Re: [PATCH 2/2] block_dev: don't take the write lock if block size doesn't change

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 28 Nov 2012, Jeff Chua wrote:

> On Wed, Nov 28, 2012 at 12:01 PM, Mikulas Patocka <mpatocka@redhat.com> wrote:
> > block_dev: don't take the write lock if block size doesn't change
> >
> > Taking the write lock has a big performance impact on the whole system
> > (because of synchronize_sched_expedited). This patch avoids taking the
> > write lock if the block size doesn't change (i.e. when mounting
> > filesystem with block size equal to the default block size).
> >
> > The logic to test if the block device is mapped was moved to a separate
> > function is_bdev_mapped to avoid code duplication.
> >
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> >
> > ---
> >  fs/block_dev.c |   25 ++++++++++++++++++-------
> >  1 file changed, 18 insertions(+), 7 deletions(-)
> >
> > Index: linux-3.7-rc7/fs/block_dev.c
> > ===================================================================
> > --- linux-3.7-rc7.orig/fs/block_dev.c   2012-11-28 04:09:01.000000000 +0100
> > +++ linux-3.7-rc7/fs/block_dev.c        2012-11-28 04:13:53.000000000 +0100
> > @@ -114,10 +114,18 @@ void invalidate_bdev(struct block_device
> >  }
> >  EXPORT_SYMBOL(invalidate_bdev);
> >
> > -int set_blocksize(struct block_device *bdev, int size)
> > +static int is_bdev_mapped(struct block_device *bdev)
> >  {
> > -       struct address_space *mapping;
> > +       int ret_val;
> > +       struct address_space *mapping = bdev->bd_inode->i_mapping;
> > +       mutex_lock(&mapping->i_mmap_mutex);
> > +       ret_val = mapping_mapped(mapping);
> > +       mutex_unlock(&mapping->i_mmap_mutex);
> > +       return ret_val;
> > +}
> >
> > +int set_blocksize(struct block_device *bdev, int size)
> > +{
> >         /* Size must be a power of two, and between 512 and PAGE_SIZE */
> >         if (size > PAGE_SIZE || size < 512 || !is_power_of_2(size))
> >                 return -EINVAL;
> > @@ -126,18 +134,21 @@ int set_blocksize(struct block_device *b
> >         if (size < bdev_logical_block_size(bdev))
> >                 return -EINVAL;
> >
> > +       /*
> > +        * If the block size doesn't change, don't take the write lock.
> > +        * We check for is_bdev_mapped anyway, for consistent behavior.
> > +        */
> > +       if (size == bdev->bd_block_size)
> > +               return is_bdev_mapped(bdev) ? -EBUSY : 0;
> > +
> >         /* Prevent starting I/O or mapping the device */
> >         percpu_down_write(&bdev->bd_block_size_semaphore);
> >
> >         /* Check that the block device is not memory mapped */
> > -       mapping = bdev->bd_inode->i_mapping;
> > -       mutex_lock(&mapping->i_mmap_mutex);
> > -       if (mapping_mapped(mapping)) {
> > -               mutex_unlock(&mapping->i_mmap_mutex);
> > +       if (is_bdev_mapped(bdev)) {
> >                 percpu_up_write(&bdev->bd_block_size_semaphore);
> >                 return -EBUSY;
> >         }
> > -       mutex_unlock(&mapping->i_mmap_mutex);
> >
> >         /* Don't change the size if it is same as current */
> >         if (bdev->bd_block_size != size) {
> 
> 
> This patch didn't really make any difference for ext2/3/4 but for
> reiserfs it does.
> 
> With the synchronize_sched_expedited() patch applied, it didn't make
> any difference.
> 
> Thanks,
> Jeff

It could make difference for computers with many cores - 
synchronize_sched_expedited() is expensive there because it synchronizes 
all active cores, so if we can avoid it, just do it.

Mikulas