[PATCH v2] fsverity: Remove WQ_UNBOUND from fsverity read workqueue

[PATCH v2] fsverity: Remove WQ_UNBOUND from fsverity read workqueue

[Nathan Huckleberry]

WQ_UNBOUND causes significant scheduler latency on ARM64/Android.  This
is problematic for latency sensitive workloads, like I/O
post-processing.

Removing WQ_UNBOUND gives a 96% reduction in fsverity workqueue related
scheduler latency and improves app cold startup times by ~30ms.
WQ_UNBOUND was also removed from the dm-verity workqueue for the same
reason [1].

This code was tested by running Android app startup benchmarks and
measuring how long the fsverity workqueue spent in the runnable state.

Before
Total workqueue scheduler latency: 553800us
After
Total workqueue scheduler latency: 18962us

[1]: https://lore.kernel.org/all/20230202012348.885402-1-nhuck@google.com/

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
---
Changelog:
v1 -> v2:
- Added comment about WQ_UNBOUND
- Added info about related dm-verity patches in commit message

 fs/verity/verify.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/verity/verify.c b/fs/verity/verify.c
index f50e3b5b52c9..782b8b4a24c1 100644
--- a/fs/verity/verify.c
+++ b/fs/verity/verify.c
@@ -387,15 +387,15 @@ EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);
 int __init fsverity_init_workqueue(void)
 {
 	/*
-	 * Use an unbound workqueue to allow bios to be verified in parallel
-	 * even when they happen to complete on the same CPU.  This sacrifices
-	 * locality, but it's worthwhile since hashing is CPU-intensive.
-	 *
 	 * Also use a high-priority workqueue to prioritize verification work,
 	 * which blocks reads from completing, over regular application tasks.
+	 *
+	 * This workqueue is not marked as unbound for performance reasons.
+	 * Using an unbound workqueue for crypto operations causes excessive
+	 * scheduler latency on ARM64.
 	 */
 	fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
-						  WQ_UNBOUND | WQ_HIGHPRI,
+						  WQ_HIGHPRI,
 						  num_online_cpus());
 	if (!fsverity_read_workqueue)
 		return -ENOMEM;
-- 
2.40.0.rc1.284.g88254d51c5-goog

Re: [PATCH v2] fsverity: Remove WQ_UNBOUND from fsverity read workqueue

[Eric Biggers]

On Fri, Mar 10, 2023 at 11:33:25AM -0800, Nathan Huckleberry wrote:
> WQ_UNBOUND causes significant scheduler latency on ARM64/Android.  This
> is problematic for latency sensitive workloads, like I/O
> post-processing.
> 
> Removing WQ_UNBOUND gives a 96% reduction in fsverity workqueue related
> scheduler latency and improves app cold startup times by ~30ms.
> WQ_UNBOUND was also removed from the dm-verity workqueue for the same
> reason [1].
> 
> This code was tested by running Android app startup benchmarks and
> measuring how long the fsverity workqueue spent in the runnable state.
> 
> Before
> Total workqueue scheduler latency: 553800us
> After
> Total workqueue scheduler latency: 18962us
> 
> [1]: https://lore.kernel.org/all/20230202012348.885402-1-nhuck@google.com/
> 
> Signed-off-by: Nathan Huckleberry <nhuck@google.com>
> ---
> Changelog:
> v1 -> v2:
> - Added comment about WQ_UNBOUND
> - Added info about related dm-verity patches in commit message
> 
>  fs/verity/verify.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/verity/verify.c b/fs/verity/verify.c
> index f50e3b5b52c9..782b8b4a24c1 100644
> --- a/fs/verity/verify.c
> +++ b/fs/verity/verify.c
> @@ -387,15 +387,15 @@ EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);
>  int __init fsverity_init_workqueue(void)
>  {
>  	/*
> -	 * Use an unbound workqueue to allow bios to be verified in parallel
> -	 * even when they happen to complete on the same CPU.  This sacrifices
> -	 * locality, but it's worthwhile since hashing is CPU-intensive.
> -	 *
>  	 * Also use a high-priority workqueue to prioritize verification work,
>  	 * which blocks reads from completing, over regular application tasks.
> +	 *
> +	 * This workqueue is not marked as unbound for performance reasons.
> +	 * Using an unbound workqueue for crypto operations causes excessive
> +	 * scheduler latency on ARM64.
>  	 */
>  	fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
> -						  WQ_UNBOUND | WQ_HIGHPRI,
> +						  WQ_HIGHPRI,
>  						  num_online_cpus());

Applied to https://git.kernel.org/pub/scm/fs/fsverity/linux.git/log/?h=for-next

I adjusted the comment slightly so that the first paragraph doesn't start with
"Also".

- Eric

Re: [PATCH v2] fsverity: Remove WQ_UNBOUND from fsverity read workqueue

[Nathan Huckleberry]

Thanks Eric.

On Mon, Mar 13, 2023 at 3:59 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Fri, Mar 10, 2023 at 11:33:25AM -0800, Nathan Huckleberry wrote:
> > WQ_UNBOUND causes significant scheduler latency on ARM64/Android.  This
> > is problematic for latency sensitive workloads, like I/O
> > post-processing.
> >
> > Removing WQ_UNBOUND gives a 96% reduction in fsverity workqueue related
> > scheduler latency and improves app cold startup times by ~30ms.
> > WQ_UNBOUND was also removed from the dm-verity workqueue for the same
> > reason [1].
> >
> > This code was tested by running Android app startup benchmarks and
> > measuring how long the fsverity workqueue spent in the runnable state.
> >
> > Before
> > Total workqueue scheduler latency: 553800us
> > After
> > Total workqueue scheduler latency: 18962us
> >
> > [1]: https://lore.kernel.org/all/20230202012348.885402-1-nhuck@google.com/
> >
> > Signed-off-by: Nathan Huckleberry <nhuck@google.com>
> > ---
> > Changelog:
> > v1 -> v2:
> > - Added comment about WQ_UNBOUND
> > - Added info about related dm-verity patches in commit message
> >
> >  fs/verity/verify.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/verity/verify.c b/fs/verity/verify.c
> > index f50e3b5b52c9..782b8b4a24c1 100644
> > --- a/fs/verity/verify.c
> > +++ b/fs/verity/verify.c
> > @@ -387,15 +387,15 @@ EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);
> >  int __init fsverity_init_workqueue(void)
> >  {
> >       /*
> > -      * Use an unbound workqueue to allow bios to be verified in parallel
> > -      * even when they happen to complete on the same CPU.  This sacrifices
> > -      * locality, but it's worthwhile since hashing is CPU-intensive.
> > -      *
> >        * Also use a high-priority workqueue to prioritize verification work,
> >        * which blocks reads from completing, over regular application tasks.
> > +      *
> > +      * This workqueue is not marked as unbound for performance reasons.
> > +      * Using an unbound workqueue for crypto operations causes excessive
> > +      * scheduler latency on ARM64.
> >        */
> >       fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
> > -                                               WQ_UNBOUND | WQ_HIGHPRI,
> > +                                               WQ_HIGHPRI,
> >                                                 num_online_cpus());
>
> Applied to https://git.kernel.org/pub/scm/fs/fsverity/linux.git/log/?h=for-next
>
> I adjusted the comment slightly so that the first paragraph doesn't start with
> "Also".
>
> - Eric

Re: [PATCH v2] fsverity: Remove WQ_UNBOUND from fsverity read workqueue

[Eric Biggers]

On Mon, Mar 13, 2023 at 04:07:59PM -0700, Nathan Huckleberry wrote:
> >
> > Applied to https://git.kernel.org/pub/scm/fs/fsverity/linux.git/log/?h=for-next
> >
> > I adjusted the comment slightly so that the first paragraph doesn't start with
> > "Also".
> >

As discussed, this seems to be a significant enough improvement to call this a
"fix".  So I've also added:

    Fixes: 8a1d0f9cacc9 ("fs-verity: add data verification hooks for ->readpages()")
    Cc: stable@vger.kernel.org

and moved the commit to:

    https://git.kernel.org/pub/scm/fs/fsverity/linux.git/log/?h=for-current

- Eric