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

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Date:   Wed, 13 Sep 2023 11:58:28 +0800
From:   alexjlzheng@...il.com
To:     alexjlzheng@...il.com
Cc:     alexjlzheng@...cent.com, chandan.babu@...cle.com,
        david@...morbit.com, djwong@...nel.org,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v2] xfs: remove redundant batch variables for serialization

On Wed, 13 Sep 2023 11:41:15 +0800, Jinliang Zheng <alexjlzheng@...il.com> wrote:
> On Mon, 11 Sep 2023 07:49:51 +1000, Dave Chinner <david@...morbit.com> wrote:
> > 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?
> 
> Hi, Dave,
> 
> Thank you for your reply.
> 
> I have tested the patch using _filebench_ and _fio_.
> 
> In fact, corner cases often occur when the addition function and comparison
> function use different batch thresholds. This is why the batch parameter of
> the addition function must be chosen carefully when we use a additon function
> with a variable parameter *percpu_counter_batch*.

There is a typo here, it should be:

This is why the batch parameter of the addition function must be chosen
carefully when we use a compare function with a variable parameter
*percpu_counter_batch*.

Sorry, please forgive my carelessness.

> 
> The percpu_counter_compare() with variable parameter *percpu_counter_batch*
> was replaced with __percpu_counter_compare() with fixed parameter
> *XFS_FDBLOCKS_BATCH* by commit 8c1903d3081a ("xfs: inode and free block
> counters need to use __percpu_counter_compare").
> 
> Then there is no need to detect the batch threshold. Just use the fixed
> *XFS_FDBLOCKS_BATCH* to simplify the code.
> 
> > 
> > Cheers,
> > 
> > Dave.
> > -- 
> > Dave Chinner
> > david@...morbit.com
> 
> Best regards，
> 
> Jinliang Zheng.

Best regards,

Jinliang Zheng.