linux-kernel - Re: [PATCH] xfs: optimise xfs_mod

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Date:   Wed, 20 Nov 2019 22:56:44 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Shaokun Zhang <zhangshaokun@...ilicon.com>,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yang Guo <guoyang2@...wei.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Subject: Re: [PATCH] xfs: optimise xfs_mod_icount/ifree when delta < 0

> +static void
>  xfs_sb_mod8(
>  	uint8_t			*field,
>  	int8_t			delta)
>  {
>  	int8_t			counter = *field;
>  
> +	if (!delta)
> +		return;
>  	counter += delta;
> +	ASSERT(counter >= 0);
>  	*field = counter;
>  }

I'd almost find it easier to keep the != 0 check in the caller, and
in fact also move the assert there and just kill these helpers, e.g.

	if (tp->t_imaxpct_delta != 0) {
		mp->m_sb.sb_imax_pct += tp->t_imaxpct_delta;
		ASSERT(mp->m_sb.sb_imax_pct >= 0);
	}
	if (tp->t_rextsize_delta != 0) {
		mp->m_sb.sb_rextsize += tp->t_rextsize_delta;
		ASSERT(mp->m_sb.sb_rextsize >= 0);
	}

While this is 2 more lines per counter it is a lot more explicit and
easier to follow.

>  	if (idelta) {
> -		error = xfs_mod_icount(mp, idelta);
> -		if (error)
> -			goto out_undo_fdblocks;
> +		percpu_counter_add_batch(&mp->m_icount, idelta,
> +					 XFS_ICOUNT_BATCH);
> +		if (idelta < 0)
> +			ASSERT(__percpu_counter_compare(&mp->m_icount, 0,
> +							XFS_ICOUNT_BATCH) >= 0)

And here I wonder if keeping single use helpers wouldn't have been a
little nicer.  Not that it really matters.