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Re: [PATCH 1/2] percpu_counter: batch size aware __percpu_counter_compar

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 1/2] percpu_counter: batch size aware __percpu_counter_compare()
From: Tejun Heo <tj@xxxxxxxxxx>
Date: Thu, 14 May 2015 11:02:30 -0400
Cc: xfs@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, peterz@xxxxxxxxxxxxx
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Sender: Tejun Heo <htejun@xxxxxxxxx>
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On Thu, May 14, 2015 at 10:55:53AM +1000, Dave Chinner wrote:
> percpu_counter: batch size aware __percpu_counter_compare()
> 
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> XFS uses non-stanard batch sizes for avoiding frequent global
> counter updates on it's allocated inode counters, as they increment
> or decrement in batches of 64 inodes. Hence the standard percpu
> counter batch of 32 means that the counter is effectively a global
> counter. Currently Xfs uses a batch size of 128 so that it doesn't
> take the global lock on every single modification.
> 
> However, Xfs also needs to compare accurately against zero, which
> means we need to use percpu_counter_compare(), and that has a
> hard-coded batch size of 32, and hence will spuriously fail to
> detect when it is supposed to use precise comparisons and hence
> the accounting goes wrong.
> 
> Add __percpu_counter_compare() to take a custom batch size so we can
> use it sanely in XFS and factor percpu_counter_compare() to use it.
> 
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Please feel free to route the patch however you see fit.

Thanks.

-- 
tejun

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