[PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned up

[PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned up

[Dennis Zhou]

The timer function, zstd_reclaim_timer_fn(), reschedules itself under
certain conditions. When cleaning up, take the lock and remove all
workspaces. This prevents the timer from rearming itself. Lastly, switch
to del_timer_sync() to ensure that the timer function can't trigger as
we're unloading.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
---
v2:
- cleanup workspaces and then disable the timer

 fs/btrfs/zstd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
index 3e418a3aeb11..6b9e29d050f3 100644
--- a/fs/btrfs/zstd.c
+++ b/fs/btrfs/zstd.c
@@ -195,8 +195,7 @@ static void zstd_cleanup_workspace_manager(void)
 	struct workspace *workspace;
 	int i;
 
-	del_timer(&wsm.timer);
-
+	spin_lock(&wsm.lock);
 	for (i = 0; i < ZSTD_BTRFS_MAX_LEVEL; i++) {
 		while (!list_empty(&wsm.idle_ws[i])) {
 			workspace = container_of(wsm.idle_ws[i].next,
@@ -206,6 +205,9 @@ static void zstd_cleanup_workspace_manager(void)
 			wsm.ops->free_workspace(&workspace->list);
 		}
 	}
+	spin_unlock(&wsm.lock);
+
+	del_timer_sync(&wsm.timer);
 }
 
 /*
-- 
2.17.1

Re: [PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned up

[David Sterba]

On Fri, Feb 22, 2019 at 02:53:48PM -0500, Dennis Zhou wrote:
> The timer function, zstd_reclaim_timer_fn(), reschedules itself under
> certain conditions. When cleaning up, take the lock and remove all
> workspaces. This prevents the timer from rearming itself. Lastly, switch
> to del_timer_sync() to ensure that the timer function can't trigger as
> we're unloading.
> 
> Signed-off-by: Dennis Zhou <dennis@kernel.org>

Reviewed-by: David Sterba <dsterba@suse.com>

> ---
> v2:
> - cleanup workspaces and then disable the timer
> 
>  fs/btrfs/zstd.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
> index 3e418a3aeb11..6b9e29d050f3 100644
> --- a/fs/btrfs/zstd.c
> +++ b/fs/btrfs/zstd.c
> @@ -195,8 +195,7 @@ static void zstd_cleanup_workspace_manager(void)
>  	struct workspace *workspace;
>  	int i;
>  
> -	del_timer(&wsm.timer);
> -
> +	spin_lock(&wsm.lock);
>  	for (i = 0; i < ZSTD_BTRFS_MAX_LEVEL; i++) {
>  		while (!list_empty(&wsm.idle_ws[i])) {
>  			workspace = container_of(wsm.idle_ws[i].next,
> @@ -206,6 +205,9 @@ static void zstd_cleanup_workspace_manager(void)
>  			wsm.ops->free_workspace(&workspace->list);

I've noticed while reading the code, why do you use the indirect call
here? The wsm.ops points to btrfs_zstd_compress so free_workspace is
always zstd_free_workspace.

The compiler is usually smart to replace such things by direct call if
the type has not escaped, but this is not true for btrfs_compress_op so
the indirect function call must be preserved.

Re: [PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned up

[Dennis Zhou]

On Wed, Feb 27, 2019 at 05:44:41PM +0100, David Sterba wrote:
> On Fri, Feb 22, 2019 at 02:53:48PM -0500, Dennis Zhou wrote:
> > The timer function, zstd_reclaim_timer_fn(), reschedules itself under
> > certain conditions. When cleaning up, take the lock and remove all
> > workspaces. This prevents the timer from rearming itself. Lastly, switch
> > to del_timer_sync() to ensure that the timer function can't trigger as
> > we're unloading.
> > 
> > Signed-off-by: Dennis Zhou <dennis@kernel.org>
> 
> Reviewed-by: David Sterba <dsterba@suse.com>
> 

Thanks!

> > ---
> > v2:
> > - cleanup workspaces and then disable the timer
> > 
> >  fs/btrfs/zstd.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
> > index 3e418a3aeb11..6b9e29d050f3 100644
> > --- a/fs/btrfs/zstd.c
> > +++ b/fs/btrfs/zstd.c
> > @@ -195,8 +195,7 @@ static void zstd_cleanup_workspace_manager(void)
> >  	struct workspace *workspace;
> >  	int i;
> >  
> > -	del_timer(&wsm.timer);
> > -
> > +	spin_lock(&wsm.lock);
> >  	for (i = 0; i < ZSTD_BTRFS_MAX_LEVEL; i++) {
> >  		while (!list_empty(&wsm.idle_ws[i])) {
> >  			workspace = container_of(wsm.idle_ws[i].next,
> > @@ -206,6 +205,9 @@ static void zstd_cleanup_workspace_manager(void)
> >  			wsm.ops->free_workspace(&workspace->list);
> 
> I've noticed while reading the code, why do you use the indirect call
> here? The wsm.ops points to btrfs_zstd_compress so free_workspace is
> always zstd_free_workspace.
> 
> The compiler is usually smart to replace such things by direct call if
> the type has not escaped, but this is not true for btrfs_compress_op so
> the indirect function call must be preserved.

I don't have a strong reason to use the indirect call here. It was just
to make it consistent for everyone to use the indirection. This at least
is in the cleanup path, so I don't think performance is that important?
But I don't feel strongly for or against calling zstd_free_workspace()
directly.

Thanks,
Dennis

Re: [PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned up

[David Sterba]

On Wed, Feb 27, 2019 at 01:29:16PM -0500, Dennis Zhou wrote:
> > I've noticed while reading the code, why do you use the indirect call
> > here? The wsm.ops points to btrfs_zstd_compress so free_workspace is
> > always zstd_free_workspace.
> > 
> > The compiler is usually smart to replace such things by direct call if
> > the type has not escaped, but this is not true for btrfs_compress_op so
> > the indirect function call must be preserved.
> 
> I don't have a strong reason to use the indirect call here. It was just
> to make it consistent for everyone to use the indirection. This at least
> is in the cleanup path, so I don't think performance is that important?

It's not just that, the timer uses it too and there are indirect calls
of the alloc_workspace callback. The indirection is not used by lzo nor
zlib code, so I don't see what 'everyone' you mean. In the generic
compression code it makes sense, I see that.

> But I don't feel strongly for or against calling zstd_free_workspace()
> directly.

I feel strongly about not using the indirection when not necessary :)

Re: [PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned up

[Dennis Zhou]

On Wed, Feb 27, 2019 at 07:36:50PM +0100, David Sterba wrote:
> On Wed, Feb 27, 2019 at 01:29:16PM -0500, Dennis Zhou wrote:
> > > I've noticed while reading the code, why do you use the indirect call
> > > here? The wsm.ops points to btrfs_zstd_compress so free_workspace is
> > > always zstd_free_workspace.
> > > 
> > > The compiler is usually smart to replace such things by direct call if
> > > the type has not escaped, but this is not true for btrfs_compress_op so
> > > the indirect function call must be preserved.
> > 
> > I don't have a strong reason to use the indirect call here. It was just
> > to make it consistent for everyone to use the indirection. This at least
> > is in the cleanup path, so I don't think performance is that important?
> 
> It's not just that, the timer uses it too and there are indirect calls
> of the alloc_workspace callback. The indirection is not used by lzo nor
> zlib code, so I don't see what 'everyone' you mean. In the generic
> compression code it makes sense, I see that.
> 
> > But I don't feel strongly for or against calling zstd_free_workspace()
> > directly.
> 
> I feel strongly about not using the indirection when not necessary :)

Great :). I sent you a patch just now to remove the indirection [1].

[1] https://lore.kernel.org/linux-btrfs/20190227212128.38491-1-dennis@kernel.org

Thanks,
Dennis