Messages in this thread | | | Date | Thu, 31 Mar 2016 09:37:43 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg() |
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On Wed, Mar 30, 2016 at 06:42:20PM -0700, Steve Muckle wrote: > On 03/30/2016 12:35 PM, Peter Zijlstra wrote: > > On Mon, Mar 28, 2016 at 12:38:26PM -0700, Steve Muckle wrote: > >> Without covering all the paths where CFS utilization changes it's > >> possible to have to wait up to a tick to act on some changes, since the > >> tick is the only guaranteed regularly-occurring instance of the hook. > >> That's an unacceptable amount of latency IMO... > > > > Note that even with your patches that might still be the case. Remote > > wakeups might not happen on the destination CPU at all, so it might not > > be until the next tick (which always happens locally) that we'll > > 'observe' the utilization change brought with the wakeups. > > > > We could force all the remote wakeups to IPI the destination CPU, but > > that comes at a significant performance cost. > > What about only IPI'ing the destination when the utilization change is > known to require a higher CPU frequency?
Can't, the way the wakeup path is constructed we would be sending the IPI way before we know about utilization.
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