Btrfs devel - Sep 2013 - [PATCH 1/2] rwsem: add rwsem_is_contended V3

Btrfs needs a simple way to know if it needs to let go of it''s read
lock on a
rwsem.  Introduce rwsem_is_contended to check to see if there are any waiters on
this rwsem currently.  This is just a hueristic, it is meant to be light and not
100% accurate and called by somebody already holding on to the rwsem in either
read or write.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
---
V2->V3: fixed the comment and simplified the function as per Peter''s
suggestions.

V1->V2: took everybodys suggestions and simplified it to just one function in
rwsem.h so it works for both the spinlock case and non-spinlock case.

 include/linux/rwsem.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 0616ffe..03f3b05 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -75,6 +75,17 @@ do {								\
 } while (0)
 
 /*
+ * This is the same regardless of which rwsem implementation that is being
used.
+ * It is just a heuristic meant to be called by somebody alreadying holding the
+ * rwsem to see if somebody from an incompatible type is wanting access to the
+ * lock.
+ */
+static inline int rwsem_is_contended(struct rw_semaphore *sem)
+{
+	return !list_empty(&sem->wait_list);
+}
+
+/*
  * lock for reading
  */
 extern void down_read(struct rw_semaphore *sem);
-- 
1.8.3.1

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 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

* Josef Bacik <jbacik@fusionio.com> wrote:
> Btrfs needs a simple way to know if it needs to let go of it''s
read lock on a
> rwsem.  Introduce rwsem_is_contended to check to see if there are any
waiters on
> this rwsem currently.  This is just a hueristic, it is meant to be light
and not
> 100% accurate and called by somebody already holding on to the rwsem in
either
> read or write.  Thanks,
> 
> Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Acked-by: Ingo Molnar <mingo@kernel.org>

Thanks,

	Ingo

Thanks for addressing this issue, Josef!

On Thu, Sep 26, 2013 at 4:26 PM, 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. 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 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
>
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