Re: [PATCH] RFC: loop: Avoid calling blk_mq_freeze_queue() when possible.

[Bart Van Assche <bvanassche@acm.org>]

On 8/14/19 3:32 AM, Martijn Coenen wrote:
> Since Android Q, the creation and configuration of loop devices is in
> the critical path of device boot. We found that the configuration of
> loop devices is pretty slow, because many ioctl()'s involve freezing the
> block queue, which in turn needs to wait for an RCU grace period. On
> Android devices we've observed up to 60ms for the creation and
> configuration of a single loop device; as we anticipate creating many
> more in the future, we'd like to avoid this delay.
> 
> This allows LOOP_SET_BLOCK_SIZE to be called before the loop device has
> been bound; since the block queue is not running at that point, we can
> avoid the expensive freezing of the queue.
> 
> On a recent x86, this patch yields the following results:
> 
> ===
> Call LOOP_SET_BLOCK_SIZE on /dev/loop0 before being bound
> ===
> ~# time ./set_block_size
> 
> real 0m0.002s
> user 0m0.000s
> sys  0m0.002s
> 
> ===
> Call LOOP_SET_BLOCK_SIZE on /dev/loop0 after being bound
> ===
> ~# losetup /dev/loop0 fs.img
> ~# time ./set_block_size
> 
> real 0m0.008s
> user 0m0.000s
> sys  0m0.002s
> 
> Over many runs, this is a 4x improvement.
> 
> This is RFC because technically it is a change in behavior; before,
> calling LOOP_SET_BLOCK_SIZE on an unbound device would return ENXIO, and
> userspace programs that left it in their code despite the returned
> error, would now suddenly see the requested value effectuated. I'm not
> sure whether this is acceptable.
> 
> An alternative might be a CONFIG option to set the default block size to
> another value than 512. Another alternative I considered is allowing the
> block device to be created with a "frozen" queue, where we can manually
> unfreeze the queue when all the configuration is done. This would be a
> much larger code change, though.

Hi Martijn,

Is the loop driver used in Android Q to make a file on a filesystem 
visible as a block device or rather to make a subset of a block device 
visible as a block device? In the latter case, have you considered to 
use the dm-linear driver instead? I expect that the overhead per I/O of 
dm-linear will be lower than that of the loop driver.

Bart.