On Wed, 16 Sep 2020 at 13:00, Mel Gorman <mgor...@suse.de> wrote: > > On Wed, Sep 16, 2020 at 12:31:03PM +0800, Aubrey Li wrote: > > Added idle cpumask to track idle cpus in sched domain. When a CPU > > enters idle, its corresponding bit in the idle cpumask will be set, > > and when the CPU exits idle, its bit will be cleared. > > > > When a task wakes up to select an idle cpu, scanning idle cpumask > > has low cost than scanning all the cpus in last level cache domain, > > especially when the system is heavily loaded. > > > > The following benchmarks were tested on a x86 4 socket system with > > 24 cores per socket and 2 hyperthreads per core, total 192 CPUs: > > > > This still appears to be tied to turning the tick off. An idle CPU > available for computation does not necessarily have the tick turned off > if it's for short periods of time. When nohz is disabled or a machine is > active enough that CPUs are not disabling the tick, select_idle_cpu may > fail to select an idle CPU and instead stack tasks on the old CPU. > > The other subtlety is that select_idle_sibling() currently allows a > SCHED_IDLE cpu to be used as a wakeup target. The CPU is not really > idle as such, it's simply running a low priority task that is suitable > for preemption. I suspect this patch breaks that.
Yes, good point. I completely missed this > > -- > Mel Gorman > SUSE Labs
