.

	-Mike