Messages in this thread | | | Date | Wed, 17 May 2017 13:46:35 +0100 | From | Matt Fleming <> | Subject | Re: hackbench vs select_idle_sibling; was: [tip:sched/core] sched/fair, cpumask: Export for_each_cpu_wrap() |
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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.
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