From: Michal Hocko <mhocko@suse.com>
To: NeilBrown <neilb@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Theodore Ts'o <tytso@mit.edu>,
Andreas Dilger <adilger.kernel@dilger.ca>,
"Darrick J. Wong" <djwong@kernel.org>,
Matthew Wilcox <willy@infradead.org>,
Mel Gorman <mgorman@suse.com>,
linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective.
Date: Wed, 15 Sep 2021 13:56:38 +0200 [thread overview]
Message-ID: <YUHfdtth69qKvk8r@dhcp22.suse.cz> (raw)
In-Reply-To: <163157838437.13293.15392955714346973750.stgit@noble.brown>
On Tue 14-09-21 10:13:04, Neil Brown wrote:
> Only 4 subsystems call set_bdi_congested() or clear_bdi_congested():
> block/pktcdvd, fs/ceph fs/fuse fs/nfs
>
> It may make sense to use congestion_wait() or wait_iff_congested()
> within these subsystems, but they have no value outside of these.
>
> Add documentation comments to these functions to discourage further use.
This is an unfortunate state. The MM layer still relies on the API.
While adding a documentation to clarify the current status can stop more
usage I am wondering what is a real alternative. My experience tells me
that a lack of real alternative will lead to new creative ways of doing
things instead.
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
> include/linux/backing-dev.h | 7 +++++++
> mm/backing-dev.c | 9 +++++++++
> 2 files changed, 16 insertions(+)
>
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index ac7f231b8825..cc9513840351 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -153,6 +153,13 @@ static inline int wb_congested(struct bdi_writeback *wb, int cong_bits)
> return wb->congested & cong_bits;
> }
>
> +/* NOTE congestion_wait() and wait_iff_congested() are
> + * largely useless except as documentation.
> + * congestion_wait() will (almost) always wait for the given timeout.
> + * wait_iff_congested() will (almost) never wait, but will call
> + * cond_resched().
> + * Were possible an alternative waiting strategy should be found.
> + */
> long congestion_wait(int sync, long timeout);
> long wait_iff_congested(int sync, long timeout);
>
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 4a9d4e27d0d9..53472ab38796 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -1023,6 +1023,11 @@ EXPORT_SYMBOL(set_bdi_congested);
> * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
> * write congestion. If no backing_devs are congested then just wait for the
> * next write to be completed.
> + *
> + * NOTE: in the current implementation, hardly any backing_devs are ever
> + * marked as congested, and write-completion is rarely reported (see calls
> + * to clear_bdi_congested). So this should not be assumed to ever wake before
> + * the timeout.
> */
> long congestion_wait(int sync, long timeout)
> {
> @@ -1054,6 +1059,10 @@ EXPORT_SYMBOL(congestion_wait);
> * The return value is 0 if the sleep is for the full timeout. Otherwise,
> * it is the number of jiffies that were still remaining when the function
> * returned. return_value == timeout implies the function did not sleep.
> + *
> + * NOTE: in the current implementation, hardly any backing_devs are ever
> + * marked as congested, and write-completion is rarely reported (see calls
> + * to clear_bdi_congested). So this should not be assumed to sleep at all.
> */
> long wait_iff_congested(int sync, long timeout)
> {
>
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-09-15 11:56 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-14 0:13 [PATCH 0/6] congestion_wait() and GFP_NOFAIL NeilBrown
2021-09-14 0:13 ` [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective NeilBrown
2021-09-15 11:56 ` Michal Hocko [this message]
2021-09-16 22:13 ` NeilBrown
2021-09-14 0:13 ` [PATCH 5/6] XFS: remove congestion_wait() loop from kmem_alloc() NeilBrown
2021-09-14 1:31 ` Dave Chinner
2021-09-14 3:27 ` NeilBrown
2021-09-14 6:05 ` Dave Chinner
2021-09-14 0:13 ` [PATCH 3/6] EXT4: Remove ENOMEM/congestion_wait() loops NeilBrown
2021-09-14 16:34 ` Mel Gorman
2021-09-14 21:48 ` NeilBrown
2021-09-15 12:06 ` Michal Hocko
2021-09-15 22:35 ` NeilBrown
2021-09-16 0:37 ` Dave Chinner
2021-09-16 6:52 ` Michal Hocko
2021-09-14 23:55 ` Dave Chinner
2021-09-15 8:59 ` Mel Gorman
2021-09-15 12:20 ` Michal Hocko
2021-09-15 14:35 ` Mel Gorman
2021-09-15 22:38 ` Dave Chinner
2021-09-16 9:00 ` Mel Gorman
2021-09-15 0:28 ` Theodore Ts'o
2021-09-15 5:25 ` NeilBrown
2021-09-15 17:02 ` Theodore Ts'o
2021-09-14 0:13 ` [PATCH 1/6] MM: improve documentation for __GFP_NOFAIL NeilBrown
2021-09-15 11:51 ` Michal Hocko
2021-09-14 0:13 ` [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc_pages() NeilBrown
2021-09-14 2:08 ` Dave Chinner
2021-09-14 2:35 ` NeilBrown
2021-09-14 5:33 ` Dave Chinner
2021-09-14 16:45 ` Mel Gorman
2021-09-14 21:13 ` NeilBrown
2021-09-14 0:13 ` [PATCH 4/6] EXT4: remove congestion_wait from ext4_bio_write_page, and simplify NeilBrown
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