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

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Date:   Mon, 18 Sep 2023 12:33:44 +0800
From:   alexjlzheng@...il.com
To:     david@...morbit.com
Cc:     alexjlzheng@...il.com, alexjlzheng@...cent.com,
        chandan.babu@...cle.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 Thu, 14 Sep 2023 09:44:42 +1000, Dave Chinner <david@...morbit.com> wrote:
> On Wed, Sep 13, 2023 at 11:41:15AM +0800, 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_.
> 
> What about all the ENOSPC and shutdown tests in fstests?

Sorry for the late reply.

I tested this patch using xfstests last week, and all enospc and shutdown
related tests passed.

> 
> If you haven't exercised ENOSPC conditions in your testing, then you
> haven't actually tested whether the new code can accurately and
> correctly detect ENOSPC conditions....
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com

Best regards,

Jinliang Zheng.