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authorJosef Bacik <jbacik@fusionio.com>2013-09-19 10:02:11 -0400
committerChris Mason <clm@fb.com>2014-01-28 13:20:10 -0800
commitc9ea7b24ce5863d65efb1134319cede160674d41 (patch)
tree5170ec6305d2e9770cf92a456d2bd78ab18751b2 /scripts/Makefile.modpost
parent4a444b1f06d259ce938a47048840260f71a91c84 (diff)
Btrfs: stop caching thread if extent_commit_sem is contended
We can starve out the transaction commit with a bunch of caching threads all running at the same time. This is because we will only drop the extent_commit_sem if we need_resched(), which isn't likely to happen since we will be reading a lot from the disk so have already schedule()'ed plenty. Alex observed that he could starve out a transaction commit for up to a minute with 32 caching threads all running at once. This will allow us to drop the extent_commit_sem to allow the transaction commit to swap the commit_root out and then all the cachers will start back up. Here is an explanation provided by Igno So, just to fill in what happens in this loop: mutex_unlock(&caching_ctl->mutex); cond_resched(); goto again; where 'again:' takes caching_ctl->mutex and fs_info->extent_commit_sem again: again: mutex_lock(&caching_ctl->mutex); /* need to make sure the commit_root doesn't disappear */ down_read(&fs_info->extent_commit_sem); So, if I'm reading the code correct, there can be a fair amount of concurrency here: there may be multiple 'caching kthreads' per filesystem active, while there's one fs_info->extent_commit_sem per filesystem AFAICS. So, what happens if there are a lot of CPUs all busy holding the ->extent_commit_sem rwsem read-locked and a writer arrives? They'd all rush to try to release the fs_info->extent_commit_sem, and they'd block in the down_read() because there's a writer waiting. So there's a guarantee of forward progress. This should answer akpm's concern I think. Thanks, Acked-by: Ingo Molnar <mingo@kernel.org> Signed-off-by: Josef Bacik <jbacik@fusionio.com> Signed-off-by: Chris Mason <clm@fb.com>
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