Messages in this thread | | | From | Steve Muckle <> | Date | Fri, 19 Aug 2016 13:10:37 -0700 | Subject | Re: [PATCH] sched: fix incorrect PELT values on SMT |
| |
On Fri, Aug 19, 2016 at 04:30:39PM +0100, Morten Rasmussen wrote: > Hi Steve, > > On Thu, Aug 18, 2016 at 06:55:41PM -0700, Steve Muckle wrote: > > PELT scales its util_sum and util_avg values via > > arch_scale_cpu_capacity(). If that function is passed the CPU's sched > > domain then it will reduce the scaling capacity if SD_SHARE_CPUCAPACITY > > is set. PELT does not pass in the sd however. The other caller of > > arch_scale_cpu_capacity, update_cpu_capacity(), does. This means > > util_sum and util_avg scale beyond the CPU capacity on SMT. > > > > On an Intel i7-3630QM for example rq->cpu_capacity_orig is 589 but > > util_avg scales up to 1024. > > I can't convince myself whether this is the right thing to do. SMT is a > bit 'special' and it depends on how you model SMT capacity. > > I'm no SMT expert, but the way I understand the current SMT capacity > model is that capacity_orig represents the capacity of the SMT-thread > when all its thread-siblings are busy. The true capacity of an > SMT-thread where all thread-siblings are idle is actually 1024, but we > don't model this (it would be nightmare to track when the capacity > should change). The capacity of a core with two or more SMT-threads is > chosen to be 1024 + smt_gain, where smt_gain is supposed represent the > additional throughput we gain for the additional SMT-threads. The reason > why we don't have 1024 per thread is that we would prefer to have only > one task per core if possible. > > With util_avg scaling to 1024 a core (capacity = 2*589) would be nearly > 'full' with just one always-running task. If we change util_avg to max > out at 589, it would take two always-running tasks for the combined > utilization to match the core capacity. So we may loose some bias > towards spreading for SMT systems. > > AFAICT, group_is_overloaded() and group_has_capacity() would both be > affected by this patch. > > Interestingly, Vincent recently proposed to set the SMT-thread capacity > to 1024 which would affectively make all the current SMT code redundant. > It would make things a lot simpler, but I'm not sure if we can get away > with it. It would need discussion at least. > > Opinions?
Thanks for having a look.
The reason I pushed this patch was to address an issue with the schedutil governor - demand is effectively doubled on SMT systems due to the above scheme. But this can just be fixed for schedutil by using a max value there consistent with what __update_load_avg() is using. I'll send another patch. It looks like there's a good reason for the current PELT scaling w.r.t. SMT in the scheduler/load balancer.
thanks, Steve
| |