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

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Date:   Fri, 16 Dec 2022 10:46:28 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     tzm <tcm1030@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mempolicy: failed to disable numa balancing

On Fri, Dec 02, 2022 at 11:59:54AM -0800, Andrew Morton 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?
> 
> After eight years, I wonder if we actually need this.

I don't think the patch is right aside from coding style issues such as
real names used in signed-off-by's.

The !numabalancing_override is checking "should the default be changed?",
itt's not checking if it should be enabled specifically. A better potential
fix would be something like this? (not actually tested)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 61aa9aedb728..fc649f8509f7 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2862,10 +2862,12 @@ static void __init check_numabalancing_enable(void)
 		numabalancing_default = true;
 
 	/* Parsed by setup_numabalancing. override == 1 enables, -1 disables */
-	if (numabalancing_override)
+	if (numabalancing_override) {
 		set_numabalancing_state(numabalancing_override == 1);
+		return;
+	}
 
-	if (num_online_nodes() > 1 && !numabalancing_override) {
+	if (num_online_nodes() > 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);