Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the percpu lock

Yosry Ahmed
2023-03-23T05:15:54Z

On Wed, Mar 22, 2023 at 9:29 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Wed, Mar 22, 2023 at 9:00 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > Currently, when sleeping is not allowed during rstat flushing, we hold
> > the global rstat lock with interrupts disabled throughout the entire
> > flush operation. Flushing in an O(# cgroups * # cpus) operation, and
> > having interrupts disabled throughout is dangerous.
> >
> > For some contexts, we may not want to sleep, but can be interrupted
> > (e.g. while holding a spinlock or RCU read lock). As such, do not
> > disable interrupts throughout rstat flushing, only when holding the
> > percpu lock. This breaks down the O(# cgroups * # cpus) duration with
> > interrupts disabled to a series of O(# cgroups) durations.
> >
> > Furthermore, if a cpu spinning waiting for the global rstat lock, it
> > doesn't need to spin with interrupts disabled anymore.
> >
> > If the caller of rstat flushing needs interrupts to be disabled, it's up
> > to them to decide that, and it should be fine to hold the global rstat
> > lock with interrupts disabled. There is currently a single context that
> > may invoke rstat flushing with interrupts disabled, the
> > mem_cgroup_flush_stats() call in mem_cgroup_usage(), if called from
> > mem_cgroup_threshold().
> >
> > To make it safe to hold the global rstat lock with interrupts enabled,
> > make sure we only flush from in_task() contexts. The side effect of that
> > we read stale stats in interrupt context, but this should be okay, as
> > flushing in interrupt context is dangerous anyway as it is an expensive
> > operation, so reading stale stats is safer.
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>
> Couple of questions:
>
> 1. What exactly is cgroup_rstat_lock protecting? Can we just remove it
> altogether?

I believe it protects the global state variables that we flush into.
For example, for memcg, it protects mem_cgroup->vmstats.

I tried removing the lock and allowing concurrent flushing on
different cpus, by changing mem_cgroup->vmstats to use atomics
instead, but that turned out to be a little expensive. Also,
cgroup_rstat_lock is already contended by different flushers
(mitigated by stats_flush_lock on the memcg side). If we remove it,
concurrent flushers contend on every single percpu lock instead, which
also seems to be expensive.

> 2. Are we really calling rstat flush in irq context?

I think it is possible through the charge/uncharge path:
memcg_check_events()->mem_cgroup_threshold()->mem_cgroup_usage(). I
added the protection against flushing in an interrupt context for
future callers as well, as it may cause a deadlock if we don't disable
interrupts when acquiring cgroup_rstat_lock.

> 3. The mem_cgroup_flush_stats() call in mem_cgroup_usage() is only
> done for root memcg. Why is mem_cgroup_threshold() interested in root
> memcg usage? Why not ignore root memcg in mem_cgroup_threshold() ?

I am not sure, but the code looks like event notifications may be set
up on root memcg, which is why we need to check thresholds.

Even if mem_cgroup_threshold() does not flush memcg stats, the purpose
of this patch is to make sure the rstat flushing code itself is not
disabling interrupts; which it currently does for any unsleepable
context, even if it is interruptible.