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Re: [dm-devel] [PATCH 0/4] Fix a crash when block device is read and block size is changed at the same time



On 2012-09-25 19:59, Jens Axboe wrote:
> On 2012-09-25 19:49, Jeff Moyer wrote:
>> Jeff Moyer <jmoyer redhat com> writes:
>>
>>> Mikulas Patocka <mpatocka redhat 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 redhat 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.

I take that back. The series doesn't apply to my current tree. Not too
unexpected, since it's some weeks old. But more importantly, please send
this is a "real" patch series. I don't want to see two implementations
of rw semaphores. I think it's perfectly fine to first do a regular rw
sem, then a last patch adding the cache friendly variant from Eric and
converting to that.

In other words, get rid of 3/4.

-- 
Jens Axboe


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