+Ivan On Mon, Aug 14, 2023 at 5:51 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > On Mon, Aug 14, 2023 at 5:48 PM Tejun Heo <tj@xxxxxxxxxx> wrote: > > > > Hello, > > > > On Mon, Aug 14, 2023 at 05:39:15PM -0700, Yosry Ahmed wrote: > > > I believe dropping unified flushing, if possible of course, may fix > > > both problems. > > > > Yeah, flushing the whole tree for every stat read will push up the big O > > complexity of the operation. It shouldn't be too bad because only what's > > updated since the last read will need flushing but if you have a really big > > machine with a lot of constantly active cgroups, you're still gonna feel it. > > So, yeah, drop that and switch the global lock to mutex and we should all be > > good? > > I hope so, but I am not sure. > > The unified flushing was added initially to mitigate a thundering herd > problem from concurrent in-kernel flushers (e.g. concurrent reclaims), > but back then flushing was atomic so we had to keep the spinlock held > for a long time. I think it should be better now, but I am hoping > Shakeel will chime in since he added the unified flushing originally. > > We also need to agree on what to do about stats_flushing_threshold and > flush_next_time since they're both global now (since all flushing is > global). > I thought we already reached the decision on how to proceed here. Let me summarize what I think we should do: 1. Completely remove the sync flush from stat files read from userspace. 2. Provide a separate way/interface to explicitly flush stats for users who want more accurate stats and can pay the cost. This is similar to the stat_refresh interface. 3. Keep the 2 sec periodic stats flusher.
