Messages in this thread | | | Subject | Re: [RFC] sched/fair: Use wake_q length as a hint for wake_wide | From | Atish Patra <> | Date | Wed, 20 Sep 2017 16:17:31 -0500 |
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On 09/20/2017 03:23 PM, Joel Fernandes wrote: > On Wed, Sep 20, 2017 at 2:33 AM, Brendan Jackman > <brendan.jackman@arm.com> wrote: >> >> On Wed, Sep 20 2017 at 05:06, Joel Fernandes wrote: >>>> On Tue, Sep 19, 2017 at 3:05 AM, Brendan Jackman >>>> <brendan.jackman@arm.com> wrote: >>>>> On Mon, Sep 18 2017 at 22:15, Joel Fernandes wrote: >>> [..] >>>>>>> IIUC, if wake_affine() behaves correctly this trick wouldn't be >>>>>>> necessary on SMP systems, so it might be best guarded by the presence >>>>>> >>>>>> Actually wake_affine doesn't check for balance if previous/next cpu >>>>>> are within the same shared cache domain. The difference is some time >>>>>> ago it would return true for shared cache but now it returns false as >>>>>> of 4.14-rc1: >>>>>> http://elixir.free-electrons.com/linux/v4.14-rc1/source/kernel/sched/fair.c#L5466 >>>>>> >>>>>> Since it would return false in the above wake up cases for task 1 and >>>>>> 2, it would then run select_idle_sibling on the previous CPU which is >>>>>> also within the big cluster, so I don't think it will make a >>>>>> difference in this case... Infact what it returns probably doesn't >>>>>> matter. >>>>> >>>>> So my paragraph here was making a leap in reasoning, let me try to fill >>>>> the gap: On SMP these tasks never need to move around. If by some chance >>>>> they did get coscheduled, the first load balance would spread them out and >>>>> then every time they wake up from then on, prev_cpu is the sensible >>>>> choice. So it will look something like: >>>>> >>>>> v CPU v ->time-> >>>>> >>>>> ------------- >>>>> { 0 (SAME) 11111111111 >>>>> cache { ------------- >>>>> { 1 (SAME) 222222222222| >>>>> ------------- >>>>> { 2 (SAME) 33333333333 >>>>> cache { ------------- >>>>> { 3 (SAME) 44444444444 >>>>> ------------- >>>>> >>>>> So here, task 2 wakes up the other guys and when it's doing tasks 3 and >>>>> 4, prev_cpu and smp_processor_id() don't share a cache, so IIUC its' >>>>> basically wake_affine's job to decide between prev_cpu and >>>>> smp_processor_id(). So "if wake_affine is behaving correctly" the >>>>> problem that this patch aims to solve (i.e. the fact that we overload >>>>> the waker's LLC domain because of bias towards prev_cpu) does not arise >>>>> on SMP. >>>>> >>>> >>>> Yes SMP, but your patch is for solving a problem for non-SMP. So your >>>> original statement about wake_affine solving any problem for SMP is >>>> not relevant I feel :-P. I guess you can just kill this para from the >>>> commit message to prevent confusion. >>> >>> Ok I take that back, you were talking about guarding this feature by >>> the SD_ASYM_CPUCAPACITY flag. >>> >>> I don't think that protection would be helpful because you can have >>> the same issue if the tasks do different amount of work on SMP. So in >>> that case some threads might still complete before the others and you >>> run into the same thing. >> >> Well assuming we're still talking about one task per CPU, if you have >> tasks doing different amount of work there's still no reason to move the >> longer-running threads around. The only reason that happens in my >> example is because of the asym capacity. > > Yes but you can very well have RT pressure and things that temporarily > change the capacity equality. Also this is a simple benchmark and for > any reason you have more than 1 task running on those other CPUs and > then the idle CPUs run some of the tasks and you run into a similar > situation that might need your patch.. > The patch would be helpful only if it doesn't cross NUMA boundary. right ?
If NUMA comes into picture, not sure searching across NUMA may hurt more than help, especially in this case. > Also one more note, the SD_ASYM_CPUCAPACITY protection is still not > needed because SD_BALANCE_WAKE isn't turned on for > !SD_ASYM_CPUCAPACITY from what I learnt from discussions with Mike, I > believe its this piece of code in sd_init that actually enables it: > > if (sd->flags & SD_ASYM_CPUCAPACITY) { > struct sched_domain *t = sd; > > for_each_lower_domain(t) > t->flags |= SD_BALANCE_WAKE; > } > > But it will always try to search within LLC group if want_affine is set irrespective of the SD_BALANCE_WAKE flag.
if (affine_sd) { sd = NULL; /* Prefer wake_affine over balance flags */ if (cpu == prev_cpu) goto pick_cpu;
if (wake_affine(affine_sd, p, prev_cpu, sync)) new_cpu = cpu; }
if (!sd) { pick_cpu: if (sd_flag & SD_BALANCE_WAKE) /* XXX always ? */ new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
SD_ASYM_CPUCAPACITY protection may still be required if it is a non-issue for big SMP servers.
Regards, Atish > thanks, > > - Joel >
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