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Mon, 03 Apr 2023 15:05:31 -0700 (PDT) MIME-Version: 1.0 References: <20230403220337.443510-1-yosryahmed@google.com> In-Reply-To: <20230403220337.443510-1-yosryahmed@google.com> From: Yosry Ahmed Date: Mon, 3 Apr 2023 15:04:55 -0700 Message-ID: Subject: Re: [PATCH mm-unstable RFC 0/5] cgroup: eliminate atomic rstat To: Alexander Viro , Christian Brauner , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 71D1214001E X-Stat-Signature: em87g6gjpfcid34adia6nkw31pfgnhrx X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1680559533-261582 X-HE-Meta: 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 lX5ZxneP QOfQDcLP/1O/y2CvFDJPeHeimqVb/Z+EvIuf55WYOfvrgKXELwU2ZlB2UmkjBouFlHUYCSdGHDo1GaO8FhdFk2xd34f6hdWedDGjfWcjoDTF2AN35G54dmKV87BSTz2DZTg/z7jkE3q2HgdjkYKOAZA3rseX3l3v34aDAzavxuZEb94LTHqp8uY7KPgxXS6q2lndeBHcxgL0maoyw3gnWvs4RcCYXQwqeIe/r/iaNCwHW85LZFJ+5IsT4VeH7IyZCjYbjKOxlsMUeLvRxkSEZQX3cyuCSD5ezkgEKJowhM0MrpMm4lBJfDi/1KM5Ymv6Aa+6msBH8FylfqIM= 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 Mon, Apr 3, 2023 at 3:03=E2=80=AFPM Yosry Ahmed = wrote: > > A previous patch series ([1] currently in mm-unstable) changed most .. and I naturally forgot to link this: [1] https://lore.kernel.org/linux-mm/20230330191801.1967435-1-yosryahmed@go= ogle.com/ > atomic rstat flushing contexts to become non-atomic. This was done to > avoid an expensive operation that scales with # cgroups and # cpus to > happen with irqs disabled and scheduling not permitted. There were two > remaining atomic flushing contexts after that series. This series tries > to eliminate them as well, eliminating atomic rstat flushing completely. > > The two remaining atomic flushing contexts are: > (a) wb_over_bg_thresh()->mem_cgroup_wb_stats() > (b) mem_cgroup_threshold()->mem_cgroup_usage() > > For (a), flushing needs to be atomic as wb_writeback() calls > wb_over_bg_thresh() with a spinlock held. However, it seems like the > call to wb_over_bg_thresh() doesn't need to be protected by that > spinlock, so this series proposes a refactoring that moves the call > outside the lock criticial section and makes the stats flushing > in mem_cgroup_wb_stats() non-atomic. > > For (b), flushing needs to be atomic as mem_cgroup_threshold() is called > with irqs disabled. We only flush the stats when calculating the root > usage, as it is approximated as the sum of some memcg stats (file, anon, > and optionally swap) instead of the conventional page counter. This > series proposes changing this calculation to use the global stats > instead, eliminating the need for a memcg stat flush. > > After these 2 contexts are eliminated, we no longer need > mem_cgroup_flush_stats_atomic() or cgroup_rstat_flush_atomic(). We can > remove them and simplify the code. > > Yosry Ahmed (5): > writeback: move wb_over_bg_thresh() call outside lock section > memcg: flush stats non-atomically in mem_cgroup_wb_stats() > memcg: calculate root usage from global state > memcg: remove mem_cgroup_flush_stats_atomic() > cgroup: remove cgroup_rstat_flush_atomic() > > fs/fs-writeback.c | 16 +++++++---- > include/linux/cgroup.h | 1 - > include/linux/memcontrol.h | 5 ---- > kernel/cgroup/rstat.c | 26 ++++-------------- > mm/memcontrol.c | 54 ++++++++------------------------------ > 5 files changed, 27 insertions(+), 75 deletions(-) > > -- > 2.40.0.348.gf938b09366-goog >