Re: [PATCH 2/2] memcg: do not drain charge pcp caches on remote isolated cpus

[Michal Hocko <mhocko@suse.com>]

On Sat 18-03-23 11:23:50, Hillf Danton wrote:
> On 17 Mar 2023 14:44:48 +0100 Michal Hocko <mhocko@suse.com>
> > Leonardo Bras has noticed that pcp charge cache draining might be
> > disruptive on workloads relying on 'isolated cpus', a feature commonly
> > used on workloads that are sensitive to interruption and context
> > switching such as vRAN and Industrial Control Systems.
> > 
> > There are essentially two ways how to approach the issue. We can either
> > allow the pcp cache to be drained on a different rather than a local cpu
> > or avoid remote flushing on isolated cpus.
> > 
> > The current pcp charge cache is really optimized for high performance
> > and it always relies to stick with its cpu. That means it only requires
> > local_lock (preempt_disable on !RT) and draining is handed over to pcp
> > WQ to drain locally again.
> > 
> > The former solution (remote draining) would require to add an additional
> > locking to prevent local charges from racing with the draining. This
> > adds an atomic operation to otherwise simple arithmetic fast path in the
> > try_charge path. Another concern is that the remote draining can cause a
> > lock contention for the isolated workloads and therefore interfere with
> > it indirectly via user space interfaces.
> > 
> > Another option is to avoid draining scheduling on isolated cpus
> > altogether. That means that those remote cpus would keep their charges
> > even after drain_all_stock returns. This is certainly not optimal either
> > but it shouldn't really cause any major problems. In the worst case
> > (many isolated cpus with charges - each of them with MEMCG_CHARGE_BATCH
> > i.e 64 page) the memory consumption of a memcg would be artificially
> > higher than can be immediately used from other cpus.
> > 
> > Theoretically a memcg OOM killer could be triggered pre-maturely.
> > Currently it is not really clear whether this is a practical problem
> > though. Tight memcg limit would be really counter productive to cpu
> > isolated workloads pretty much by definition because any memory
> > reclaimed induced by memcg limit could break user space timing
> > expectations as those usually expect execution in the userspace most of
> > the time.
> > 
> > Also charges could be left behind on memcg removal. Any future charge on
> > those isolated cpus will drain that pcp cache so this won't be a
> > permanent leak.
> > 
> > Considering cons and pros of both approaches this patch is implementing
> > the second option and simply do not schedule remote draining if the
> > target cpu is isolated. This solution is much more simpler. It doesn't
> > add any new locking and it is more more predictable from the user space
> > POV. Should the pre-mature memcg OOM become a real life problem, we can
> > revisit this decision.
> 
> JFYI feel free to take a look at the non-housekeeping CPUs [1].
> 
> [1] https://lore.kernel.org/lkml/20230223150624.GA29739@lst.de/

Such an approach would require remote draining and I hope I have
explained why that is not a preferred way in this case. Other than that
I do agree with Christoph that a generic approach would be really nice.

-- 
Michal Hocko
SUSE Labs