Re: [PATCH v2 0/5] mm: support parallel free of memory

[Aaron Lu]

On Fri, Mar 17, 2017 at 08:47:08AM +0100, Michal Hocko wrote:
> On Thu 16-03-17 11:36:21, Tim Chen wrote:
> [...]
> > Perhaps we can only do this expedited exit only when there are idle cpus around.
> > We can use the root sched domain's overload indicator for such a quick check.
> 
> This is not so easy, I am afraid. Those CPUs might be idle for a good
> reason (power saving etc.). You will never know by simply checking

Is it that those CPUs are deliberately put into idle mode to save power?
IIRC, idle injection driver could be used to do this and if so, the
injected idle task is a realtime one so the spawned kworker will not be
able to preempt(disturb) it.

> one metric. This is why doing these optimistic parallelization
> optimizations is far from trivial. This is not the first time somebody
> wants to do this.  People are trying to make THP migration faster
> doing the similar thing. I guess we really need a help from the
> scheduler to do this properly, though. I've been thinking about an API
> (e.g. try_to_run_in_backgroun) which would evaluate all these nasty
> details and either return with -EBUSY or kick the background thread to
> accomplish the work if the system is reasonably idle. I am not really
> sure whether such an API is viable though.  Peter, what do you think?

I would very much like to know what these nasty details are and what
'reasonably idle' actually means, I think they are useful to understand
the problem and define the API.

I totally agree that we shouldn't distrub the system by starting more
workers/threads to do spin work or to make a process utilizing more CPU
or other resources than allowed by its cgroup.

Thanks.