Hello, On Mon, Aug 14, 2023 at 05:28:22PM -0700, Yosry Ahmed wrote: > > So, the original design used mutex for synchronize flushing with the idea > > being that updates are high freq but reads are low freq and can be > > relatively slow. Using rstats for mm internal operations changed this > > assumption quite a bit and we ended up switching that mutex with a lock. > > Naive question, do mutexes handle thundering herd problems better than > spinlocks? I would assume so but I am not sure. I don't know. We can ask Waiman if that becomes a problem. > > * Flush-side, maybe we can break flushing into per-cpu or whatnot but > > there's no avoiding the fact that flushing can take quite a while if there > > are a lot to flush whether locks are split or not. I wonder whether it'd > > be possible to go back to mutex for flushing and update the users to > > either consume the cached values or operate in a sleepable context if > > synchronous read is necessary, which is the right thing to do anyway given > > how long flushes can take. > > Unfortunately it cannot be broken down into per-cpu as all flushers > update the same per-cgroup counters, so we need a bigger locking > scope. Switching to atomics really hurts performance. Breaking down > the lock to be per-cgroup is doable, but since we need to lock both > the parent and the cgroup, flushing top-level cgroups (which I assume > is most common) will lock the root anyway. Plus, there's not much point in flushing in parallel, so I don't feel too enthusiastic about splitting flush locking. > All flushers right now operate in sleepable context, so we can go > again to the mutex if you think this will make things better. The Yes, I think that'd be more sane. > slowness problem reported recently is in a sleepable context, it's > just too slow for userspace if I understand correctly. I mean, there's a certain amount of work to do. There's no way around it if you wanna read the counters synchronously. The only solution there would be using a cached value or having some sort of auto-flushing mechanism so that the amount to flush don't build up too much - e.g. keep a count of the number of entries to flush and trigger flush if it goes over some threshold. Thanks. -- tejun
