linux-kernel - Re: [PATCH 14/19] mm: Create/affine kswapd to its preferred node

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Message-ID: <c89d07bd-979c-4a69-b73d-53f7e91b09f0@suse.cz>
Date: Wed, 7 Aug 2024 19:05:53 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Frederic Weisbecker <frederic@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>
Cc: Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 14/19] mm: Create/affine kswapd to its preferred node

On 8/7/24 18:02, Frederic Weisbecker wrote:
> kswapd is dedicated to a specific node. As such it wants to be
> preferrably affine to it, memory and CPUs-wise.
> 
> Use the proper kthread API to achieve that. As a bonus it takes care of
> CPU-hotplug events and CPU-isolation on its behalf.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

But there's a bug to fix:

> ---
>  mm/vmscan.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index cfa839284b92..b8ee3bf8fa29 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -7159,10 +7159,6 @@ static int kswapd(void *p)
>  	unsigned int highest_zoneidx = MAX_NR_ZONES - 1;
>  	pg_data_t *pgdat = (pg_data_t *)p;
>  	struct task_struct *tsk = current;
> -	const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
> -
> -	if (!cpumask_empty(cpumask))
> -		set_cpus_allowed_ptr(tsk, cpumask);
>  
>  	/*
>  	 * Tell the memory management that we're a "memory allocator",
> @@ -7331,7 +7327,7 @@ void __meminit kswapd_run(int nid)
>  
>  	pgdat_kswapd_lock(pgdat);
>  	if (!pgdat->kswapd) {
> -		pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
> +		pgdat->kswapd = kthread_create_on_node(kswapd, pgdat, nid, "kswapd%d", nid);
>  		if (IS_ERR(pgdat->kswapd)) {
>  			/* failure at boot is fatal */
>  			pr_err("Failed to start kswapd on node %d，ret=%ld\n",
> @@ -7339,6 +7335,7 @@ void __meminit kswapd_run(int nid)
>  			BUG_ON(system_state < SYSTEM_RUNNING);
>  			pgdat->kswapd = NULL;
>  		}
> +		wake_up_process(pgdat->kswapd);

		should be in an else { } branch as it only ends up with BUG() during early
boot and not when hotplugging later

>  	}
>  	pgdat_kswapd_unlock(pgdat);
>  }