On Fri, Aug 11, 2023 at 7:08 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > > Hi all, > > (sorry for late response as I was away) > > On Fri, Aug 11, 2023 at 1:40 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > [...] > > > > > > > > > > Last note, for /proc/vmstat we have /proc/sys/vm/stat_refresh to trigger > > > > > an explicit refresh. For those users who really need more accurate > > > > > numbers we might consider interface like that. Or allow to write to stat > > > > > file and do that in the write handler. > > > > > > > > This wouldn't be my first option, but if that's the only way to get > > > > accurate stats I'll take it. > > > > > > To be honest, this would be my preferable option because of 2 reasons. > > > a) we do not want to guarantee to much on the precision front because > > > that would just makes maintainability much more harder with different > > > people having a different opinion of how much precision is enough and b) > > > it makes the more rare (need precise) case the special case rather than > > > the default. > > > > How about we go with the proposed approach in this patch (or the mutex > > approach as it's much cleaner), and if someone complains about slow > > reads we revert the change and introduce the refresh API? We might > > just get away with making all reads accurate and avoid the hassle of > > updating some userspace readers to do write-then-read. We don't know > > for sure that something will regress. > > > > What do you think? > > Actually I am with Michal on this one. As I see multiple regression > reports for reading the stats, I am inclined towards rate limiting the > sync stats flushing from user readable interfaces (through > mem_cgroup_flush_stats_ratelimited()) and providing a separate > interface as suggested by Michal to explicitly flush the stats for > users ok with the cost. Since we flush the stats every 2 seconds, most > of the users should be fine and the users who care about accuracy can > pay for it. I am worried that writing to a stat for flushing then reading will increase the staleness window which we are trying to reduce here. Would it be acceptable to add a separate interface to explicitly read flushed stats without having to write first? If the distinction disappears in the future we can just short-circuit both interfaces.
