From: Johannes Weiner <hannes@cmpxchg.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Shakeel Butt <shakeelb@google.com>, Tejun Heo <tj@kernel.org>,
Josef Bacik <josef@toxicpanda.com>, Jens Axboe <axboe@kernel.dk>,
Zefan Li <lizefan.x@bytedance.com>,
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
Subject: Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the percpu lock
Date: Thu, 23 Mar 2023 13:33:43 -0400 [thread overview]
Message-ID: <20230323173343.GF739026@cmpxchg.org> (raw)
In-Reply-To: <CAJD7tkbGCgk9VkGdec0=AdHErds4XQs1LzJMhqVryXdjY5PVAg@mail.gmail.com>
On Thu, Mar 23, 2023 at 09:17:33AM -0700, Yosry Ahmed wrote:
> On Thu, Mar 23, 2023 at 9:10 AM Shakeel Butt <shakeelb@google.com> wrote:
> >
> > On Thu, Mar 23, 2023 at 8:46 AM Shakeel Butt <shakeelb@google.com> wrote:
> > >
> > > On Thu, Mar 23, 2023 at 8:43 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> > > >
> > > > On Thu, Mar 23, 2023 at 8:40 AM Shakeel Butt <shakeelb@google.com> wrote:
> > > > >
> > > > > On Thu, Mar 23, 2023 at 6:36 AM Yosry Ahmed <yosryahmed@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_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.
> > > > > >
> > > > > > Right, but I think this would be orthogonal to this patch series.
> > > > > >
> > > > >
> > > > > I don't think we can make cgroup_rstat_lock a non-irq-disabling lock
> > > > > without either breaking a link between mem_cgroup_threshold and
> > > > > cgroup_rstat_lock or make mem_cgroup_threshold work without disabling
> > > > > 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 we only
> > > > acquire cgroup_rstat_lock from non-irq context it should be fine 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_lock
> > > 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 recommended
> > to actually remove all such users instead of silently
> > ignoring/bypassing the functionality.
+1
It shouldn't silently skip the requested operation, rather it
shouldn't be requested from an incompatible context.
> > 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.
Can you do
/* Note: stale usage data when called from irq context!! */
if (in_task())
mem_cgroup_flush_stats()
directly in the callsite? Maybe even include the whole callchain in
the comment that's currently broken and needs fixing/removing.
next prev parent reply other threads:[~2023-03-23 17:35 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-23 4:00 [RFC PATCH 0/7] Make rstat flushing IRQ and sleep friendly Yosry Ahmed
2023-03-23 4:00 ` [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the percpu lock Yosry Ahmed
2023-03-23 4:29 ` Shakeel Butt
2023-03-23 5:15 ` Yosry Ahmed
2023-03-23 6:33 ` Shakeel Butt
2023-03-23 13:35 ` Yosry Ahmed
2023-03-23 15:40 ` Shakeel Butt
2023-03-23 15:42 ` Yosry Ahmed
2023-03-23 15:46 ` Shakeel Butt
2023-03-23 16:09 ` Shakeel Butt
2023-03-23 16:17 ` Yosry Ahmed
2023-03-23 16:29 ` Shakeel Butt
2023-03-23 16:36 ` Yosry Ahmed
2023-03-23 16:45 ` Shakeel Butt
2023-03-23 16:51 ` Yosry Ahmed
2023-03-23 19:09 ` Shakeel Butt
2023-03-23 17:33 ` Johannes Weiner [this message]
2023-03-23 18:09 ` Yosry Ahmed
2023-03-23 18:19 ` Johannes Weiner
2023-03-24 1:39 ` Tejun Heo
2023-03-24 7:22 ` Yosry Ahmed
2023-03-24 14:12 ` Waiman Long
2023-03-24 22:50 ` Yosry Ahmed
2023-03-25 1:54 ` Tejun Heo
2023-03-25 2:17 ` Yosry Ahmed
2023-03-25 4:30 ` Shakeel Butt
2023-03-25 4:37 ` Yosry Ahmed
2023-03-25 4:46 ` Shakeel Butt
2023-03-27 23:23 ` Yosry Ahmed
2023-03-29 18:53 ` Tejun Heo
2023-03-29 19:22 ` Hugh Dickins
2023-03-29 20:00 ` Tejun Heo
2023-03-29 20:38 ` Hugh Dickins
2023-03-30 4:26 ` Yosry Ahmed
2023-03-31 1:51 ` Tejun Heo
2023-03-23 4:00 ` [RFC PATCH 2/7] memcg: do not disable interrupts when holding stats_flush_lock Yosry Ahmed
2023-03-23 4:32 ` Shakeel Butt
2023-03-23 5:16 ` Yosry Ahmed
2023-03-23 4:00 ` [RFC PATCH 3/7] cgroup: rstat: remove cgroup_rstat_flush_irqsafe() Yosry Ahmed
2023-03-23 15:43 ` Johannes Weiner
2023-03-23 15:45 ` Yosry Ahmed
2023-03-23 4:00 ` [RFC PATCH 4/7] memcg: sleep during flushing stats in safe contexts Yosry Ahmed
2023-03-23 15:56 ` Johannes Weiner
2023-03-23 16:01 ` Yosry Ahmed
2023-03-23 17:27 ` Johannes Weiner
2023-03-23 18:07 ` Yosry Ahmed
2023-03-23 19:35 ` Shakeel Butt
2023-03-23 4:00 ` [RFC PATCH 5/7] vmscan: memcg: sleep when flushing stats during reclaim Yosry Ahmed
2023-03-23 4:00 ` [RFC PATCH 6/7] workingset: memcg: sleep when flushing stats in workingset_refault() Yosry Ahmed
2023-03-23 15:50 ` Johannes Weiner
2023-03-23 16:02 ` Yosry Ahmed
2023-03-23 16:00 ` Johannes Weiner
2023-03-23 16:02 ` Yosry Ahmed
2023-03-23 4:00 ` [RFC PATCH 7/7] memcg: do not modify rstat tree for zero updates Yosry Ahmed
2023-03-23 4:10 ` [RFC PATCH 0/7] Make rstat flushing IRQ and sleep friendly Shakeel Butt
2023-03-23 5:07 ` Yosry Ahmed
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