Date:	Sun, 4 May 2008 21:11:33 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Mike Anderson <andmike@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Alasdair Graeme Kergon <agk@...hat.com>
Subject: Re: [PATCH] Optimize lock in queue unplugging

On Wed, Apr 30 2008, Mikulas Patocka wrote:
> 
> 
> On Wed, 30 Apr 2008, Jens Axboe wrote:
> 
> >On Tue, Apr 29 2008, Mike Anderson wrote:
> >>Jens Axboe <jens.axboe@...cle.com> 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.

That is because the md/dm don't set the plugged flag which I think they
should. So we fix that instead so that plugging works the same from the
block core or from a driver instead of adding work-arounds in the block
unplug handler. Adding a check for plugged in the plug handler is a
hack, I don't see how you can argue against that.

-- 
Jens Axboe

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