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From:   Andreas Dilger <adilger@dilger.ca>
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Subject: Re: [RFC PATCH v1 08/50] fs/ext4/ialloc.c: Replace % with
 reciprocal_scale() TO BE VERIFIED
Date:   Sat, 28 Mar 2020 18:10:11 -0600
In-Reply-To: <20200328231536.GA11951@SDF.ORG>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Theodore Ts'o <tytso@mit.edu>,
        linux-ext4 <linux-ext4@vger.kernel.org>
To:     George Spelvin <lkml@SDF.ORG>
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 <9A60C390-349E-4A90-A812-F04EB5A82136@dilger.ca>
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On Mar 28, 2020, at 5:15 PM, George Spelvin <lkml@SDF.ORG> wrote:
>=20
> On Sat, Mar 28, 2020 at 04:56:17PM -0600, Andreas Dilger wrote:
>>=20
>> So I think the current patch is fine.  The for-loop construct of
>> using "++g =3D=3D ngroups && (g =3D 0)" to wrap "g" around is new to =
me,
>> but looks correct.
>>=20
>> Reviewed-by: Andreas Dilger <adilger@dilger.ca>
>=20
> Thank you.  Standing back and looking from higher altitude, I missed
> a second modulo at fallback_retry: which should be made consistent,
> so I need a one re-spin.
>=20
> Also, we could, if desired, eliminate the i variable entirely
> using the fact that we have a copy of the starting position cached
> in parent_group.  I.e.
>=20
> 		g =3D parent_group =3D reciprocal_scale(grp, ngroups);
> -		for (i =3D 0; i < ngroups; i++, ++g =3D=3D ngroups && (g =
=3D 0)) {
> +		do {
> 			...
> -		}
> +			if (++g =3D=3D ngroups)
> +				g =3D 0;
> +		} while (g !=3D parent_group);
>=20
> Or perhaps the following would be simpler, replacing the modulo
> with a conditional subtract:
>=20
> -		g =3D parent_group =3D reciprocal_scale(grp, ngroups);
> +		parent_group =3D reciprocal_scale(grp, ngroups);
> -		for (i =3D 0; i < ngroups; i++, ++g =3D=3D ngroups && (g =
=3D 0)) {
> +		for (i =3D 0; i < ngroups; i++) {
> +			g =3D parent_group + i;
> +			if (g >=3D ngroups)
> +				g -=3D ngroups;
>=20
> The conditional branch starts out always false, and ends up always =
true,
> but except for a few bobbles when it switches, branch prediction =
should
> handle it very well.
>=20
> Any preference?

I was looking at whether we could use a for-loop without "i"?  Something =
like:

	for (g =3D parent_group + 1; g !=3D parent_group; ++g >=3D =
ngroups && (g =3D 0))

The initial group is parent_group + 1, to avoid special-casing when the
initial parent_group =3D 0 (which would prevent the loop from =
terminating).

Cheers, Andreas


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