Re: hackbench vs select_idle_sibling; was: [tip:sched/core] sched/fair, cpumask: Export for_each_cpu_wrap()

Chris Mason
2017-05-17T14:51:04Z

On 05/17/2017 06:53 AM, Peter Zijlstra wrote:
> On Mon, May 15, 2017 at 02:03:11AM -0700, tip-bot for Peter Zijlstra wrote:
>> sched/fair, cpumask: Export for_each_cpu_wrap()
>
>> -static int cpumask_next_wrap(int n, const struct cpumask *mask, int start, int *wrapped)
>> -{
>
>> - next = find_next_bit(cpumask_bits(mask), nr_cpumask_bits, n+1);
>
>> -}
>
> OK, so this patch fixed an actual bug in the for_each_cpu_wrap()
> implementation. The above 'n+1' should be 'n', and the effect is that
> it'll skip over CPUs, potentially resulting in an iteration that only
> sees every other CPU (for a fully contiguous mask).
>
> This in turn causes hackbench to further suffer from the regression
> introduced by commit:
>
> 4c77b18cf8b7 ("sched/fair: Make select_idle_cpu() more aggressive")
>
> So its well past time to fix this.
>
> Where the old scheme was a cliff-edge throttle on idle scanning, this
> introduces a more gradual approach. Instead of stopping to scan
> entirely, we limit how many CPUs we scan.
>
> Initial benchmarks show that it mostly recovers hackbench while not
> hurting anything else, except Mason's schbench, but not as bad as the
> old thing.
>
> It also appears to recover the tbench high-end, which also suffered like
> hackbench.
>
> I'm also hoping it will fix/preserve kitsunyan's interactivity issue.
>
> Please test..

We'll get some tests going here too.

-chris