Re: [PATCH 1/3] direct-io: only inc/dec inode->i_dio_count for file systems

[Jens Axboe <axboe@fb.com>]

On 04/15/2015 04:36 PM, Dave Chinner wrote:
> On Wed, Apr 15, 2015 at 04:01:36PM -0600, Jens Axboe wrote:
>> do_blockdev_direct_IO() increments and decrements the inode
>> ->i_dio_count for each IO operation. It does this to protect against
>> truncate of a file. Block devices don't need this sort of protection.
>>
>> For a capable multiqueue setup, this atomic int is the only shared
>> state between applications accessing the device for O_DIRECT, and it
>> presents a scaling wall for that. In my testing, as much as 30% of
>> system time is spent incrementing and decrementing this value. A mixed
>> read/write workload improved from ~2.5M IOPS to ~9.6M IOPS, with
>> better latencies too. Before:
> .....
>> diff --git a/fs/inode.c b/fs/inode.c
>> index f00b16f45507..c4901c40ad65 100644
>> --- a/fs/inode.c
>> +++ b/fs/inode.c
>> @@ -1946,18 +1946,31 @@ void inode_dio_wait(struct inode *inode)
>>   EXPORT_SYMBOL(inode_dio_wait);
>>
>>   /*
>> - * inode_dio_done - signal finish of a direct I/O requests
>> + * inode_dio_begin - signal start of a direct I/O requests
>>    * @inode: inode the direct I/O happens on
>>    *
>>    * This is called once we've finished processing a direct I/O request,
>>    * and is used to wake up callers waiting for direct I/O to be quiesced.
>>    */
>> -void inode_dio_done(struct inode *inode)
>> +void inode_dio_inc(struct inode *inode)
>
> function name does not match docbook comment....

Oops, will fix that up.

>> +{
>> +	atomic_inc(&inode->i_dio_count);
>> +}
>> +EXPORT_SYMBOL(inode_dio_inc);
>> +
>> +/*
>> + * inode_dio_dec - signal finish of a direct I/O requests
>> + * @inode: inode the direct I/O happens on
>> + *
>> + * This is called once we've finished processing a direct I/O request,
>> + * and is used to wake up callers waiting for direct I/O to be quiesced.
>> + */
>> +void inode_dio_dec(struct inode *inode)
>>   {
>>   	if (atomic_dec_and_test(&inode->i_dio_count))
>>   		wake_up_bit(&inode->i_state, __I_DIO_WAKEUP);
>>   }
>> -EXPORT_SYMBOL(inode_dio_done);
>> +EXPORT_SYMBOL(inode_dio_dec);
>
> Bikeshedding: I think this would be better suited to inode_dio_begin()
> and inode_dio_end() because now we are trying to say "this is where
> the DIO starts, and this is where it ends". It's not really
> "reference counting" interface, we're trying to annotate the
> boundaries of where DIO iis protected against truncate....

I don't really care, if people like begin/end more than inc/dec, I'm 
happy with that.

> And, realistically, if we are pushing this up into the filesystems
> again, we should push it up into *all* filesystems and get rid of it
> completely from the DIO layer. That way no new twisty passages in
> the direct IO code are needed.

Lets please keep that for a potential round 2. It's not like I'm piling 
lots of hacks on, it's two one-liner changes. It's not adding a lot to 
the entropy of direct-io.c. I've been carrying this patch for years now, 
I really don't want to sign up for futzing around in direct-io.c, nor is 
that a reasonable requirement imho.

-- 
Jens Axboe