Re: [PATCH v4 04/11] sched/fair: rework load_balance

From: Vincent Guittot
Date: Tue Nov 12 2019 - 05:58:39 EST

Le Friday 08 Nov 2019 à 18:37:30 (+0000), Mel Gorman a écrit :
> On Fri, Nov 08, 2019 at 05:35:01PM +0100, Vincent Guittot wrote:
> > > Fair enough, netperf hits the corner case where it does not work but
> > > that is also true without your series.
> >
> > I run mmtest/netperf test on my setup. It's a mix of small positive or
> > negative differences (see below)
> >
> > <SNIP>
> >
> > netperf-tcp
> > 5.3-rc2 5.3-rc2
> > tip +rwk+fix
> > Hmean 64 871.30 ( 0.00%) 860.90 * -1.19%*
> > Hmean 128 1689.39 ( 0.00%) 1679.31 * -0.60%*
> > Hmean 256 3199.59 ( 0.00%) 3241.98 * 1.32%*
> > Hmean 1024 9390.47 ( 0.00%) 9268.47 * -1.30%*
> > Hmean 2048 13373.95 ( 0.00%) 13395.61 * 0.16%*
> > Hmean 3312 16701.30 ( 0.00%) 17165.96 * 2.78%*
> > Hmean 4096 15831.03 ( 0.00%) 15544.66 * -1.81%*
> > Hmean 8192 19720.01 ( 0.00%) 20188.60 * 2.38%*
> > Hmean 16384 23925.90 ( 0.00%) 23914.50 * -0.05%*
> > Stddev 64 7.38 ( 0.00%) 4.23 ( 42.67%)
> > Stddev 128 11.62 ( 0.00%) 10.13 ( 12.85%)
> > Stddev 256 34.33 ( 0.00%) 7.94 ( 76.88%)
> > Stddev 1024 35.61 ( 0.00%) 116.34 (-226.66%)
> > Stddev 2048 285.30 ( 0.00%) 80.50 ( 71.78%)
> > Stddev 3312 304.74 ( 0.00%) 449.08 ( -47.36%)
> > Stddev 4096 668.11 ( 0.00%) 569.30 ( 14.79%)
> > Stddev 8192 733.23 ( 0.00%) 944.38 ( -28.80%)
> > Stddev 16384 553.03 ( 0.00%) 299.44 ( 45.86%)
> >
> > 5.3-rc2 5.3-rc2
> > tip +rwk+fix
> > Duration User 138.05 140.95
> > Duration System 1210.60 1208.45
> > Duration Elapsed 1352.86 1352.90
> >
>
> This roughly matches what I've seen. The interesting part to me for
> netperf is the next section of the report that reports the locality of
> numa hints. With netperf on a 2-socket machine, it's generally around
> 50% as the client/server are pulled apart. Because netperf is not
> heavily memory bound, it doesn't have much impact on the overall
> performance but it's good at catching the cross-node migrations.

Ok. I didn't want to make my reply too long. I have put them below for
the netperf-tcp results:
5.3-rc2 5.3-rc2
tip +rwk+fix
Ops NUMA alloc hit 60077762.00 60387907.00
Ops NUMA alloc miss 0.00 0.00
Ops NUMA interleave hit 0.00 0.00
Ops NUMA alloc local 60077571.00 60387798.00
Ops NUMA base-page range updates 5948.00 17223.00
Ops NUMA PTE updates 5948.00 17223.00
Ops NUMA PMD updates 0.00 0.00
Ops NUMA hint faults 4639.00 14050.00
Ops NUMA hint local faults % 2073.00 6515.00
Ops NUMA hint local percent 44.69 46.37
Ops NUMA pages migrated 1528.00 4306.00
Ops AutoNUMA cost 23.27 70.45


>
> > >
> > > > I agree that additional patches are probably needed to improve load
> > > > balance at NUMA level and I expect that this rework will make it
> > > > simpler to add.
> > > > I just wanted to get the output of some real use cases before defining
> > > > more numa level specific conditions. Some want to spread on there numa
> > > > nodes but other want to keep everything together. The preferred node
> > > > and fbq_classify_group was the only sensible metrics to me when he
> > > > wrote this patchset but changes can be added if they make sense.
> > > >
> > >
> > > That's fair. While it was possible to address the case before your
> > > series, it was a hatchet job. If the changelog simply notes that some
> > > special casing may still be required for SD_NUMA but it's outside the
> > > scope of the series, then I'd be happy. At least there is a good chance
> > > then if there is follow-up work that it won't be interpreted as an
> > > attempt to reintroduce hacky heuristics.
> > >
> >
> > Would the additional comment make sense for you about work to be done
> > for SD_NUMA ?
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 0ad4b21..7e4cb65 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6960,11 +6960,34 @@ enum fbq_type { regular, remote, all };
> > * group. see update_sd_pick_busiest().
> > */
> > enum group_type {
> > + /*
> > + * The group has spare capacity that can be used to process more work.
> > + */
> > group_has_spare = 0,
> > + /*
> > + * The group is fully used and the tasks don't compete for more CPU
> > + * cycles. Nevetheless, some tasks might wait before running.
> > + */
> > group_fully_busy,
> > + /*
> > + * One task doesn't fit with CPU's capacity and must be migrated on a
> > + * more powerful CPU.
> > + */
> > group_misfit_task,
> > + /*
> > + * One local CPU with higher capacity is available and task should be
> > + * migrated on it instead on current CPU.
> > + */
> > group_asym_packing,
> > + /*
> > + * The tasks affinity prevents the scheduler to balance the load across
> > + * the system.
> > + */
> > group_imbalanced,
> > + /*
> > + * The CPU is overloaded and can't provide expected CPU cycles to all
> > + * tasks.
> > + */
> > group_overloaded
> > };
>
> Looks good.
>
> >
> > @@ -8563,7 +8586,11 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> >
> > /*
> > * Try to use spare capacity of local group without overloading it or
> > - * emptying busiest
> > + * emptying busiest.
> > + * XXX Spreading tasks across numa nodes is not always the best policy
> > + * and special cares should be taken for SD_NUMA domain level before
> > + * spreading the tasks. For now, load_balance() fully relies on
> > + * NUMA_BALANCING and fbq_classify_group/rq to overide the decision.
> > */
> > if (local->group_type == group_has_spare) {
> > if (busiest->group_type > group_fully_busy) {
>
> Perfect. Any patch in that are can then update the comment and it
> should be semi-obvious to the next reviewer that it's expected.
>
> Thanks Vincent.
>
> --
> Mel Gorman
> SUSE Labs