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Thu, 23 Mar 2023 08:46:45 -0700 (PDT) MIME-Version: 1.0 References: <20230323040037.2389095-1-yosryahmed@google.com> <20230323040037.2389095-2-yosryahmed@google.com> In-Reply-To: From: Shakeel Butt Date: Thu, 23 Mar 2023 08:46:34 -0700 Message-ID: Subject: Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the percpu lock To: Yosry Ahmed Cc: Tejun Heo , Josef Bacik , Jens Axboe , Zefan Li , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Andrew Morton , Vasily Averin , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: B5B2D40018 X-Stat-Signature: 1gpmzgtqdy7shy5dybnbn75ts3pkaifd X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1679586406-988659 X-HE-Meta: U2FsdGVkX1/S3b8SGfF3bUAevKYAvjlgpRl7cBG1BTJVa0eXj8gsIBigVHjP0FPiAa3rpxyU89ek5H2MgnXjHTG8rG0OQz8Q2qnw7S4Ng3C27+OBlLh+Wg7oXcjBjdGkQ/FA9APxJ42bcBTSyX1+Ief/CVfXRYjcboQDY1q1cUo1NAYtVKeCj/WwymYOtZVIwFLzBpX421mFKZGvY+LK5zQJ+tDZ8DGb+0WQ1MT7vuTF4oyRhq4bpYyC2NbOSfVIeRkRgdAz9cH77J/ZCrh52BgTEdQnrkS3mPjY0xbm4JCEN2lskSWJEeBH62lvNOpsoiABW7Ni7qwQ22tKuTDxN0oSYocisRaumuEJJczMw+UlukFDmK7M6qWSXUJyAZsl+bIbKBS1uVcIzew8qt8qL/CC1u4UXKSL1ceNiVMjVrXhBwlDoM2SI8U+U4P0x2uo2RwAgrussgKwQDLN+9W6OQj/HWjVb0LGwqxNB/TTmQ0Fc7QE1CifLdOXqbOn/SqvZMMTKre6zxz5vs1eC3WX7cyfS3EP/vrhedtt5ufisUcrZXx+3X0k4CxtOF5B9PKzPnD2SdoMbq3D+kKWVYGgSWaLynLhI/6lDZImjtMLmZBM+dAKzUebocFK0mRjh02fBV8+k9C3jp6F1K5/QPFG2BCm9kSZ8jixUKMvP3kOl5k0XZ5EpBorUHl+jJ6RCtMMK4Ck6aTVrc67UBCV6CuJKU9GuxS8yVqg7BkTJgfTGX/5cZ3JQ6mNoIAw3OtL9PpdJ7Ufesu7Wv7YdyUCTcm1KvbA+ype5cYAchX59NkP1FquPmhr3awZLb7uadgs5XuQIubQV+7x52i5O8VzpQMAXMxZska4pCkJn13y0RvHn1DBuOgAMWDkrlulkKae7FbM4/Z/n3fMGPdLKRunOJs9OrpIXhc3JG4Q2kARdc9dcdw9hv7bJ8IBy/12W0izdCafVXEq4iNLzIUYG7yyfqh /vf4hLSv SJZTRgeuMgVMy1ppruG/OTlfv9kaFpNW97Ce1MTKaKKkjTzQWn7P5aw3wS11J1KDrCghO8PFY+QxgmigIJdFwKVoNPce7+zdMdyF03hexrRAtlODUyVFYRpJ6igQhyRv8ZyEK/kiqvtbaVqTZHv/ytgZRp333p7o3xhR0Ks8TuNjfSLOMRJF8KQUF6gmNJq4UI3Go5SZE0BKLBDxUs9YwbJlgovwXC29Afqn/MITt0SmTZ7x26RI49HA44PgR/D5C+zOenWP+RYs6log= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Mar 23, 2023 at 8:43=E2=80=AFAM Yosry Ahmed = wrote: > > On Thu, Mar 23, 2023 at 8:40=E2=80=AFAM Shakeel Butt wrote: > > > > On Thu, Mar 23, 2023 at 6:36=E2=80=AFAM Yosry Ahmed 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 di= sable > > > > > interrupts when acquiring cgroup_rstat_lock. > > > > > > > > > > > 3. The mem_cgroup_flush_stats() call in mem_cgroup_usage() is o= nly > > > > > > done for root memcg. Why is mem_cgroup_threshold() interested i= n 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.