Re: task isolation discussion at Linux Plumbers

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 7 Nov 2016 17:55:47 +0100 (CET)
Thomas Gleixner <tglx@linutronix.de> wrote:

> On Sat, 5 Nov 2016, Chris Metcalf wrote:
> > == Remote statistics ==
> > 
> > We discussed the possibility of remote statistics gathering, i.e. load
> > average etc.  The idea would be that we could have housekeeping
> > core(s) periodically iterate over the nohz cores to load their rq
> > remotely and do update_current etc.  Presumably it should be possible
> > for a single housekeeping core to handle doing this for all the
> > nohz_full cores, as we only need to do it quite infrequently.
> > 
> > Thomas suggested that this might be the last remaining thing that
> > needed to be done to allow disabling the current behavior of falling
> > back to a 1 Hz clock in nohz_full.
> > 
> > I believe Thomas said he had a patch to do this already.  
> 
> No, Riek was working on that.

Rik series made it possible to have remote tick sampling. That is,
calling account_process_tick(cpu) from a housekeeping CPU, so that
tick accounting is done for a remote "cpu". The series was intended
to improve overhead on nohz_full CPUs.

However, to get rid of the 1Hz tick, we need to do the same thing
for scheduler_tick(). I'm not sure if this has been attempted yet,
and if it's at all possible.

> > == Remote LRU cache drain ==
> > 
> > One of the issues with task isolation currently is that the LRU cache
> > drain must be done prior to entering userspace, but it requires
> > interrupts enabled and thus can't be done atomically.  My previous
> > patch series have handled this by checking with interrupts disabled,
> > but then looping around with interrupts enabled to try to drain the
> > LRU pagevecs.  Experimentally this works, but it's not provable that
> > it terminates, which is worrisome.  Andy suggested adding a percpu
> > flag to disable creation of deferred work like LRU cache pages.
> > 
> > Thomas suggested using an RT "local lock" to guard the LRU cache
> > flush; he is planning on bringing the concept to mainline in any case.
> > However, after some discussion we converged on simply using a spinlock
> > to guard the appropriate resources.  As a result, the
> > lru_add_drain_all() code that currently queues work on each remote cpu
> > to drain it, can instead simply acquire the lock and drain it remotely.
> > This means that a task isolation task no longer needs to worry about
> > being interrupted by SMP function call IPIs, so we don't have to deal
> > with this in the task isolation framework any more.
> > 
> > I don't recall anyone else volunteering to tackle this, so I will plan
> > to look at it.  The patch to do that should be orthogonal to the
> > revised task isolation patch series.  
> 
> I offered to clean up the patch from RT. I'll do that in the next days.

Yes, the RT kernel got a patch that fixes this. I wonder if the same
idea could work for vm_stat (that is, gathering those stats from
housekeeping CPUs vs. having to queue deferrable work to do it).