From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] mm: memcg: provide accurate stats for userspace reads Date: Wed, 9 Aug 2023 14:58:41 +0200 Message-ID: References: <20230809045810.1659356-1-yosryahmed@google.com> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1691585922; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=QVWr8XXVikYCh9tW7i2JMcKFQHyA2Ul51zZ7VG7gDMk=; b=qQ/BYlEykYjWP3ve2rOyS1R2CVAD0zcH+XiaHYHXrTSyZBaOWu5MEx9YCNGQLo34CDGhiq TlsnFjOvhVaRV6Y75dFlwrNBeMiX9QtdlVT24EHYvyYM2ig/6Y8uxc/G9h4IscJRDSrIhb kQn1NeGt9IJHsLituqZTT0ZUKdtS8Fs= Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="windows-1252" To: Yosry Ahmed Cc: Johannes Weiner , Roman Gushchin , Shakeel Butt , Andrew Morton , Muchun Song , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org On Wed 09-08-23 05:31:04, Yosry Ahmed wrote: > On Wed, Aug 9, 2023 at 1:51=E2=80=AFAM Michal Hocko wrote: > > > > On Wed 09-08-23 04:58:10, Yosry Ahmed wrote: > > > Over time, the memcg code added multiple optimizations to the stats > > > flushing path that introduce a tradeoff between accuracy and > > > performance. In some contexts (e.g. dirty throttling, refaults, etc),= a > > > full rstat flush of the stats in the tree can be too expensive. Such > > > optimizations include [1]: > > > (a) Introducing a periodic background flusher to keep the size of the > > > update tree from growing unbounded. > > > (b) Allowing only one thread to flush at a time, and other concurrent > > > flushers just skip the flush. This avoids a thundering herd problem > > > when multiple reclaim/refault threads attempt to flush the stats at > > > once. > > > (c) Only executing a flush if the magnitude of the stats updates exce= eds > > > a certain threshold. > > > > > > These optimizations were necessary to make flushing feasible in > > > performance-critical paths, and they come at the cost of some accuracy > > > that we choose to live without. On the other hand, for flushes invoked > > > when userspace is reading the stats, the tradeoff is less appealing > > > This code path is not performance-critical, and the inaccuracies can > > > affect userspace behavior. For example, skipping flushing when there = is > > > another ongoing flush is essentially a coin flip. We don't know if the > > > ongoing flush is done with the subtree of interest or not. > > > > I am not convinced by this much TBH. What kind of precision do you > > really need and how much off is what we provide? > > > > More expensive read of stats from userspace is quite easy to notice > > and usually reported as a regression. So you should have a convincing > > argument that an extra time spent is really worth it. AFAIK there are > > many monitoring (top like) tools which simply read those files regularly > > just to show numbers and they certainly do not need a high level of > > precision. >=20 > We used to spend this time before commit fd25a9e0e23b ("memcg: unify > memcg stat flushing") which generalized the "skip if ongoing flush" > for all stat flushing. As far I know, the problem was contention on > the flushing lock which also affected critical paths like refault. >=20 > The problem is that the current behavior is indeterministic, if cpu A > tries to flush stats and cpu B is already doing that, cpu A will just > skip. At that point, the cgroup(s) that cpu A cares about may have > been fully flushed, partially flushed (in terms of cpus), or not > flushed at all. We have no idea. We just know that someone else is > flushing something. IOW, in some cases the flush request will be > completely ignored and userspace will read stale stats (up to 2s + the > periodic flusher runtime). Yes, that is certainly true but why does that matter? Stats are always a snapshot of the past. Do we get an inconsistent image that would be actively harmful. > Some workloads need to read up-to-date stats as feedback to actions > (e.g. after proactive reclaim, or for userspace OOM killing purposes), > and reading such stale stats causes regressions or misbehavior by > userspace. Please tell us more about those and why should all others that do not require such a precision should page that price as well. > > [...] > > > @@ -639,17 +639,24 @@ static inline void memcg_rstat_updated(struct m= em_cgroup *memcg, int val) > > > } > > > } > > > > > > -static void do_flush_stats(void) > > > +static void do_flush_stats(bool full) > > > { > > > + if (!atomic_read(&stats_flush_ongoing) && > > > + !atomic_xchg(&stats_flush_ongoing, 1)) > > > + goto flush; > > > + > > > /* > > > - * We always flush the entire tree, so concurrent flushers can = just > > > - * skip. This avoids a thundering herd problem on the rstat glo= bal lock > > > - * from memcg flushers (e.g. reclaim, refault, etc). > > > + * We always flush the entire tree, so concurrent flushers can = choose to > > > + * skip if accuracy is not critical. Otherwise, wait for the on= going > > > + * flush to complete. This avoids a thundering herd problem on = the rstat > > > + * global lock from memcg flushers (e.g. reclaim, refault, etc). > > > */ > > > - if (atomic_read(&stats_flush_ongoing) || > > > - atomic_xchg(&stats_flush_ongoing, 1)) > > > - return; > > > - > > > + while (full && atomic_read(&stats_flush_ongoing) =3D=3D 1) { > > > + if (!cond_resched()) > > > + cpu_relax(); > > > > You are reinveting a mutex with spinning waiter. Why don't you simply > > make stats_flush_ongoing a real mutex and make use try_lock for !full > > flush and normal lock otherwise? >=20 > So that was actually a spinlock at one point, when we used to skip if > try_lock failed. AFAICS cgroup_rstat_flush is allowed to sleep so spinlocks are not really possible. > We opted for an atomic because the lock was only used > in a try_lock fashion. The problem here is that the atomic is used to > ensure that only one thread actually attempts to flush at a time (and > others skip/wait), to avoid a thundering herd problem on > cgroup_rstat_lock. >=20 > Here, what I am trying to do is essentially equivalent to "wait until > the lock is available but don't grab it". If we make > stats_flush_ongoing a mutex, I am afraid the thundering herd problem > will be reintroduced for stats_flush_ongoing this time. You will have potentially many spinners for something that might take quite a lot of time (sleep) if there is nothing else to schedule. I do not think this is a proper behavior. Really, you shouldn't be busy waiting for a sleeper. > I am not sure if there's a cleaner way of doing this, but I am > certainly open for suggestions. I also don't like how the spinning > loop looks as of now. mutex_try_lock for non-critical flushers and mutex_lock of syncing ones. We can talk a custom locking scheme if that proves insufficient or problematic. --=20 Michal Hocko SUSE Labs