On Fri, Mar 6, 2020 at 12:40 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Thu, Mar 05, 2020 at 02:11:49PM -0800, Paul Turner wrote: > > The goal is to improve jitter since we're constantly periodically > > preempting other classes to run the watchdog. Even on a single CPU > > this is measurable as jitter in the us range. But, what increases the > > motivation is this disruption has been recently magnified by CPU > > "gifts" which require evicting the whole core when one of the siblings > > schedules one of these watchdog threads. > > > > The majority outcome being asserted here is that we could actually > > exercise pick_next_task if required -- there are other potential > > things this will catch, but they are much more braindead generally > > speaking (e.g. a bug in pick_next_task itself). > > I still utterly hate what the patch does though; there is no way I'll > have watchdog code hook in the scheduler like this. That's just asking > for trouble. > > Why isn't it sufficient to sample the existing context switch counters > from the watchdog? And why can't we fix that? We could go to pick next and repick the same task. There won't be a context switch but we still want to hold the watchdog. I assume such a counter also needs to be per cpu and inside the rq lock. There doesn't seem to be an existing one that fits this purpose.
