[PATCH v3] xfs: re-enable xfsaild idle mode and fix associated races

[Brian Foster]

On 07/01/2012 08:07 PM, Dave Chinner wrote:
> On Thu, Jun 28, 2012 at 06:52:56AM -0400, Brian Foster wrote:
>> xfsaild idle mode logic currently leads to a couple hangs:
>>
>> 1.) If xfsaild is rescheduled in during an incremental scan
>>     (i.e., tout != 0) and the target has been updated since
>>     the previous run, we can hit the new target and go into
>>     idle mode with a still populated ail.
>> 2.) A wake up is only issued when the target is pushed forward.
>>     The wake up can race with xfsaild if it is currently in the
>>     process of entering idle mode, causing future wake up
>>     events to be lost.
>>
>> These hangs have been reproduced and verified as fixed by
>> running xfstests 273 in a loop on a slightly modified upstream
>> kernel. The kernel is modified to re-enable idle mode as
>> previously implemented (when count == 0) and with a revert of
>> commit 670ce93f, which includes performance improvements that
>> make this harder to reproduce.
>>
>> The solution, the algorithm for which has been outlined by
>> Dave Chinner, is to modify xfsaild to enter idle mode only when
>> the ail is empty and the push target has not been moved forward
>> since the last push.
>>
>> Signed-off-by: Brian Foster <bfoster at redhat.com>
> 
> Looks OK to me, and hasn't caused any problems here.
> 
> Final question - did you confirm with powertop that the xfsaild is
> no longer causing wakeups a minute or two after you stop writing to
> the filesystem? (I haven't yet)
> 

I hadn't tested with powertop, but I had some tracepoints hacked in
around the idle/wake cases to verify the thread was actually scheduling
out. FWIW, I just gave powertop a quick test and it appears to work as
expected...

With current upstream on my rhel6.3 VM, I see the following after
running a 'touch /mnt/file;sync' and letting the fs idle for a bit:

   0.5% ( 19.9)      xfsaild/vdb1 : xfsaild (process_timeout)

and this drops off completely with the patch applied. Thanks for the tip.

Brian

> Reviewed-by: Dave Chinner <dchinner at redhat.com>
>