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Date: Tue, 22 Jan 2013 09:03:15 +0100
From: Mike Galbraith <bitbucket@...ine.de>
To: Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com,
peterz@...radead.org, mingo@...nel.org, a.p.zijlstra@...llo.nl
Subject: Re: [RFC PATCH 0/2] sched: simplify the select_task_rq_fair()
On Tue, 2013-01-22 at 11:43 +0800, Michael Wang wrote:
> On 01/21/2013 05:44 PM, Mike Galbraith wrote:
> > On Mon, 2013-01-21 at 17:22 +0800, Michael Wang wrote:
> >> On 01/21/2013 05:09 PM, Mike Galbraith wrote:
> >>> On Mon, 2013-01-21 at 15:45 +0800, Michael Wang wrote:
> >>>> On 01/21/2013 03:09 PM, Mike Galbraith wrote:
> >>>>> On Mon, 2013-01-21 at 07:42 +0100, Mike Galbraith wrote:
> >>>>>> On Mon, 2013-01-21 at 13:07 +0800, Michael Wang wrote:
> >>>>>
> >>>>>>> May be we could try change this back to the old way later, after the aim
> >>>>>>> 7 test on my server.
> >>>>>>
> >>>>>> Yeah, something funny is going on.
> >>>>>
> >>>>> Never entering balance path kills the collapse. Asking wake_affine()
> >>>>> wrt the pull as before, but allowing us to continue should no idle cpu
> >>>>> be found, still collapsed. So the source of funny behavior is indeed in
> >>>>> balance_path.
> >>>>
> >>>> Below patch based on the patch set could help to avoid enter balance path
> >>>> if affine_sd could be found, just like the old logical, would you like to
> >>>> take a try and see whether it could help fix the collapse?
> >>>
> >>> No, it does not.
> >>
> >> Hmm...what have changed now compared to the old logical?
> >
> > What I did earlier to confirm the collapse originates in balance_path is
> > below. I just retested to confirm.
> >
> > Tasks jobs/min jti jobs/min/task real cpu
> > 1 435.34 100 435.3448 13.92 3.76 Mon Jan 21 10:24:00 2013
> > 1 440.09 100 440.0871 13.77 3.76 Mon Jan 21 10:24:22 2013
> > 1 440.41 100 440.4070 13.76 3.75 Mon Jan 21 10:24:45 2013
...
> >
> > That was with your change backed out, and the q/d below applied.
>
> So that change will help to solve the issue? good to know :)
>
> But it will invoke wake_affine() with out any delay, the benefit
> of the patch set will be reduced a lot...
Yeah, I used size large hammer.
> I think this change help to solve the issue because it avoid jump
> into balance path when wakeup for any cases, I think we can do
> some change like below to achieve this and meanwhile gain benefit
> from delay wake_affine().
Yup, I killed it all the way dead. I'll see what below does.
I don't really see the point of the wake_affine() change in this set
though. Its purpose is to decide if a pull is ok or not. If we don't
need its opinion when we look for an (momentarily?) idle core in
this_domain, we shouldn't need it at all, and could just delete it. If
we ever enter balance_path, we can't possibly induce imbalance without
there being something broken in that path, no?
BTW, it could well be that an unpatched kernel will collapse as well if
WAKE_BALANCE is turned on. I've never tried that on a largish box, as
doing any of the wakeup time optional stuff used to make tbench scream.
> Since the issue could not been reproduced on my side, I don't know
> whether the patch benefit or not, so if you are willing to send out
> a formal patch, I would be glad to include it in my patch set ;-)
Just changing to scan prev_cpu before considering pulling would put a
big dent in the bouncing cow problem, but that's the intriguing thing
about this set.. can we have the tbench and pgbench big box gain without
a lot of pain to go with it? Small boxen will surely benefit, pretty
much can't be hurt, but what about all those fast/light tasks that won't
hop across nodes to red hot data?
No formal patch is likely to result from any testing I do atm at least.
I'm testing your patches because I see potential, I really want it to
work out, but have to see it do that with my own two beady eyeballs ;-)
> And another patch below below is a debug one, which will print out
> all the sbm info, so we could check whether it was initialized
> correctly, just use command "dmesg | grep WYT" to show the map.
>
> Regards,
> Michael Wang
>
> ---
> kernel/sched/fair.c | 42 +++++++++++++++++++++++++-----------------
> 1 files changed, 25 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2aa26c1..4361333 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3250,7 +3250,7 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
> }
>
> /*
> - * Try and locate an idle CPU in the sched_domain.
> + * Try and locate an idle CPU in the sched_domain, return -1 if failed.
> */
> static int select_idle_sibling(struct task_struct *p, int target)
> {
> @@ -3292,13 +3292,13 @@ static int select_idle_sibling(struct task_struct *p, int target)
>
> target = cpumask_first_and(sched_group_cpus(sg),
> tsk_cpus_allowed(p));
> - goto done;
> + return target;
> next:
> sg = sg->next;
> } while (sg != sd->groups);
> }
> -done:
> - return target;
> +
> + return -1;
> }
>
> /*
> @@ -3342,40 +3342,48 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
> * may has already been cached on prev_cpu, and usually
> * they require low latency.
> *
> - * So firstly try to locate an idle cpu shared the cache
> + * Therefor, balance path in such case will cause damage
> + * and bring benefit synchronously, wakeup on prev_cpu
> + * may better than wakeup on a new lower load cpu for the
> + * cached memory, and we never know.
> + *
> + * So the principle is, try to find an idle cpu as close to
> + * prev_cpu as possible, if failed, just take prev_cpu.
> + *
> + * Firstly try to locate an idle cpu shared the cache
> * with prev_cpu, it has the chance to break the load
> * balance, fortunately, select_idle_sibling() will search
> * from top to bottom, which help to reduce the chance in
> * some cases.
> */
> new_cpu = select_idle_sibling(p, prev_cpu);
> - if (idle_cpu(new_cpu))
> + if (new_cpu != -1)
> goto unlock;
>
> /*
> * No idle cpu could be found in the topology of prev_cpu,
> - * before jump into the slow balance_path, try search again
> - * in the topology of current cpu if it is the affine of
> - * prev_cpu.
> + * before take the prev_cpu, try search again in the
> + * topology of current cpu if it is the affine of prev_cpu.
> */
> - if (cpu == prev_cpu ||
> - !sbm->affine_map[prev_cpu] ||
> + if (cpu == prev_cpu || !sbm->affine_map[prev_cpu] ||
> !cpumask_test_cpu(cpu, tsk_cpus_allowed(p)))
> - goto balance_path;
> + goto take_prev;
>
> new_cpu = select_idle_sibling(p, cpu);
> - if (!idle_cpu(new_cpu))
> - goto balance_path;
> -
> /*
> * Invoke wake_affine() finally since it is no doubt a
> * performance killer.
> */
> - if (wake_affine(sbm->affine_map[prev_cpu], p, sync))
> + if ((new_cpu != -1) &&
> + wake_affine(sbm->affine_map[prev_cpu], p, sync))
> goto unlock;
> +
> +take_prev:
> + new_cpu = prev_cpu;
> + goto unlock;
> }
>
> -balance_path:
> + /* Balance path. */
> new_cpu = (sd_flag & SD_BALANCE_WAKE) ? prev_cpu : cpu;
> sd = sbm->sd[type][sbm->top_level[type]];
>
> --
> 1.7.4.1
>
> DEBUG PATCH:
>
> ---
> kernel/sched/core.c | 30 ++++++++++++++++++++++++++++++
> 1 files changed, 30 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0c63303..f251f29 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5578,6 +5578,35 @@ static void update_top_cache_domain(int cpu)
> static int sbm_max_level;
> DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_balance_map, sbm_array);
>
> +static void debug_sched_balance_map(int cpu)
> +{
> + int i, type, level = 0;
> + struct sched_balance_map *sbm = &per_cpu(sbm_array, cpu);
> +
> + printk("WYT: sbm of cpu %d\n", cpu);
> +
> + for (type = 0; type < SBM_MAX_TYPE; type++) {
> + if (type == SBM_EXEC_TYPE)
> + printk("WYT: \t exec map\n");
> + else if (type == SBM_FORK_TYPE)
> + printk("WYT: \t fork map\n");
> + else if (type == SBM_WAKE_TYPE)
> + printk("WYT: \t wake map\n");
> +
> + for (level = 0; level < sbm_max_level; level++) {
> + if (sbm->sd[type][level])
> + printk("WYT: \t\t sd %x, idx %d, level %d, weight %d\n", sbm->sd[type][level], level, sbm->sd[type][level]->level, sbm->sd[type][level]->span_weight);
> + }
> + }
> +
> + printk("WYT: \t affine map\n");
> +
> + for_each_possible_cpu(i) {
> + if (sbm->affine_map[i])
> + printk("WYT: \t\t affine with cpu %x in sd %x, weight %d\n", i, sbm->affine_map[i], sbm->affine_map[i]->span_weight);
> + }
> +}
> +
> static void build_sched_balance_map(int cpu)
> {
> struct sched_balance_map *sbm = &per_cpu(sbm_array, cpu);
> @@ -5688,6 +5717,7 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
> * destroy_sched_domains() already do the work.
> */
> build_sched_balance_map(cpu);
> + debug_sched_balance_map(cpu);
> rcu_assign_pointer(rq->sbm, sbm);
> }
>
--
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