Re: [dm-devel] [PATCH v2 2/6] dm bufio: Add DM_BUFIO_CLIENT_NO_SLEEP flag

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Jul 27 2022 at 11:25P -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> Hi
> 
> I'd like to ask - why not use dm_bufio_trylock from an interrupt context?
> 
> I would just add a new function "dm_bufio_get_trylock" that is equivalent 
> to "dm_bufio_get", except that it uses dm_bufio_trylock - and if it fails, 
> fallback to process context.
> 
> I think using dm_bufio_trylock would be less hacky than introducing a 
> new dm_bufio flag that changes mutex to spinlock.

OK, I can drop the bufio hacks (patches 1 and 2) and replace with a
dm_bufio_get_trylock and see if that resolves the cryptsetup testing
issues Milan is reporting.  But on the flip side I feel like trylock
is far more prone to fail for at least one of a series of cached
buffers needed (via _get). And so it'll punt to workqueue more often
and _not_ provide the desired performance improvement.  BUT.. we shall
see.

All said, I'm now dropping this patchset from the upcoming 5.20 merge.
This all clearly needs more development time.

Huck: I'll run with Mikulas's suggestion and try to get the cryptsetup
tests passing. But I'll leave performance testing to you.

Thanks,
Mike



> On Tue, 26 Jul 2022, Mike Snitzer wrote:
> 
> > From: Nathan Huckleberry <nhuck@google.com>
> > 
> > Add an optional flag that ensures dm_bufio_client does not sleep
> > (primary focus is to service dm_bufio_get without sleeping). This
> > allows the dm-bufio cache to be queried from interrupt context.
> > 
> > To ensure that dm-bufio does not sleep, dm-bufio must use a spinlock
> > instead of a mutex. Additionally, to avoid deadlocks, special care
> > must be taken so that dm-bufio does not sleep while holding the
> > spinlock.
> > 
> > But again: the scope of this no_sleep is initially confined to
> > dm_bufio_get, so __alloc_buffer_wait_no_callback is _not_ changed to
> > avoid sleeping because __bufio_new avoids allocation for NF_GET.
> > 
> > Signed-off-by: Nathan Huckleberry <nhuck@google.com>
> > Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> > ---
> >  drivers/md/dm-bufio.c    | 22 +++++++++++++++++++---
> >  include/linux/dm-bufio.h |  5 +++++
> >  2 files changed, 24 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> > index ad5603eb12e3..486179d1831c 100644
> > --- a/drivers/md/dm-bufio.c
> > +++ b/drivers/md/dm-bufio.c
> > @@ -81,6 +81,8 @@
> >   */
> >  struct dm_bufio_client {
> >  	struct mutex lock;
> > +	spinlock_t spinlock;
> > +	unsigned long spinlock_flags;
> >  
> >  	struct list_head lru[LIST_SIZE];
> >  	unsigned long n_buffers[LIST_SIZE];
> > @@ -90,6 +92,7 @@ struct dm_bufio_client {
> >  	s8 sectors_per_block_bits;
> >  	void (*alloc_callback)(struct dm_buffer *);
> >  	void (*write_callback)(struct dm_buffer *);
> > +	bool no_sleep;
> >  
> >  	struct kmem_cache *slab_buffer;
> >  	struct kmem_cache *slab_cache;
> > @@ -167,17 +170,26 @@ struct dm_buffer {
> >  
> >  static void dm_bufio_lock(struct dm_bufio_client *c)
> >  {
> > -	mutex_lock_nested(&c->lock, dm_bufio_in_request());
> > +	if (c->no_sleep)
> > +		spin_lock_irqsave_nested(&c->spinlock, c->spinlock_flags, dm_bufio_in_request());
> > +	else
> > +		mutex_lock_nested(&c->lock, dm_bufio_in_request());
> >  }
> >  
> >  static int dm_bufio_trylock(struct dm_bufio_client *c)
> >  {
> > -	return mutex_trylock(&c->lock);
> > +	if (c->no_sleep)
> > +		return spin_trylock_irqsave(&c->spinlock, c->spinlock_flags);
> > +	else
> > +		return mutex_trylock(&c->lock);
> >  }
> >  
> >  static void dm_bufio_unlock(struct dm_bufio_client *c)
> >  {
> > -	mutex_unlock(&c->lock);
> > +	if (c->no_sleep)
> > +		spin_unlock_irqrestore(&c->spinlock, c->spinlock_flags);
> > +	else
> > +		mutex_unlock(&c->lock);
> >  }
> >  
> >  /*----------------------------------------------------------------*/
> > @@ -1748,12 +1760,16 @@ struct dm_bufio_client *dm_bufio_client_create(struct block_device *bdev, unsign
> >  	c->alloc_callback = alloc_callback;
> >  	c->write_callback = write_callback;
> >  
> > +	if (flags & DM_BUFIO_CLIENT_NO_SLEEP)
> > +		c->no_sleep = true;
> > +
> >  	for (i = 0; i < LIST_SIZE; i++) {
> >  		INIT_LIST_HEAD(&c->lru[i]);
> >  		c->n_buffers[i] = 0;
> >  	}
> >  
> >  	mutex_init(&c->lock);
> > +	spin_lock_init(&c->spinlock);
> >  	INIT_LIST_HEAD(&c->reserved_buffers);
> >  	c->need_reserved_buffers = reserved_buffers;
> >  
> > diff --git a/include/linux/dm-bufio.h b/include/linux/dm-bufio.h
> > index e21480715255..15d9e15ca830 100644
> > --- a/include/linux/dm-bufio.h
> > +++ b/include/linux/dm-bufio.h
> > @@ -17,6 +17,11 @@
> >  struct dm_bufio_client;
> >  struct dm_buffer;
> >  
> > +/*
> > + * Flags for dm_bufio_client_create
> > + */
> > +#define DM_BUFIO_CLIENT_NO_SLEEP 0x1
> > +
> >  /*
> >   * Create a buffered IO cache on a given device
> >   */
> > -- 
> > 2.32.1 (Apple Git-133)
> > 
> > --
> > dm-devel mailing list
> > dm-devel@redhat.com
> > https://listman.redhat.com/mailman/listinfo/dm-devel
> > 
> --
> dm-devel mailing list
> dm-devel@redhat.com
> https://listman.redhat.com/mailman/listinfo/dm-devel
> 

--
dm-devel mailing list
dm-devel@redhat.com
https://listman.redhat.com/mailman/listinfo/dm-devel