Messages in this thread Patch in this message | | | From | Josef Bacik <> | Subject | [PATCH 2/2] Btrfs: stop caching thread if extent_commit_sem is contended | Date | Mon, 13 Jan 2014 12:18:51 -0500 |
| |
From: Josef Bacik <jbacik@fusionio.com>
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> --- fs/btrfs/extent-tree.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index 1c82bea..3d19dcc 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -442,7 +442,8 @@ next: if (ret) break; - if (need_resched()) { + if (need_resched() || + rwsem_is_contended(&fs_info->extent_commit_sem)) { caching_ctl->progress = last; btrfs_release_path(path); up_read(&fs_info->extent_commit_sem); -- 1.8.3.1
| |