Re: [PATCH 16/19] sched/numa: Detect if node actively handling migration

Srikar Dronamraju
2018-06-06T15:32:14Z

> >
> > All tasks will not be stuck at task/cpu A.
> >
> > "[PATCH 10/19] sched/numa: Stop multiple tasks from moving to the
> > cpu..." the first task to set cpu A as swap target will ensure
> > subsequent tasks wont be allowed to set cpu A as target for swap till
> > it
> > finds a better task/cpu. Because of this there a very very small
> > chance
> > of a second task unable to find a task to swap.
>
> Would it not be better for task_numa_compare to skip
> from consideration CPUs that somebody else is already
> trying to migrate a task to, but still search for the
> best location, instead of settling for a worse location
> to migrate to?

Yes its better to skip cpus if they are already in migration.
And we are already doing it with the above patch. However as I said
earlier

- Task T1 sets Cpu 1 as best_cpu,
- Task T2 finds cpu1 and skips Cpu1
- Task T1 finds cpu2 slightly better than cpu1.
- Task T1 resets cpu1 as best_cpu, sets best_cpu as cpu2.
- Task T2 finds cpu2 and skips cpu2
- Task T1 finds cpu3 as best_cpu slightly better than cpu2.
- Task T1 resets cpu2 as best_cpu, sets best_cpu as cpu3.
- Task T2 finds cpu3 and skips cpu3

So after this T1 was able to find a cpu but T2 couldn't find a cpu even
though there were 3 cpus that were available for 2 task to swap.

Again, this is too corner case, that I am okay to drop.