On 17 Mar 2023 14:44:48 +0100 Michal Hocko <mhocko@xxxxxxxx> > 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@xxxxxx/
