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

Shakeel Butt
2023-03-23T06:34:12Z

On Wed, Mar 22, 2023 at 10:15 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
[...]
> > 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.

We should add a comment on what it is protecting. I think block rstat
are fine but memcg and bpf would need this.

>
> > 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.

This is something we should deprecate as root memcg's usage is ill defined.

>
> 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.

Basically I am saying we should aim for VM_BUG_ON(!in_task()) in the
flush function rather than adding should_skip_flush() which does not
stop potential new irq flushers.