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

[Vincent Guittot <vincent.guittot@linaro.org>]

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