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Thu, 20 Apr 2023 11:53:32 -0700 (PDT) MIME-Version: 1.0 References: <20230403220337.443510-1-yosryahmed@google.com> <20230403220337.443510-2-yosryahmed@google.com> In-Reply-To: <20230403220337.443510-2-yosryahmed@google.com> From: Shakeel Butt Date: Thu, 20 Apr 2023 11:53:21 -0700 Message-ID: Subject: Re: [PATCH mm-unstable RFC 1/5] writeback: move wb_over_bg_thresh() call outside lock section To: Yosry Ahmed , Jan Kara , Jens Axboe Cc: Alexander Viro , Christian Brauner , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Andrew Morton , 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-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 0DE9F4000F X-Stat-Signature: 1qgwrctw1gt8kgp59gzyi3pbc1hxa9zw X-HE-Tag: 1682016812-379875 X-HE-Meta: U2FsdGVkX189I7FZ6/ymifGBhsBx7a9YP1NhS6x1ylaxMAzts+0JIjRftBf8ts8eHMWxV+2RXUCspGOdvt9riN9nMmcY19vAcAOWv+CDbmDZ6ZlLkT2VJ8+Xa/9d7y69muzWgh77Nze1V4WU1EK0lKrpdNEI0pFOc/IwFiYbptAgVhHMIN5XaYZPp97f0MDzNbBMLROcwBBE8N0FvXYStWxrIakeDkydchTZC0uqNi8cHDUIoqoz35At+HW25GtK+lOlAblv8NRy2EJds9SRacb/tYMJR2y7l0BWIQR7ZAoK1c0MQiUATJwZ8wUMDDoKl1wNIsSQl5UiEGwA9rGF36KGOOOZhWk0q1h0XDRn4NIl5C7bDZ5U++o9eNtgLm0sB0kik8h3F3IYh44MsvW6t63aubWfmOfSx1ToNc14YjfPpO0jkbWE4hHuH3RZ/AgbSG01fqrsdXtIcL1yr7uy5eDWWaRhKZ9rX1xvjrJZT88l1ggl1MBzeeLBzTCr+gSHSEf0ZJ0VwxWMp8TQkClMcj/rooNCUOJ5pfc99p5EwEx5RZKiyh5onw7RIjSA9miS+PR8CWsRkAYDJQP412IaAAMxYbD1hzY21CYi0SvkEdGcTeu4FmXeYdRhBRTZIoi4nkeO8aJ4Rn5E+nEN5ct3GWeKaa/mILOQpfEnhsrew+Ec2Kz5O+5KP5MiLXmu1h+beIx4Vwv/1asFBfxHY4A6kAaZO9OK3ZtpqMMlsE/HWTLGjodXN6MzbemvL26IEg3IcPiQZ53DUuokpgBQVqZmTaMreKb1glKXhf+LtouBBlvM59ZoUpApaeR/fWygs7RgDA0AUJ7v5T6o6EJe6N5J2Hpgz0cSwpKXNZ6c5zza6vl3nJgbyc99TAigz0UmMQGbdtJEeAQ9lzZ4gtpELSf2g1xFqrUvOsO+b6/ZbgFzM/Nv+OXe8bExxlXYVUQ+Qry8G9J7cEsUMjwh58WkUY5 8NmCh2iL YfxLaC9NKoWiRTMSfBdT/AUADJvmPMMMqtmuM71dnJ7aXraANH0HsyDMz0onYl48Cf1wtjEc+1rWIihq8t4LcrREgjQrpUGuIGEHkq8Y9a6mYZXS+Bcje82ccy9yPhcgSefrAd0cODPRRKM3tKq/FC6xhuX3t7ZbHClhSrv6YuFQrf7tQxHl8Jy4/Fn9+dPkV7KPFazV4UAM91tlEevee/LcrNfDGBGlZJomjcZqr5dzJbAKWyt+oivuEwfW6moMaZLI08w1S84qVxO1iIs5ZCFsaT0LoNmyB7dinuFBZNr++mlI= 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: +Jens & Jan The patch looks good but it would be nice to pass this patch through the eyes of experts of this area. On Mon, Apr 3, 2023 at 3:03=E2=80=AFPM Yosry Ahmed = wrote: > > wb_over_bg_thresh() calls mem_cgroup_wb_stats() which invokes an rstat > flush, which can be expensive on large systems. Currently, > wb_writeback() calls wb_over_bg_thresh() within a lock section, so we > have to make the rstat flush atomically. On systems with a lot of > cpus/cgroups, this can cause us to disable irqs for a long time, > potentially causing problems. > > Move the call to wb_over_bg_thresh() outside the lock section in > preparation to make the rstat flush in mem_cgroup_wb_stats() non-atomic. > The list_empty(&wb->work_list) should be okay outside the lock section > of wb->list_lock as it is protected by a separate lock (wb->work_lock), > and wb_over_bg_thresh() doesn't seem like it is modifying any of the b_* > lists the wb->list_lock is protecting. Also, the loop seems to be > already releasing and reacquring the lock, so this refactoring looks > safe. > > Signed-off-by: Yosry Ahmed > --- > fs/fs-writeback.c | 16 +++++++++++----- > 1 file changed, 11 insertions(+), 5 deletions(-) > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > index 195dc23e0d831..012357bc8daa3 100644 > --- a/fs/fs-writeback.c > +++ b/fs/fs-writeback.c > @@ -2021,7 +2021,6 @@ static long wb_writeback(struct bdi_writeback *wb, > struct blk_plug plug; > > blk_start_plug(&plug); > - spin_lock(&wb->list_lock); > for (;;) { > /* > * Stop writeback when nr_pages has been consumed > @@ -2046,6 +2045,9 @@ static long wb_writeback(struct bdi_writeback *wb, > if (work->for_background && !wb_over_bg_thresh(wb)) > break; > > + > + spin_lock(&wb->list_lock); > + > /* > * Kupdate and background works are special and we want t= o > * include all inodes that need writing. Livelock avoidan= ce is > @@ -2075,13 +2077,19 @@ static long wb_writeback(struct bdi_writeback *wb= , > * mean the overall work is done. So we keep looping as l= ong > * as made some progress on cleaning pages or inodes. > */ > - if (progress) > + if (progress) { > + spin_unlock(&wb->list_lock); > continue; > + } > + > /* > * No more inodes for IO, bail > */ > - if (list_empty(&wb->b_more_io)) > + if (list_empty(&wb->b_more_io)) { > + spin_unlock(&wb->list_lock); > break; > + } > + > /* > * Nothing written. Wait for some inode to > * become available for writeback. Otherwise > @@ -2093,9 +2101,7 @@ static long wb_writeback(struct bdi_writeback *wb, > spin_unlock(&wb->list_lock); > /* This function drops i_lock... */ > inode_sleep_on_writeback(inode); > - spin_lock(&wb->list_lock); > } > - spin_unlock(&wb->list_lock); > blk_finish_plug(&plug); > > return nr_pages - work->nr_pages; > -- > 2.40.0.348.gf938b09366-goog >