Re: [PATCH 2/7] nohz: New tick dependency mask

[Peter Zijlstra]

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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