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

To: Brian Foster <bfoster@xxxxxxxxxx>
Subject: Re: [PATCH v3] xfs: re-enable xfsaild idle mode and fix associated races
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 2 Jul 2012 10:07:12 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <1340880776-45730-1-git-send-email-bfoster@xxxxxxxxxx>
References: <1340880776-45730-1-git-send-email-bfoster@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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@xxxxxxxxxx>

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)

Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>

-- 
Dave Chinner
david@xxxxxxxxxxxxx

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