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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 23 Mar 2023 09:45:20 -0700
Message-ID: <CALvZod5USCtNtnPuYRbRv_psBCNytQWWQ592TFsJLfrLpyLJmw@mail.gmail.com>
Subject: Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the
 percpu lock
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Josef Bacik <josef@toxicpanda.com>,
        Jens Axboe <axboe@kernel.dk>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vasily Averin <vasily.averin@linux.dev>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        bpf@vger.kernel.org
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On Thu, Mar 23, 2023 at 9:37=E2=80=AFAM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> On Thu, Mar 23, 2023 at 9:29=E2=80=AFAM Shakeel Butt <shakeelb@google.com=
> wrote:
> >
> > On Thu, Mar 23, 2023 at 9:18=E2=80=AFAM Yosry Ahmed <yosryahmed@google.=
com> wrote:
> > >
> > > On Thu, Mar 23, 2023 at 9:10=E2=80=AFAM Shakeel Butt <shakeelb@google=
.com> wrote:
> > > >
> > > > On Thu, Mar 23, 2023 at 8:46=E2=80=AFAM Shakeel Butt <shakeelb@goog=
le.com> wrote:
> > > > >
> > > > > On Thu, Mar 23, 2023 at 8:43=E2=80=AFAM Yosry Ahmed <yosryahmed@g=
oogle.com> wrote:
> > > > > >
> > > > > > On Thu, Mar 23, 2023 at 8:40=E2=80=AFAM Shakeel Butt <shakeelb@=
google.com> wrote:
> > > > > > >
> > > > > > > On Thu, Mar 23, 2023 at 6:36=E2=80=AFAM Yosry Ahmed <yosryahm=
ed@google.com> wrote:
> > > > > > > >
> > > > > > > [...]
> > > > > > > > > >
> > > > > > > > > > > 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_cgrou=
p_usage(). I
> > > > > > > > > > added the protection against flushing in an interrupt c=
ontext for
> > > > > > > > > > future callers as well, as it may cause a deadlock if w=
e don't disable
> > > > > > > > > > interrupts when acquiring cgroup_rstat_lock.
> > > > > > > > > >
> > > > > > > > > > > 3. The mem_cgroup_flush_stats() call in mem_cgroup_us=
age() is only
> > > > > > > > > > > done for root memcg. Why is mem_cgroup_threshold() in=
terested in root
> > > > > > > > > > > memcg usage? Why not ignore root memcg in mem_cgroup_=
threshold() ?
> > > > > > > > > >
> > > > > > > > > > I am not sure, but the code looks like event notificati=
ons may be set
> > > > > > > > > > up on root memcg, which is why we need to check thresho=
lds.
> > > > > > > > >
> > > > > > > > > This is something we should deprecate as root memcg's usa=
ge is ill defined.
> > > > > > > >
> > > > > > > > Right, but I think this would be orthogonal to this patch s=
eries.
> > > > > > > >
> > > > > > >
> > > > > > > I don't think we can make cgroup_rstat_lock a non-irq-disabli=
ng lock
> > > > > > > without either breaking a link between mem_cgroup_threshold a=
nd
> > > > > > > cgroup_rstat_lock or make mem_cgroup_threshold work without d=
isabling
> > > > > > > irqs.
> > > > > > >
> > > > > > > So, this patch can not be applied before either of those two =
tasks are
> > > > > > > done (and we may find more such scenarios).
> > > > > >
> > > > > >
> > > > > > Could you elaborate why?
> > > > > >
> > > > > > My understanding is that with an in_task() check to make sure w=
e only
> > > > > > acquire cgroup_rstat_lock from non-irq context it should be fin=
e to
> > > > > > acquire cgroup_rstat_lock without disabling interrupts.
> > > > >
> > > > > From mem_cgroup_threshold() code path, cgroup_rstat_lock will be =
taken
> > > > > with irq disabled while other code paths will take cgroup_rstat_l=
ock
> > > > > with irq enabled. This is a potential deadlock hazard unless
> > > > > cgroup_rstat_lock is always taken with irq disabled.
> > > >
> > > > Oh you are making sure it is not taken in the irq context through
> > > > should_skip_flush(). Hmm seems like a hack. Normally it is recommen=
ded
> > > > to actually remove all such users instead of silently
> > > > ignoring/bypassing the functionality.
> > >
> > > It is a workaround, we simply accept to read stale stats in irq
> > > context instead of the expensive flush operation.
> > >
> > > >
> > > > So, how about removing mem_cgroup_flush_stats() from
> > > > mem_cgroup_usage(). It will break the known chain which is taking
> > > > cgroup_rstat_lock with irq disabled and you can add
> > > > WARN_ON_ONCE(!in_task()).
> > >
> > > This changes the behavior in a more obvious way because:
> > > 1. The memcg_check_events()->mem_cgroup_threshold()->mem_cgroup_usage=
()
> > > path is also exercised in a lot of paths outside irq context, this
> > > will change the behavior for any event thresholds on the root memcg.
> > > With proposed skipped flushing in irq context we only change the
> > > behavior in a small subset of cases.
> > >
> > > I think we can skip flushing in irq context for now, and separately
> > > deprecate threshold events for the root memcg. When that is done we
> > > can come back and remove should_skip_flush() and add a VM_BUG_ON or
> > > WARN_ON_ONCE instead. WDYT?
> > >
> > > 2. mem_cgroup_usage() is also used when reading usage from userspace.
> > > This should be an easy workaround though.
> >
> > This is a cgroup v1 behavior and to me it is totally reasonable to get
> > the 2 second stale root's usage. Even if you want to skip flushing in
> > irq, do that in the memcg code and keep VM_BUG_ON/WARN_ON_ONCE in the
> > rstat core code. This way we will know if other subsystems are doing
> > the same or not.
>
> We can do that. Basically in mem_cgroup_usage() have:
>
> /* Some useful comment */
> if (in_task())
>     mem_cgroup_flush_stats();
>
> and in cgroup_rstat_flush() have:
> WARN_ON_ONCE(!in_task());
>
> I am assuming VM_BUG_ON is not used outside mm code.
>
> The only thing that worries me is that if there is another unlikely
> path somewhere that flushes stats in irq context we may run into a
> deadlock. I am a little bit nervous about not skipping flushing if
> !in_task() in cgroup_rstat_flush().

I think it is a good thing. We will find such scenarios and fix those
instead of hiding them forever or keeping the door open for new such
scenarios.