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? Morten
