From: NeilBrown <neilb@suse.de>
To: 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>
Cc: 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: [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective.
Date: Tue, 14 Sep 2021 10:13:04 +1000 [thread overview]
Message-ID: <163157838437.13293.15392955714346973750.stgit@noble.brown> (raw)
In-Reply-To: <163157808321.13293.486682642188075090.stgit@noble.brown>
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.
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)
{
next prev parent reply other threads:[~2021-09-14 0:28 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 ` NeilBrown [this message]
2021-09-15 11:56 ` [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective Michal Hocko
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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