linux-kernel - Re: [PATCH] mm/mempolicy: failed to disable numa balancing

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Message-ID: <CAHS8izMAR7SVanWf90vSCbnUDYZPvZQQ7G7wcqjkgxQjicz=GQ@mail.gmail.com>
Date:   Fri, 2 Dec 2022 19:44:40 -0800
From:   Mina Almasry <almasrymina@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     tzm <tcm1030@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH] mm/mempolicy: failed to disable numa balancing

On Fri, Dec 2, 2022 at 12:00 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Fri,  2 Dec 2022 22:16:30 +0800 tzm <tcm1030@....com> wrote:
>
> > It will be failed to  disable numa balancing policy permanently by passing
> > <numa_balancing=disable> to boot cmdline parameters.
> > The numabalancing_override variable is int and 1 for enable -1 for disable.
> > So, !enumabalancing_override will always be true, which cause this bug.
>
> That's really old code!
>
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -2865,7 +2865,7 @@ static void __init check_numabalancing_enable(void)
> >       if (numabalancing_override)
> >               set_numabalancing_state(numabalancing_override == 1);
> >
> > -     if (num_online_nodes() > 1 && !numabalancing_override) {
> > +     if (num_online_nodes() > 1 && (numabalancing_override == 1)) {
> >               pr_info("%s automatic NUMA balancing. Configure with numa_balancing= or the kernel.numa_balancing sysctl\n",
> >                       numabalancing_default ? "Enabling" : "Disabling");
> >               set_numabalancing_state(numabalancing_default);
>
> Looks right to me.  Mel?
>

Maybe I'm missing something, but it looks wrong to me?

numabalancing_override is default initialized to 0, I think,
indicating that no override exists.
numabalancing_override == 1 indicates it has been overridden to true.
numabalancing_override == -1 indicates that it has been overridden to false.

The above code reads to me:

if (override_exists)
    set_numabalancing_state(override_value)

if (num_online_nodes() > ! && !override_exists)
    set_numabalancing_state(numabalancing_default)

A more clear fix for readability would be an early return between
these 2 if statements I think.

> After eight years, I wonder if we actually need this.
>