linux-kernel - Re: [PATCH v2] xfs: remove redundant batch variables for serialization

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Date:   Mon, 11 Sep 2023 07:49:51 +1000
From:   Dave Chinner <david@...morbit.com>
To:     alexjlzheng@...il.com
Cc:     chandan.babu@...cle.com, djwong@...nel.org,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jinliang Zheng <alexjlzheng@...cent.com>
Subject: Re: [PATCH v2] xfs: remove redundant batch variables for
 serialization

On Sat, Sep 09, 2023 at 03:17:51PM +0800, alexjlzheng@...il.com wrote:
> From: Jinliang Zheng <alexjlzheng@...cent.com>
> 
> Historically, when generic percpu counters were introduced in xfs for
> free block counters by commit 0d485ada404b ("xfs: use generic percpu
> counters for free block counter"), the counters used a custom batch
> size. In xfs_mod_freecounter(), originally named xfs_mod_fdblocks(),
> this patch attempted to serialize the program using a smaller batch size
> as parameter to the addition function as the counter approaches 0.
> 
> Commit 8c1903d3081a ("xfs: inode and free block counters need to use
> __percpu_counter_compare") pointed out the error in commit 0d485ada404b
> ("xfs: use generic percpu counters for free block counter") mentioned
> above and said that "Because the counters use a custom batch size, the
> comparison functions need to be aware of that batch size otherwise the
> comparison does not work correctly". Then percpu_counter_compare() was
> replaced with __percpu_counter_compare() with parameter
> XFS_FDBLOCKS_BATCH.
> 
> After commit 8c1903d3081a ("xfs: inode and free block counters need to
> use __percpu_counter_compare"), the existence of the batch variable is
> no longer necessary, so this patch is proposed to simplify the code by
> removing it.

Hmmmm. Fiddling with percpu counter batch thresholds can expose
unexpected corner case behaviours.  What testing have you done on
this change?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com