linux-kernel - Re: [PATCH v3 RESEND] xfs: remove redundant batch variables for serialization

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Date: Thu, 18 Apr 2024 08:30:27 +1000
From: Dave Chinner <david@...morbit.com>
To: Jinliang Zheng <alexjlzheng@...il.com>
Cc: djwong@...nel.org, alexjlzheng@...cent.com, chandan.babu@...cle.com,
	linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v3 RESEND] xfs: remove redundant batch variables for
 serialization

On Wed, Apr 17, 2024 at 11:54:38PM +0800, Jinliang Zheng wrote:
> On Wed, 17 Apr 2024 08:27:13 -0700, djwong@...nel.org wrote:
> > On Wed, Apr 17, 2024 at 08:07:35PM +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.
> > > 
> > > Signed-off-by: Jinliang Zheng <alexjlzheng@...cent.com>
> > > ---
> > > Changelog:
> > > 
> > > v3: Resend for the second time 
> > > 
> > > v2: https://lore.kernel.org/linux-xfs/20230918043344.890817-1-alexjlzheng@tencent.com/
> > > 
> > > v1: https://lore.kernel.org/linux-xfs/20230908235713.GP28202@frogsfrogsfrogs/T/#t
> > 
> > ...you still haven't answered my question from V1: What problem are you
> > solving with this patch?
> 
> Hi, thank you for your reply. :)
> 
> I'm trying to simplify the code. When percpu_counter_add_batch() and
> __percpu_counter_compare() use the same batch size, percpu_counter can count
> correctly, so there is no need to reduce the batch size to 1, which will cause
> unnecessary serialization.

One of the reasons the batch size gets set to 1 when we are really
near to enospc is so that anyone else reading the counter will see
the update immediately via percpu_counter_read*(). i.e. this forces
the global count to be updated immediately rather than use the
percpu accumulators.

This behaviour helps external, unsynchronised visibility be more
accurate (e.g. in speculative preallocation) as we get very close to
ENOSPC. We don't care if we serialise near ENOSPC - we're going to
be going through allocation slow paths anyway, so there's no benefit
to be gained by maintaining wide concurrency when we are this close
to ENOSPC.

There is, however, benefit to near-ENOSPC allocation behaviour by
being more accurate, hence the batch size change when we near the
zero threshold.

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