Re: [PATCH] Optimize lock in queue unplugging

[Mikulas Patocka]

On Wed, 30 Apr 2008, Jens Axboe wrote:

> On Tue, Apr 29 2008, Mike Anderson wrote:
> > Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:
> > > On Tue, Apr 29 2008, Mikulas Patocka wrote:
> > > > Hi
> > > >
> > > > Mike Anderson was doing an OLTP benchmark on a computer with 48 physical
> > > > disks mapped to one logical device via device mapper.
> > > >
> > > > He found that there was a slowdown on request_queue->lock in function
> > > > generic_unplug_device. The slowdown is caused by the fact that when some
> > > > code calls unplug on the device mapper, device mapper calls unplug on all
> > > > physical disks. These unplug calls take the lock, find that the queue is
> > > > already unplugged, release the lock and exit.
> > > >
> > > > With the below patch, performance of the benchmark was increased by 18%
> > > > (the whole OLTP application, not just block layer microbenchmarks).
> > > >
> > > > So I'm submitting this patch for upstream. I think the patch is correct,
> > > > because when more threads call simultaneously plug and unplug, it is
> > > > unspecified, if the queue is or isn't plugged (so the patch can't make
> > > > this worse). And the caller that plugged the queue should unplug it
> > > > anyway. (if it doesn't, there's 3ms timeout).
> > >
> > > Where were these unplug calls coming from? The block layer will
> > > generally only unplug when it is already unplugged, so if you are seeing
> > > so many unplug calls that the patch redues overhead by as much
> > > described, perhaps the callsite is buggy?
> >
> > I do not have direct access the the benchmark setup, but here is the data
> > I have received.
> >
> > The oprofile data was showing ll_rw_blk::generic_unplug_device() as a top
> > routine at 13% of the samples. Annotation of the samples shows hits on
> > spin_lock_irq(q->queue_lock).
> >
> > Here are some sample call traces:
> >
> > Call trace #1
> >
> > kernel:  [<ffffffff80058c6c>] generic_unplug_device+0x5d/0xc6
> > kernel:  [<ffffffff8820ea3e>] :dm_mod:dm_table_unplug_all+0x33/0x41
> > kernel:  [<ffffffff8820cc85>] :dm_mod:dm_unplug_all+0x1d/0x28
> > kernel:  [<ffffffff8005a78a>] blk_backing_dev_unplug+0x56/0x5b
> > kernel:  [<ffffffff80014cdc>] sync_buffer+0x36/0x3f
> > kernel:  [<ffffffff800629a4>] __wait_on_bit+0x40/0x6f
> > kernel:  [<ffffffff80014ca6>] sync_buffer+0x0/0x3f
> > kernel:  [<ffffffff80062a3f>] out_of_line_wait_on_bit+0x6c/0x78
> > kernel:  [<ffffffff8009c474>] wake_bit_function+0x0/0x23
> > kernel:  [<ffffffff88034c85>] :jbd:journal_commit_transaction+0x91f/0x1086
> > kernel:  [<ffffffff8003d038>] lock_timer_base+0x1b/0x3c
> > kernel:  [<ffffffff8803840e>] :jbd:kjournald+0xc1/0x213
> > kernel:  [<ffffffff8009c446>] autoremove_wake_function+0x0/0x2e
> > kernel:  [<ffffffff8009c283>] keventd_create_kthread+0x0/0x61
> > kernel:  [<ffffffff8803834d>] :jbd:kjournald+0x0/0x213
> > kernel:  [<ffffffff8009c283>] keventd_create_kthread+0x0/0x61
> > kernel:  [<ffffffff800321d5>] kthread+0xfe/0x132
> > kernel:  [<ffffffff8005cfb1>] child_rip+0xa/0x11
> > kernel:  [<ffffffff8009c283>] keventd_create_kthread+0x0/0x61
> > kernel:  [<ffffffff800320d7>] kthread+0x0/0x132
> > kernel:  [<ffffffff8005cfa7>] child_rip+0x0/0x11
> >
> > Call trace #2
> > kernel:  [<ffffffff80058c6c>] generic_unplug_device+0x5d/0xc6
> > kernel:  [<ffffffff8820ea3e>] :dm_mod:dm_table_unplug_all+0x33/0x41
> > kernel:  [<ffffffff8820cc85>] :dm_mod:dm_unplug_all+0x1d/0x28
> > kernel:  [<ffffffff8005a78a>] blk_backing_dev_unplug+0x56/0x5b
> > kernel:  [<ffffffff800e8bfe>] __blockdev_direct_IO+0x889/0xaa2
> > kernel:  [<ffffffff88050800>] :ext3:ext3_direct_IO+0xf3/0x18b
> > kernel:  [<ffffffff8804ec84>] :ext3:ext3_get_block+0x0/0xe3
> > kernel:  [<ffffffff800be6bb>] generic_file_direct_IO+0xbd/0xfb
> > kernel:  [<ffffffff8001e637>] generic_file_direct_write+0x60/0xf2
> > kernel:  [<ffffffff80015cfd>] __generic_file_aio_write_nolock+0x2b7/0x3b8
> > kernel:  [<ffffffff8002134f>] generic_file_aio_write+0x65/0xc1
> > kernel:  [<ffffffff8804c192>] :ext3:ext3_file_write+0x16/0x91
> > kernel:  [<ffffffff80017944>] do_sync_write+0xc7/0x104
> > kernel:  [<ffffffff8009c446>] autoremove_wake_function+0x0/0x2e
> > kernel:  [<ffffffff80111400>] free_msg+0x22/0x3c
> > kernel:  [<ffffffff800161c4>] vfs_write+0xce/0x174
> > kernel:  [<ffffffff8004194c>] sys_pwrite64+0x50/0x70
> > kernel:  [<ffffffff8005cde9>] error_exit+0x0/0x84
> > kernel:  [<ffffffff8005c116>] system_call+0x7e/0x83
>
> So it's basically dm calling into blk_unplug() all the time, which
> doesn't check if the queue is plugged. The reason why I didn't like the
> initial patch is that ->unplug_fn() really should not be called unless
> the queue IS plugged. So how about this instead:
>
> http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=c44993018887e82abd49023e92e8d8b6000e03ed
>
> That's a lot more appropriate, imho.
>
> --
> Jens Axboe

This doesn't seem correct to me. The difference between blk_unplug and 
generic_unplug_device is that blk_unplug is called on every type of device 
and generic_unplug_device (pointed to by q->unplug_fn) is a method that is 
called on low-level disk devices.

dm and md redefine q->unplug_fn to point to their own method. On dm and 
md, blk_unplug is called, but generic_unplug_device is not.

So if you have this setup
dm-linear(unplugged) -> disk(plugged)

then, with your patch, a call to blk_unplug(dm-linear) will not unplug the 
disk. With my patch, a call to blk_unplug(dm-linear) will unplug the disk 
--- it calls q->unplug_fn that points to dm_unplug_all, that calls 
blk_unplug again on the disk and that calls generic_unplug_device on disk 
queue.

Mikulas
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