Messages in this thread | | | From | Brendan Jackman <> | Subject | Re: wake_wide mechanism clarification | Date | Thu, 03 Aug 2017 11:53:19 +0100 |
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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.
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)
> This would make affine misses really expensive, but we can probably negate this > by tracking per task how often it misses the target, and use that to adjust when > we do wake_affine in the future for that task. Still experimenting some, I just > found out a few hours ago I need to rework some of this to fix my cpu imbalance > problem with cgroups, so once I get something working I'll throw it your way to > take a look. Thanks,
Cheers, Brendan
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