Re: wake_wide mechanism clarification

[Atish Patra]

On 08/03/2017 10:05 AM, Brendan Jackman wrote:
> On Thu, Aug 03 2017 at 13:15, Josef Bacik wrote:
> > On Thu, Aug 03, 2017 at 11:53:19AM +0100, Brendan Jackman wrote:
> > > Hi,
> > >
> > > On Fri, Jun 30 2017 at 17:55, Josef Bacik wrote:
> > > > On Fri, Jun 30, 2017 at 07:02:20PM +0200, Mike Galbraith wrote:
> > > > > On Fri, 2017-06-30 at 10:28 -0400, Josef Bacik wrote:
> > > > > > On Thu, Jun 29, 2017 at 08:04:59PM -0700, Joel Fernandes wrote:
> > > > > >
> > > > > > > That makes sense that we multiply slave's flips by a factor because
> > > > > > > its low, but I still didn't get why the factor is chosen to be
> > > > > > > llc_size instead of something else for the multiplication with slave
> > > > > > > (slave * factor).
> > > > >
> > > > > > Yeah I don't know why llc_size was chosen...
> > > > >
> > > > > static void update_top_cache_domain(int cpu)
> > > > > {
> > > > > struct sched_domain_shared *sds = NULL;
> > > > > struct sched_domain *sd;
> > > > > int id = cpu;
> > > > > int size = 1;
> > > > >
> > > > > sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
> > > > > if (sd) {
> > > > > id = cpumask_first(sched_domain_span(sd));
> > > > > size = cpumask_weight(sched_domain_span(sd));
> > > > > sds = sd->shared;
> > > > > }
> > > > >
> > > > > rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
> > > > > per_cpu(sd_llc_size, cpu) = size;
> > > > >
> > > > > The goal of wake wide was to approximate when pulling would be a futile
> > > > > consolidation effort and counterproductive to scaling. 'course with
> > > > > ever increasing socket size, any 1:N waker is ever more likely to run
> > > > > out of CPU for its one and only self (slamming into scaling wall)
> > > > > before it needing to turn its minions loose to conquer the world.
> > > > >
> > > > > Something else to consider: network interrupt waking multiple workers
> > > > > at high frequency. If the waking CPU is idle, do you really want to
> > > > > place a worker directly in front of a tattoo artist, or is it better
> > > > > off nearly anywhere but there?
> > > > >
> > > > > If the box is virtual, with no topology exposed (or real but ancient)
> > > > > to let select_idle_sibling() come to the rescue, two workers can even
> > > > > get tattooed simultaneously (see sync wakeup).
> > > >
> > > > Heuristics are hard, news at 11.  I think messing with wake_wide() itself is too
> > > > big of a hammer, we probably need a middle ground.  I'm messing with it right
> > > > now so it's too early to say for sure, but i _suspect_ the bigger latencies we
> > > > see are not because we overload the cpu we're trying to pull to, but because
> > > > when we fail to do the wake_affine() we only look at siblings of the affine_sd
> > > > instead of doing the full "find the idlest cpu in the land!" thing.
> > >
> > > This is the problem I've been hitting lately. My use case is 1 task per
> > > CPU on ARM big.LITTLE (asymmetrical CPU capacity). The workload is 1
> > > task per CPU, they all do X amount of work then pthread_barrier_wait
> > > (i.e. sleep until the last task finishes its X and hits the barrier). On
> > > big.LITTLE, the tasks which get a "big" CPU finish faster, and then
> > > those CPUs pull over the tasks that are still running:
> > >
> > >      v CPU v           ->time->
> > >
> > >                     -------------
> > >    0  (big)         11111  /333
> > >                     -------------
> > >    1  (big)         22222   /444|
> > >                     -------------
> > >    2  (LITTLE)      333333/
> > >                     -------------
> > >    3  (LITTLE)      444444/
> > >                     -------------
> > >
> > > Now when task 4 hits the barrier (at |) and wakes the others up, there
> > > are 4 tasks with prev_cpu=<big> and 0 tasks with
> > > prev_cpu=<little>. Assuming that those wakeups happen on CPU4,
> > > regardless of wake_affine, want_affine means that we'll only look in
> > > sd_llc (cpus 0 and 1), so tasks will be unnecessarily coscheduled on the
> > > bigs until the next load balance, something like this:
> > >
> > >      v CPU v           ->time->
> > >
> > >                     ------------------------
> > >    0  (big)         11111  /333  31313\33333
> > >                     ------------------------
> > >    1  (big)         22222   /444|424\4444444
> > >                     ------------------------
> > >    2  (LITTLE)      333333/          \222222
> > >                     ------------------------
> > >    3  (LITTLE)      444444/            \1111
> > >                     ------------------------
> > >                                  ^^^
> > >                            underutilization
> > >
> > > > I _think_
> > > > the answer is to make select_idle_sibling() try less hard to find something
> > > > workable and only use obviously idle cpu's in the affine sd, and fall back to
> > > > the full load balance esque search.
> > >
> > > So this idea of allowing select_idle_sibling to fail, and falling back
> > > to the slow path, would help me too, I think.
> >
> > Unfortunately this statement of mine was wrong, I had it in my head that we
> > would fall back to a find the idlest cpu thing provided we failed to wake
> > affine, but we just do select_idle_sibling() and expect the load balancer to
> > move things around as needed.
>
> Ah yes, when wake_affine() returns false, we still do
> select_idle_sibling (except in prev_cpu's sd_llc instead of
> smp_processor_id()'s), and that is the problem faced by my workload. I
> thought you were suggesting to change the flow so that
> select_idle_sibling can say "I didn't find any idle siblings - go to the
> find_idlest_group path".
>
> > > This is also why I was playing with your
> > > don't-affine-recently-balanced-tasks patch[1], which also helps my case
> > > since it prevents want_affine for tasks 3 and 4 (which were recently
> > > moved by an active balance).
> > >
> > > [1] https://marc.info/?l=linux-kernel&m=150003849602535&w=2
> > >     (also linked elsewhere in this thread)
> >
> > Would you try peter's sched/experimental branch and see how that affects your
> > workload?  I'm still messing with my patches and I may drop this one as it now
> > appears to be too aggressive with the new set of patches.  Thanks,
>
> Sure, I'll take a look at those, thanks. I guess the idea of caching
> values in LB and then using them in wakeup[2] is a lighter-handed way of
> achieving the same thing as last_balance_ts? It won't solve my problem
> directly since we'll still only look in sd_llc, but I think it could be
> a basis for a way to say "go find_idlest_group path on these tasks" at
> the beginning of select_task_rq_fair.
>
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=sched/experimental&id=5b4ed509027a5b6f495e6fe871cae850d5762bef
>
> Thanks,
> Brendan

Would it be better if it searches for idle cpus in next higher domain 
(if that is not NUMA) instead of doing find_idlest_group ?

The above approach [2] will still try to search a idle cpu in the llc 
domain of new_cpu. If it finds one great. Otherwise, let's search for an 
idle cpu in next higher domain(if that is not NUMA) excluding the 
current domain which we have already searched. I think it would help in 
cases where LLC < NUMA.


Regards,
Atish