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

From: Matt Fleming
Date: Wed May 17 2017 - 08:46:55 EST

On Wed, 17 May, at 12:53:50PM, 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..

Tests queued up at SUSE. I'll let you know the results as soon as
they're ready -- it'll be at least a couple of days.