From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from dkim1.fusionio.com ([66.114.96.53]:42066 "EHLO dkim1.fusionio.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751391Ab3IZN0l (ORCPT ); Thu, 26 Sep 2013 09:26:41 -0400 Received: from mx2.fusionio.com (unknown [10.101.1.160]) by dkim1.fusionio.com (Postfix) with ESMTP id E96417C0686 for ; Thu, 26 Sep 2013 07:26:40 -0600 (MDT) From: Josef Bacik To: CC: , , , , Subject: [PATCH 2/2] Btrfs: stop caching thread if extent_commit_sem is contended Date: Thu, 26 Sep 2013 09:26:36 -0400 Message-ID: <1380201996-17445-2-git-send-email-jbacik@fusionio.com> In-Reply-To: <1380201996-17445-1-git-send-email-jbacik@fusionio.com> References: <1380201996-17445-1-git-send-email-jbacik@fusionio.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-btrfs-owner@vger.kernel.org List-ID: 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 Signed-off-by: Josef Bacik --- 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 cfb3cf7..cc074c34 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