Messages in this thread | | | Date | Wed, 2 Dec 2015 13:48:06 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/7] nohz: New tick dependency mask |
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On Tue, Dec 01, 2015 at 11:20:28PM +0100, Frederic Weisbecker wrote: > On Tue, Dec 01, 2015 at 09:41:09PM +0100, Peter Zijlstra wrote: > > On Fri, Nov 13, 2015 at 03:22:04PM +0100, Frederic Weisbecker wrote: > > > The tick dependency is evaluated on every IRQ. This is a batch of checks > > > which determine whether it is safe to stop the tick or not. These checks > > > are often split in many details: posix cpu timers, scheduler, sched clock, > > > perf events. Each of which are made of smaller details: posix cpu > > > timer involves checking process wide timers then thread wide timers. Perf > > > involves checking freq events then more per cpu details. > > > > > > Checking these details asynchronously every time we update the full > > > dynticks state bring avoidable overhead and a messy layout. > > > > > > Lets introduce instead tick dependency masks: one for system wide > > > dependency (unstable sched clock), one for CPU wide dependency (sched, > > > perf), and task/signal level dependencies. The subsystems are responsible > > > of setting and clearing their dependency through a set of APIs that will > > > take care of concurrent dependency mask modifications and kick targets > > > to restart the relevant CPU tick whenever needed. > > > > Maybe better explain why we need the per task and per signal thingy? > > I'll detail that some more in the changelog. The only user of the per > task/per signal tick dependency is posix cpu timer. I've been first > proposing a global tick dependency as soon as any posix cpu timer is > armed.
> It simplified everything but some reviewers complained (eg: > some users might want to run posix timers on housekeepers without > bothering full dynticks CPUs). I could remove the per signal > dependency with dispatching it through all threads in the group each > time there is an update but that's the best I can think of.
Right, I remember some of that. Seems worth preserving these reasons. Maybe even in code comments near the definition of these things.
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