Date: Fri, 20 Sep 2013 07:12:47 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Josef Bacik <jbacik@fusionio.com>
Cc: linux-btrfs@vger.kernel.org, walken@google.com,
        linux-kernel@vger.kernel.org, mingo@elte.hu, peter@hurleysoftware.com,
        akpm@linux-foundation.org
Subject: Re: [PATCH 2/2] Btrfs: stop caching thread if extetn_commit_sem is
 contended
Message-ID: <20130920051247.GC1486@gmail.com>
References: <1379605688-987-1-git-send-email-jbacik@fusionio.com>
 <1379605688-987-2-git-send-email-jbacik@fusionio.com>
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* Josef Bacik <jbacik@fusionio.com> wrote:

> 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.  Thanks,
> 
> 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 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);

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.

If this analysis is correct then:

  Acked-by: Ingo Molnar <mingo@kernel.org>

Thanks,

	Ingo
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