On Fri 25-08-23 20:43:02, Michal Hocko wrote: > On Fri 25-08-23 11:21:16, Yosry Ahmed wrote: > > On Fri, Aug 25, 2023 at 11:17 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Fri 25-08-23 08:14:54, Yosry Ahmed wrote: > > > > On Fri, Aug 25, 2023 at 12:05 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > [...] > > > > > I might be wrong but the whole discussion so far suggests that the > > > > > global rstat lock should be reconsidered. From my personal experience > > > > > global locks easily triggerable from the userspace are just a receip for > > > > > problems. Stats reading shouldn't be interfering with the system runtime > > > > > as much as possible and they should be deterministic wrt runtime as > > > > > well. > > > > > > > > The problem is that the global lock also serializes the global > > > > counters that we flush to. I will talk from the memcg flushing > > > > perspective as that's what I am familiar with. I am not sure how much > > > > this is transferable to other flushers. > > > > > > > > On the memcg side (see mem_cgroup_css_rstat_flush()), the global lock > > > > synchronizes access to multiple counters, for this discussion what's > > > > most important are: > > > > - The global stat counters of the memcg being flushed (e.g. > > > > memcg->vmstats->state). > > > > - The pending stat counters of the parent being flushed (e.g. > > > > parent->vmstats->state_pending). > > > > > > I haven't digested the rest of the email yet (Friday brain, sorry) but I > > > do not think you are adressing this particular part so let me ask before > > > I dive more into the following. I really do not follow the serialization > > > requirement here because the lock doesn't really serialize the flushing, > > > does it? At least not in a sense of a single caller to do the flushing > > > atomicaly from other flushers. It is possible that the current flusher > > > simply drops the lock midway and another one retakes the lock and > > > performs the operation again. So what additional flushing > > > synchronization does it provide and why cannot parallel flushers simply > > > compete over pcp spinlocks? > > > > > > So what am I missing? > > > > Those counters are non-atomic. The lock makes sure we don't have two > > concurrent flushers updating the same counter locklessly and > > non-atomically, which would be possible if we flush the same cgroup on > > two different cpus in parallel. > > pcp lock (cpu_lock) guarantees the very same, doesn't it? Nope, it doesn't. I really need to have a deeper look. -- Michal Hocko SUSE Labs
