Re: [PATCH V3 RESEND RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task

[Andrew Jones <drjones@redhat.com>]

On Fri, Jan 25, 2013 at 04:10:25PM +0530, Raghavendra K T wrote:
> * Ingo Molnar <mingo@kernel.org> [2013-01-24 11:32:13]:
> 
> > 
> > * Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:
> > 
> > > From: Peter Zijlstra <peterz@infradead.org>
> > > 
> > > In case of undercomitted scenarios, especially in large guests
> > > yield_to overhead is significantly high. when run queue length of
> > > source and target is one, take an opportunity to bail out and return
> > > -ESRCH. This return condition can be further exploited to quickly come
> > > out of PLE handler.
> > > 
> > > (History: Raghavendra initially worked on break out of kvm ple handler upon
> > >  seeing source runqueue length = 1, but it had to export rq length).
> > >  Peter came up with the elegant idea of return -ESRCH in scheduler core.
> > > 
> > > Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> > > Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
> > > Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> > > Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
> > > Acked-by: Andrew Jones <drjones@redhat.com>
> > > Tested-by: Chegu Vinod <chegu_vinod@hp.com>
> > > ---
> > > 
> > >  kernel/sched/core.c |   25 +++++++++++++++++++------
> > >  1 file changed, 19 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 2d8927f..fc219a5 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -4289,7 +4289,10 @@ EXPORT_SYMBOL(yield);
> > >   * It's the caller's job to ensure that the target task struct
> > >   * can't go away on us before we can do any checks.
> > >   *
> > > - * Returns true if we indeed boosted the target task.
> > > + * Returns:
> > > + *	true (>0) if we indeed boosted the target task.
> > > + *	false (0) if we failed to boost the target.
> > > + *	-ESRCH if there's no task to yield to.
> > >   */
> > >  bool __sched yield_to(struct task_struct *p, bool preempt)
> > >  {
> > > @@ -4303,6 +4306,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
> > >  
> > >  again:
> > >  	p_rq = task_rq(p);
> > > +	/*
> > > +	 * If we're the only runnable task on the rq and target rq also
> > > +	 * has only one task, there's absolutely no point in yielding.
> > > +	 */
> > > +	if (rq->nr_running == 1 && p_rq->nr_running == 1) {
> > > +		yielded = -ESRCH;
> > > +		goto out_irq;
> > > +	}
> > 
> > Looks good to me in principle.
> > 
> > Would be nice to get more consistent benchmark numbers. Once 
> > those are unambiguously showing that this is a win:
> > 
> >   Acked-by: Ingo Molnar <mingo@kernel.org>
> >
> 
> I ran the test with kernbench and sysbench again on 32 core mx3850
> machine with 32 vcpu guests. Results shows definite improvements.
> 
> ebizzy and dbench show similar improvement for 1x overcommit
> (note that stdev for 1x in dbench is lesser improvemet is now seen at
> only 20%)
> 
> [ all the experiments are taken out of 8 run averages ].
> 
> The patches benefit large guest undercommit scenarios, so I believe
> with large guest performance improvemnt is even significant. [ Chegu
> Vinod results show performance near to no ple cases ].

The last results you posted for dbench for the patched 1x case were
showing much better throughput than the no-ple 1x case, which is what
was strange. Is that still happening? You don't have the no-ple 1x
data here this time. The percent errors look a lot better.

Unfortunately I
> do not have a machine to test larger guest (>32).
> 
> Ingo, Please let me know if this is okay to you.
> 
> base kernel = 3.8.0-rc4
> 
> +-----------+-----------+-----------+------------+-----------+
>                 kernbench  (time in sec lower is better)
> +-----------+-----------+-----------+------------+-----------+
>     base        stdev        patched    stdev      %improve
> +-----------+-----------+-----------+------------+-----------+
> 1x   46.6028     1.8672	    42.4494     1.1390	   8.91234
> 2x   99.9074     9.1859	    90.4050     2.6131	   9.51121
> +-----------+-----------+-----------+------------+-----------+
> +-----------+-----------+-----------+------------+-----------+
>                sysbench (time in sec lower is better) 
> +-----------+-----------+-----------+------------+-----------+
>     base        stdev        patched    stdev      %improve
> +-----------+-----------+-----------+------------+-----------+
> 1x   18.7402     0.3764	    17.7431     0.3589	   5.32065
> 2x   13.2238     0.1935	    13.0096     0.3152	   1.61981
> +-----------+-----------+-----------+------------+-----------+
> 
> +-----------+-----------+-----------+------------+-----------+
>                 ebizzy  (records/sec higher is better)
> +-----------+-----------+-----------+------------+-----------+
>     base        stdev        patched    stdev      %improve
> +-----------+-----------+-----------+------------+-----------+
> 1x  2421.9000    19.1801	  5883.1000   112.7243	 142.91259
> +-----------+-----------+-----------+------------+-----------+
> 
> +-----------+-----------+-----------+------------+-----------+
>                 dbench (throughput MB/sec  higher is better)
> +-----------+-----------+-----------+------------+-----------+
>     base        stdev        patched    stdev      %improve
> +-----------+-----------+-----------+------------+-----------+
> 1x  11675.9900   857.4154	 14103.5000   215.8425	  20.79061
> +-----------+-----------+-----------+------------+-----------+
>