Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg()

[Steve Muckle]

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?