Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task enqueue/dequeue

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Jan 19, 2016 at 02:17:08PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 13, 2016 at 05:01:31PM +0100, Frederic Weisbecker wrote:
> > The full nohz CPU load is currently accounted on tick restart only.
> > But there are a few issues with this model:
> > 
> > _ On tick restart, if cpu_load[0] doesn't contain the load of the actual
> >   tickless load that just ran, we are going to account a wrong value.
> >   And it is very likely to be so given that cpu_load[0] doesn't have
> >   an opportunity to be updated between tick stop and tick restart.
> > 
> > _ If the runqueue had updates that didn't trigger a tick restart, we
> >   are going to miss those CPU load changes.
> > 
> > A solution to fix this is to update the CPU load everytime we enqueue
> > or dequeue a task in the fair runqueue and more than a jiffy occured
> > since the last update.
> 
> Would not a much better solution be to do this remotely instead of from
> one of the hottest functions in the scheduler?

The problem with doing this remotely is that we can miss past cpu loads if
there was several enqueue/dequeue operations happening while tickless.

For example if CPU 1 runs sched entity A with a load of 5 (purely theoric)
for 5 sec then it sleeps, entity B runs with a load of 1 and then CPU 2 updates
the load of CPU 1 remotely. cpu_load[0] will be accurate because it's the current
load of CPU 1 (which is the load of entity B), but the other indexes won't take
the decayed load of entity A into account.

Now we can indeed remove the queue time local update and only rely on remote
updates when needed if we can live with a light precision on target_load() and
source_load().