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Date: Mon, 13 Dec 2021 20:17:37 +0530
From: "Gautham R. Shenoy" <gautham.shenoy@....com>
To: Mel Gorman <mgorman@...hsingularity.net>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Valentin Schneider <valentin.schneider@....com>,
Aubrey Li <aubrey.li@...ux.intel.com>,
Barry Song <song.bao.hua@...ilicon.com>,
Mike Galbraith <efault@....de>,
Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when
SD_NUMA spans multiple LLCs
Hello Mel,
On Mon, Dec 13, 2021 at 01:01:31PM +0000, Mel Gorman wrote:
> On Mon, Dec 13, 2021 at 01:58:03PM +0530, Gautham R. Shenoy wrote:
> > > On a Zen3 machine running STREAM parallelised with OMP to have on instance
> > > per LLC the results and without binding, the results are
> > >
> > > 5.16.0-rc1 5.16.0-rc1
> > > vanilla sched-numaimb-v4
> > > MB/sec copy-16 166712.18 ( 0.00%) 651540.22 ( 290.82%)
> > > MB/sec scale-16 140109.66 ( 0.00%) 382254.74 ( 172.83%)
> > > MB/sec add-16 160791.18 ( 0.00%) 623073.98 ( 287.51%)
> > > MB/sec triad-16 160043.84 ( 0.00%) 633964.52 ( 296.12%)
> >
> >
> > Could you please share the size of the stream array ? These numbers
> > are higher than what I am observing.
> >
>
> 512MB
Thanks, I will try with this one.
>
> > > @@ -9280,19 +9286,14 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> > > }
> > > }
> > >
> > > -#define NUMA_IMBALANCE_MIN 2
> > > -
> > > static inline long adjust_numa_imbalance(int imbalance,
> > > - int dst_running, int dst_weight)
> > > + int dst_running, int dst_weight,
> > > + int imb_numa_nr)
> > > {
> > > if (!allow_numa_imbalance(dst_running, dst_weight))
> > > return imbalance;
> > >
> >
> > if (4 * dst_running >= dst_weight) we return imbalance here. The
> > dst_weight here corresponds to the span of the domain, while
> > dst_running is the nr_running in busiest.
> >
>
> Yes, once dst_running is high enough, no imbalance is allowed. In
> previous versions I changed this but that was a mistake and in this
> version, the threshold where imbalance is not allowed remains the same.
>
> > On Zen3, at the top most NUMA domain, the dst_weight = 256 across in
> > all the configurations of Nodes Per Socket (NPS) = 1/2/4. There are
> > two groups, where each group is a socket. So, unless there are at
> > least 64 tasks running in one of the sockets, we would not return
> > imbalance here and go to the next step.
> >
>
> Yes
>
> >
> > > - /*
> > > - * Allow a small imbalance based on a simple pair of communicating
> > > - * tasks that remain local when the destination is lightly loaded.
> > > - */
> > > - if (imbalance <= NUMA_IMBALANCE_MIN)
> > > + if (imbalance <= imb_numa_nr)
> >
> > imb_numa_nr in NPS=1 mode, imb_numa_nr would be 4. Since NUMA domains
> > don't have PREFER_SIBLING, we would be balancing the number of idle
> > CPUs. We will end up doing the imbalance, as long as the difference
> > between the idle CPUs is at least 8.
> >
> > In NPS=2, imb_numa_nr = 8 for this topmost NUMA domain. So here, we
> > will not rebalance unless the difference between the idle CPUs is 16.
> >
> > In NPS=4, imb_numa_nr = 16 for this topmost NUMA domain. So, the
> > threshold is now bumped up to 32.
> >
> > > return 0;
> >
> >
> >
> > >
> > > return imbalance;
> > > @@ -9397,7 +9398,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> > > /* Consider allowing a small imbalance between NUMA groups */
> > > if (env->sd->flags & SD_NUMA) {
> > > env->imbalance = adjust_numa_imbalance(env->imbalance,
> > > - busiest->sum_nr_running, env->sd->span_weight);
> > > + busiest->sum_nr_running, env->sd->span_weight,
> > > + env->sd->imb_numa_nr);
> > > }
> > >
> > > return;
> > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > > index d201a7052a29..bacec575ade2 100644
> > > --- a/kernel/sched/topology.c
> > > +++ b/kernel/sched/topology.c
> > > @@ -2242,6 +2242,43 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> > > }
> > > }
> > >
> > > + /*
> > > + * Calculate an allowed NUMA imbalance such that LLCs do not get
> > > + * imbalanced.
> > > + */
> > > + for_each_cpu(i, cpu_map) {
> > > + unsigned int imb = 0;
> > > + unsigned int imb_span = 1;
> > > +
> > > + for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
> > > + struct sched_domain *child = sd->child;
> > > +
> > > + if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
> > > + (child->flags & SD_SHARE_PKG_RESOURCES)) {
> > > + struct sched_domain *top = sd;
> >
> >
> > We don't seem to be using top anywhere where sd may not be used since
> > we already have variables imb and imb_span to record the
> > top->imb_numa_nr and top->span_weight.
> >
>
> Top could have been removed but we might still need it.
>
> >
> > > + unsigned int llc_sq;
> > > +
> > > + /*
> > > + * nr_llcs = (top->span_weight / llc_weight);
> > > + * imb = (child_weight / nr_llcs) >> 2
> >
> > child here is the llc. So can we use imb = (llc_weight / nr_llcs) >> 2.
> >
>
> That is be clearer.
>
> > > + *
> > > + * is equivalent to
> > > + *
> > > + * imb = (llc_weight^2 / top->span_weight) >> 2
> > > + *
> > > + */
> > > + llc_sq = child->span_weight * child->span_weight;
> > > +
> > > + imb = max(2U, ((llc_sq / top->span_weight) >> 2));
> > > + imb_span = sd->span_weight;
> >
> > On Zen3, child_weight (or llc_weight) = 16. llc_sq = 256.
> > with NPS=1
> > top = DIE.
> > top->span_weight = 128. imb = max(2, (256/128) >> 2) = 2. imb_span = 128.
> >
> > with NPS=2
> > top = NODE.
> > top->span_weight = 64. imb = max(2, (256/64) >> 2) = 2. imb_span = 64.
> >
> > with NPS=4
> > top = NODE.
> > top->span_weight = 32. imb = max(2, (256/32) >> 2) = 2. imb_span = 32.
> >
> > On Zen2, child_weight (or llc_weight) = 8. llc_sq = 64.
> > with NPS=1
> > top = DIE.
> > top->span_weight = 128. imb = max(2, (64/128) >> 2) = 2. imb_span = 128.
> >
> > with NPS=2
> > top = NODE.
> > top->span_weight = 64. imb = max(2, (64/64) >> 2) = 2. imb_span = 64.
> >
> > with NPS=4
> > top = NODE.
> > top->span_weight = 32. imb = max(2, (64/32) >> 2) = 2. imb_span = 32.
> >
> >
> > > +
> > > + sd->imb_numa_nr = imb;
> > > + } else {
> > > + sd->imb_numa_nr = imb * (sd->span_weight / imb_span);
> > > + }
> >
> > On Zen3,
> > with NPS=1
> > sd=NUMA, sd->span_weight = 256. sd->imb_numa_nr = 2 * (256/128) = 4.
> >
> > with NPS=2
> > sd=NUMA, sd->span_weight = 128. sd->imb_numa_nr = 2 * (128/64) = 4
> > sd=NUMA, sd->span_weight = 256. sd->imb_numa_nr = 2 * (256/64) = 8
> >
> > with NPS=4
> > sd=NUMA, sd->span_weight = 128. sd->imb_numa_nr = 2 * (128/32) = 8
> > sd=NUMA, sd->span_weight = 256. sd->imb_numa_nr = 2 * (256/32) = 16
> >
> >
> > For Zen2, since the imb_span and imb values are the same as the
> > corresponding NPS=x values on Zen3, the imb_numa_nr values are the
> > same as well since the corresponding sd->span_weight is the same.
> >
> > If we look at the highest NUMA domain, there are two groups in all the
> > NPS configurations. There are the same number of LLCs in each of these
> > groups across the different NPS configurations (nr_llcs=8 on Zen3, 16
> > on Zen2) . However, the imb_numa_nr at this domain varies with the NPS
> > value, since we compute the imb_numa_nr value relative to the number
> > of "top" domains that can be fit within this NUMA domain. This is
> > because the size of the "top" domain varies with the NPS value. This
> > shows up in the benchmark results.
> >
>
> This was intentional to have some scaling but based on your results, the
> scaling might be at the wrong level.
Ok.
>
> >
> >
> > The numbers with stream, tbench and YCSB +
> > Mongodb are as follows:
> >
> >
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > Stream with 16 threads.
> > built with -DSTREAM_ARRAY_SIZE=128000000, -DNTIMES=10
> > Zen3, 64C128T per socket, 2 sockets,
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > NPS=1
> > Test: tip/sched/core mel-v3 mel-v4
> > Copy: 113716.62 (0.00 pct) 218961.59 (92.55 pct) 217130.07 (90.93 pct)
> > Scale: 110996.89 (0.00 pct) 216674.73 (95.20 pct) 220765.94 (98.89 pct)
> > Add: 124504.19 (0.00 pct) 253461.32 (103.57 pct 260273.88 (109.04 pct)
> > Triad: 122890.43 (0.00 pct) 247552.00 (101.44 pct 252615.62 (105.56 pct)
> >
> >
> > NPS=2
> > Test: tip/sched/core mel-v3 mel-v4
> > Copy: 58217.00 (0.00 pct) 204630.34 (251.49 pct) 191312.73 (228.62 pct)
> > Scale: 55004.76 (0.00 pct) 212142.88 (285.68 pct) 175499.15 (219.06 pct)
> > Add: 63269.04 (0.00 pct) 254752.56 (302.64 pct) 203571.50 (221.75 pct)
> > Triad: 62178.25 (0.00 pct) 247290.80 (297.71 pct) 198988.70 (220.02 pct)
> >
> > NPS=4
> > Test: tip/sched/core mel-v3 mel-v4
> > Copy: 37986.66 (0.00 pct) 254183.87 (569.13 pct) 48748.87 (28.33 pct)
> > Scale: 35471.22 (0.00 pct) 237804.76 (570.41 pct) 48317.82 (36.21 pct)
> > Add: 39303.25 (0.00 pct) 292285.20 (643.66 pct) 54259.59 (38.05 pct)
> > Triad: 39319.85 (0.00 pct) 285284.30 (625.54 pct) 54503.98 (38.61 pct)
> >
>
> At minimum, v3 is a failure because a single pair of communicating tasks
> were getting split across NUMA domains and the allowed numa imbalance
> gets cut off too early because of the change to allow_numa_imbalance.
> So while it's a valid comparison, it's definitely not the fix.
v3 is definitely not a fix. I wasn't hinting at that. It was just to
point out the opportunity that we have.
>
> Given how you describe NPS, maybe the scaling should only start at the
> point where tasks are no longer balanced between sibling domains. Can
> you try this? I've only boot tested it at this point. It should work for
> STREAM at least but probably not great for tbench.
Thanks for the patch. I will queue this one for tonight.
>
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index bacec575ade2..1fa3e977521d 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2255,26 +2255,38 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>
> if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
> (child->flags & SD_SHARE_PKG_RESOURCES)) {
> - struct sched_domain *top = sd;
> + struct sched_domain *top, *top_p;
> unsigned int llc_sq;
>
> /*
> - * nr_llcs = (top->span_weight / llc_weight);
> - * imb = (child_weight / nr_llcs) >> 2
> + * nr_llcs = (sd->span_weight / llc_weight);
> + * imb = (llc_weight / nr_llcs) >> 2
> *
> * is equivalent to
> *
> - * imb = (llc_weight^2 / top->span_weight) >> 2
> + * imb = (llc_weight^2 / sd->span_weight) >> 2
> *
> */
> llc_sq = child->span_weight * child->span_weight;
>
> - imb = max(2U, ((llc_sq / top->span_weight) >> 2));
> - imb_span = sd->span_weight;
> -
> + imb = max(2U, ((llc_sq / sd->span_weight) >> 2));
> sd->imb_numa_nr = imb;
> +
> + /*
> + * Set span based on top domain that places
> + * tasks in sibling domains.
> + */
> + top = sd;
> + top_p = top->parent;
> + while (top_p && (top_p->flags & SD_PREFER_SIBLING)) {
> + top = top->parent;
> + top_p = top->parent;
> + }
> + imb_span = top_p ? top_p->span_weight : sd->span_weight;
> } else {
> - sd->imb_numa_nr = imb * (sd->span_weight / imb_span);
> + int factor = max(1U, (sd->span_weight / imb_span));
> +
> + sd->imb_numa_nr = imb * factor;
> }
> }
> }
--
Thanks and Regards
gautham.
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