Messages in this thread |  | | Subject | Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested | | From | David Woodhouse <> | | Date | Mon, 09 Jul 2018 13:47:14 +0100 |
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On Mon, 2018-07-09 at 05:34 -0700, Paul E. McKenney wrote: > The reason that David's latencies went from 100ms to one second is > because I made this code less aggressive about invoking resched_cpu().
Ten seconds. We saw synchronize_sched() take ten seconds in 4.15. We wouldn't have been happy with one second, but ten seconds was considered particularly suboptimal.
> The reason I did that was to allow cond_resched_rcu_qs() to be used less > without performance regressions. And just plain cond_resched() on > !PREEMPT is intended to handle the faster checks. But KVM defeats > this by checking need_resched() before invoking cond_resched().
It isn't just KVM. It's a relatively common construct to use need_resched(), then drop any local locks around cond_resched().
A bare cond_resched() will call rcu_all_qs() unconditionally, and it is kind of inconsistent that need_resched() doesn't include the corresponding condition.[unhandled content-type:application/x-pkcs7-signature] |  |