linux-kernel - Re: [patch] sched: fix SMT scheduler regression in find_busiest

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Message-ID: <20100216182346.GA19327@dirshya.in.ibm.com>
Date:	Tue, 16 Feb 2010 23:55:30 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Ma, Ling" <ling.ma@...el.com>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>, ego@...ibm.com
Subject: Re: [patch] sched: fix SMT scheduler regression in
 find_busiest_queue()

* Peter Zijlstra <peterz@...radead.org> [2010-02-16 18:28:44]:

> On Tue, 2010-02-16 at 21:29 +0530, Vaidyanathan Srinivasan wrote:
> > Agreed.  Placement control should be handled by SD_PREFER_SIBLING
> > and SD_POWER_SAVINGS flags.
> > 
> > --Vaidy
> > 
> > ---
> > 
> >     sched_smt_powersavings for threaded systems need this fix for
> >     consolidation to sibling threads to work.  Since threads have 
> >     fractional capacity, group_capacity will turn out to be one 
> >     always and not accommodate another task in the sibling thread.
> > 
> >     This fix makes group_capacity a function of cpumask_weight that
> >     will enable the power saving load balancer to pack tasks among
> >     sibling threads and keep more cores idle.
> >     
> >     Signed-off-by: Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
> > 
> > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> > index 522cf0e..ec3a5c5 100644
> > --- a/kernel/sched_fair.c
> > +++ b/kernel/sched_fair.c
> > @@ -2538,9 +2538,17 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
> >                  * In case the child domain prefers tasks go to siblings
> >                  * first, lower the group capacity to one so that we'll try
> >                  * and move all the excess tasks away.
> 
> I prefer a blank line in between two paragraphs, but even better would
> be to place this comment at the else if site.
> 
> > +                * If power savings balance is set at this domain, then
> > +                * make capacity equal to number of hardware threads to
> > +                * accomodate more tasks until capacity is reached.  The
> 
> my spell checker seems to prefer: accommodate 

ok, will fix the comment.
 
> > +                * default is fractional capacity for sibling hardware
> > +                * threads for fair use of available hardware resources.
> >                  */
> >                 if (prefer_sibling)
> >                         sgs.group_capacity = min(sgs.group_capacity, 1UL);
> > +               else if (sd->flags & SD_POWERSAVINGS_BALANCE)
> > +                       sgs.group_capacity =
> > +                               cpumask_weight(sched_group_cpus(group));
> 
> I guess we should apply cpu_active_mask so that we properly deal with
> offline siblings, except with cpumasks being the beasts they are I see
> no cheap way to do that.

The sched_domain will be rebuilt with the sched_group_cpus()
representing only online siblings right?  sched_group_cpus(group) will
always be a subset of cpu_active_mask.  Can please explain your
comment.

> >                 if (local_group) {
> >                         sds->this_load = sgs.avg_load;
> > @@ -2855,7 +2863,8 @@ static int need_active_balance(struct sched_domain *sd, int sd_idle, int idle)
> >                     !test_sd_parent(sd, SD_POWERSAVINGS_BALANCE))
> >                         return 0;
> >  
> > -               if (sched_mc_power_savings < POWERSAVINGS_BALANCE_WAKEUP)
> > +               if (sched_mc_power_savings < POWERSAVINGS_BALANCE_WAKEUP &&
> > +                   sched_smt_power_savings < POWERSAVINGS_BALANCE_WAKEUP)
> >                         return 0;
> >         }
> 
> /me still hopes for that unification patch.. :-)

I will post an RFC soon.  The main challenge has been with the order
in which we should place SD_POWER_SAVINGS flag at MC and CPU/NODE level
depending on the system topology and sched_powersavings settings.

--Vaidy
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