linux-kernel - Re: [PATCH 46/49] mm/mempolicy: replace nodes_weight with nodes_weight

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Date:   Fri, 11 Feb 2022 11:47:22 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Larry Woodman <lwoodman@...hat.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Joe Perches <joe@...ches.com>, Dennis Zhou <dennis@...nel.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Nicholas Piggin <npiggin@...il.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Alexey Klimov <aklimov@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 46/49] mm/mempolicy: replace nodes_weight with
 nodes_weight_eq

 + Larry Woodman <lwoodman@...hat.com>

On Fri, Feb 11, 2022 at 06:44:39PM +0100, Christophe JAILLET wrote:
> Le 10/02/2022 à 23:49, Yury Norov a écrit :
> > do_migrate_pages() calls nodes_weight() to compare the weight
> > of nodemask with a given number. We can do it more efficiently with
> > nodes_weight_eq() because conditional nodes_weight() may stop
> > traversing the nodemask earlier, as soon as condition is (or is not)
> > met.
> > 
> > Signed-off-by: Yury Norov <yury.norov@...il.com>
> > ---
> >   mm/mempolicy.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 7c852793d9e8..56efd00b1b6e 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -1154,7 +1154,7 @@ int do_migrate_pages(struct mm_struct *mm, const nodemask_t *from,
> >   			 *          [0-7] - > [3,4,5] moves only 0,1,2,6,7.
> >   			 */
> > -			if ((nodes_weight(*from) != nodes_weight(*to)) &&
> > +			if (!nodes_weight_eq(*from, nodes_weight(*to)) &&
> >   						(node_isset(s, *to)))
> 
> Hi,
> 
> I've not looked in details, but would it make sense to hoist the
> "(nodes_weight(*from) != nodes_weight(*to))" test out of the
> for_each_node_mask() to compute it only once?
> 
> 'from' and 'to' look unmodified in the loop.

It seems that 'from' and 'to' are untouched in the outer while()
loop as well, so we can compare weights of nodemaps only once at the
beginning.

Larry, can you please comment on that?

Thanks,
Yury