Re: [RFC PATCH 6/6] mm: Drain LRUs upon resume to userspace on nohz_full CPUs

[Michal Hocko <mhocko@suse.com>]

On Wed 03-07-24 14:52:21, Frederic Weisbecker wrote:
> Le Tue, Jun 25, 2024 at 04:20:01PM +0200, Michal Hocko a écrit :
> > On Tue 25-06-24 15:52:44, Frederic Weisbecker wrote:
> > > LRUs can be drained through several ways. One of them may add disturbances
> > > to isolated workloads while queuing a work at any time to any target,
> > > whether running in nohz_full mode or not.
> > > 
> > > Prevent from that on isolated tasks with draining LRUs upon resuming to
> > > userspace using the isolated task work framework.
> > > 
> > > It's worth noting that this is inherently racy against
> > > lru_add_drain_all() remotely queueing the per CPU drain work and
> > > therefore it prevents from the undesired disturbance only
> > > *most of the time*.
> > 
> > Can we simply not schedule flushing on remote CPUs and leave that to the
> > "return to the userspace" path?
> 
> Do you mean I should add a call on return to the userspace path or can
> I expect it to be drained at some point already?

I would make the particular per cpu cache to be drained on return to the
userspace.

> The other limitation with that task work thing is that if the task
> queueing the work actually goes to sleep and another task go on the CPU
> and does isolated work in userspace, the drain doesn't happen. Now whether
> that is a real problem or not, I have no idea.

Theoretically there is a problem because pages sitting on pcp LRU caches
cannot be migrated and some other operations will fail as well. But
practically speaking those pages should be mostly of interest to the
process allocating them most of the time. Page sharing between isolated
workloads sounds like a terrible idea to me. Maybe reality hits us in
this regards but we can deal with that when we learn about those
workloads.

So I wouldn't lose too much sleep over that. We are dealing with those
isolated workloads being broken by simple things like fork now because
that apparently adds pages on the pcp LRU cache and draining will happen
sooner or later (very often when the task is already running in the
userspace).

-- 
Michal Hocko
SUSE Labs