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Message-ID: <20100216155906.GC8777@dirshya.in.ibm.com>
Date: Tue, 16 Feb 2010 21:29:06 +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> [2010-02-15 14:00:43]:
> On Mon, 2010-02-15 at 18:05 +0530, Vaidyanathan Srinivasan wrote:
> > * Peter Zijlstra <peterz@...radead.org> [2010-02-14 11:11:58]:
> >
> > > On Sun, 2010-02-14 at 02:06 +0530, Vaidyanathan Srinivasan wrote:
> > > > > > > @@ -4119,12 +4119,23 @@ find_busiest_queue(struct sched_group *group, enum cpu_idle_type idle,
> > > > > > > continue;
> > > > > > >
> > > > > > > rq = cpu_rq(i);
> > > > > > > - wl = weighted_cpuload(i) * SCHED_LOAD_SCALE;
> > > > > > > - wl /= power;
> > > > > > > + wl = weighted_cpuload(i);
> > > > > > >
> > > > > > > + /*
> > > > > > > + * When comparing with imbalance, use weighted_cpuload()
> > > > > > > + * which is not scaled with the cpu power.
> > > > > > > + */
> > > > > > > if (capacity && rq->nr_running == 1 && wl > imbalance)
> > > > > > > continue;
> > > > > > >
> > > > > > > + /*
> > > > > > > + * For the load comparisons with the other cpu's, consider
> > > > > > > + * the weighted_cpuload() scaled with the cpu power, so that
> > > > > > > + * the load can be moved away from the cpu that is potentially
> > > > > > > + * running at a lower capacity.
> > > > > > > + */
> > > > > > > + wl = (wl * SCHED_LOAD_SCALE) / power;
> > > > > > > +
> > > > > > > if (wl > max_load) {
> > > > > > > max_load = wl;
> > > > > > > busiest = rq;
> > > > > > >
> > > > > > >
> > > >
> > > > In addition to the above fix, for sched_smt_powersavings to work, the
> > > > group capacity of the core (mc level) should be made 2 in
> > > > update_sg_lb_stats() by changing the DIV_ROUND_CLOSEST to
> > > > DIV_RPUND_UP()
> > > >
> > > > sgs->group_capacity =
> > > > DIV_ROUND_UP(group->cpu_power, SCHED_LOAD_SCALE);
> > > >
> > > > Ideally we can change this to DIV_ROUND_UP and let SD_PREFER_SIBLING
> > > > flag to force capacity to 1. Need to see if there are any side
> > > > effects of setting SD_PREFER_SIBLING at SIBLING level sched domain
> > > > based on sched_smt_powersavings flag.
> > >
> > > OK, so while I think that Suresh' patch can make sense (haven't had time
> > > to think it through), the above really sounds wrong. Things should not
> > > rely on the cpu_power value like that.
> >
> > Hi Peter,
> >
> > The reason rounding is a problem is because threads have fractional
> > cpu_power and we lose some power in DIV_ROUND_CLOSEST(). At MC level
> > a group has 2*589=1178 and group_capacity will be 1 always if
> > DIV_ROUND_CLOSEST() is used irrespective of the SD_PREFER_SIBLING
> > flag.
> >
> > We are reducing group capacity here to 1 even though we have 2 sibling
> > threads in the group. In the sched_smt_powassavings>0 case, the
> > group_capacity should be 2 to allow task consolidation to this group
> > while leaving other groups completely idle.
> >
> > DIV_ROUND_UP(group->cpu_power, SCHED_LOAD_SCALE) will ensure any spare
> > capacity is rounded up and counted.
> >
> > While, if SD_REFER_SIBLING is set,
> >
> > update_sd_lb_stats():
> > if (prefer_sibling)
> > sgs.group_capacity = min(sgs.group_capacity, 1UL);
> >
> > will ensure the group_capacity is 1 and allows spreading of tasks.
>
> We should be weakening this link between cpu_power and capacity, not
> strengthening it. What I think you want is to use
> cpumask_weight(sched_gropu_cpus(group)) or something as capacity.
Yes, this is a good suggestion and elegant solution to the problem.
I have a patch attached using this approach.
> The setup for cpu_power is that it can reflect the actual capacity for
> work, esp with todays asymmetric cpus where a socket can run on a
> different frequency we need to make sure this is so.
>
> So no, that DIV_ROUND_UP is utterly broken, as there might be many ways
> for cpu_power of multiple threads/cpus to be less than the number of
> cpus.
>
> Furthermore, for powersavings it makes sense to make the capacity a
> function of an overload argument/tunable, so that you can specify the
> threshold of packing.
>
> So really, cpu_power is a normalization factor to equally distribute
> load across cpus that have asymmetric work capacity, if you need any
> placement constraints outside of that, do _NOT_ touch cpu_power.
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.
+ * 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
+ * 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));
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;
}
--
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