Date:	Tue, 25 Sep 2012 19:59:02 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	Mikulas Patocka <mpatocka@...hat.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Jan Kara <jack@...e.cz>, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	kosaki.motohiro@...fujitsu.com, linux-fsdevel@...r.kernel.org,
	lwoodman@...hat.com, "Alasdair G. Kergon" <agk@...hat.com>
Subject: Re: [PATCH 0/4] Fix a crash when block device is read and block size
 is changed at the same time

On 2012-09-25 19:49, Jeff Moyer wrote:
> Jeff Moyer <jmoyer@...hat.com> writes:
> 
>> Mikulas Patocka <mpatocka@...hat.com> writes:
>>
>>> Hi Jeff
>>>
>>> Thanks for testing.
>>>
>>> It would be interesting ... what happens if you take the patch 3, leave 
>>> "struct percpu_rw_semaphore bd_block_size_semaphore" in "struct 
>>> block_device", but remove any use of the semaphore from fs/block_dev.c? - 
>>> will the performance be like unpatched kernel or like patch 3? It could be 
>>> that the change in the alignment affects performance on your CPU too, just 
>>> differently than on my CPU.
>>
>> It turns out to be exactly the same performance as with the 3rd patch
>> applied, so I guess it does have something to do with cache alignment.
>> Here is the patch (against vanilla) I ended up testing.  Let me know if
>> I've botched it somehow.
>>
>> So, I next up I'll play similar tricks to what you did (padding struct
>> block_device in all kernels) to eliminate the differences due to
>> structure alignment and provide a clear picture of what the locking
>> effects are.
> 
> After trying again with the same padding you used in the struct
> bdev_inode, I see no performance differences between any of the
> patches.  I tried bumping up the number of threads to saturate the
> number of cpus on a single NUMA node on my hardware, but that resulted
> in lower IOPS to the device, and hence consumption of less CPU time.
> So, I believe my results to be inconclusive.
> 
> After talking with Vivek about the problem, he had mentioned that it
> might be worth investigating whether bd_block_size could be protected
> using SRCU.  I looked into it, and the one thing I couldn't reconcile is
> updating both the bd_block_size and the inode->i_blkbits at the same
> time.  It would involve (afaiui) adding fields to both the inode and the
> block_device data structures and using rcu_assign_pointer  and
> rcu_dereference to modify and access the fields, and both fields would
> need to protected by the same struct srcu_struct.  I'm not sure whether
> that's a desirable approach.  When I started to implement it, it got
> ugly pretty quickly.  What do others think?
> 
> For now, my preference is to get the full patch set in.  I will continue
> to investigate the performance impact of the data structure size changes
> that I've been seeing.
> 
> So, for the four patches:
> 
> Acked-by: Jeff Moyer <jmoyer@...hat.com>
> 
> Jens, can you have a look at the patch set?  We are seeing problem
> reports of this in the wild[1][2].

I'll queue it up for 3.7. I can run my regular testing on the 8-way, it
has a nack for showing scaling problems very nicely in aio/dio. As long
as we're not adding per-inode cache line dirtying per IO (and the
per-cpu rw sem looks OK), then I don't think there's too much to worry
about.

-- 
Jens Axboe

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