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SubjectRe: [PATCH 2/2] Btrfs: stop caching thread if extetn_commit_sem is contended

* 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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