On Wed, Jun 26, 2024 at 2:35 PM Jesper Dangaard Brouer <hawk@xxxxxxxxxx> wrote: > > > > On 26/06/2024 00.59, Yosry Ahmed wrote: > > On Tue, Jun 25, 2024 at 3:35 PM Christoph Lameter (Ampere) <cl@xxxxxxxxx> wrote: > >> > >> On Tue, 25 Jun 2024, Yosry Ahmed wrote: > >> > >>>> In my reply above, I am not arguing to go back to the older > >>>> stats_flush_ongoing situation. Rather I am discussing what should be the > >>>> best eventual solution. From the vmstats infra, we can learn that > >>>> frequent async flushes along with no sync flush, users are fine with the > >>>> 'non-determinism'. Of course cgroup stats are different from vmstats > >>>> i.e. are hierarchical but I think we can try out this approach and see > >>>> if this works or not. > >>> > >>> If we do not do sync flushing, then the same problem that happened > >>> with stats_flush_ongoing could occur again, right? Userspace could > >>> read the stats after an event, and get a snapshot of the system before > >>> that event. > >>> > >>> Perhaps this is fine for vmstats if it has always been like that (I > >>> have no idea), or if no users make assumptions about this. But for > >>> cgroup stats, we have use cases that rely on this behavior. > >> > >> vmstat updates are triggered initially as needed by the shepherd task and > >> there is no requirement that this is triggered simultaenously. We > >> could actually randomize the intervals in vmstat_update() a bit if this > >> will help. > > > > The problem is that for cgroup stats, the behavior has been that a > > userspace read will trigger a flush (i.e. propagating updates). We > > have use cases that depend on this. If we switch to the vmstat model > > where updates are triggered independently from user reads, it > > constitutes a behavioral change. > > I implemented a variant using completions as Yosry asked for: > > https://lore.kernel.org/all/171943668946.1638606.1320095353103578332.stgit@firesoul/ Thanks! I will take a look at this a little bit later. I am wondering if you could verify if that solution fixes the problem with kswapd flushing?
