linux-kernel - Re: [PATCH v5 1/2] sched/numa: fix task swap by skipping kernel threads

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Message-ID: <zecfzttkv2ryqbusxjyo7avvkb22dnbaggt3bth2miaujk3wjo@vwwshve724jx>
Date: Fri, 23 May 2025 16:22:05 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Chen Yu <yu.c.chen@...el.com>
Cc: peterz@...radead.org, akpm@...ux-foundation.org, mkoutny@...e.com, 
	mingo@...hat.com, tj@...nel.org, hannes@...xchg.org, corbet@....net, 
	mgorman@...e.de, mhocko@...nel.org, muchun.song@...ux.dev, 
	roman.gushchin@...ux.dev, tim.c.chen@...el.com, aubrey.li@...el.com, libo.chen@...cle.com, 
	kprateek.nayak@....com, vineethr@...ux.ibm.com, venkat88@...ux.ibm.com, ayushjai@....com, 
	cgroups@...r.kernel.org, linux-doc@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, yu.chen.surf@...mail.com, Ayush Jain <Ayush.jain3@....com>
Subject: Re: [PATCH v5 1/2] sched/numa: fix task swap by skipping kernel
 threads

On Fri, May 23, 2025 at 08:51:01PM +0800, Chen Yu wrote:
> From: Libo Chen <libo.chen@...cle.com>
> 
> Task swapping is triggered when there are no idle CPUs in
> task A's preferred node. In this case, the NUMA load balancer
> chooses a task B on A's preferred node and swaps B with A. This
> helps improve NUMA locality without introducing load imbalance
> between nodes. In the current implementation, B's NUMA node
> preference is not mandatory. That is to say, a kernel thread
> might be incorrectly chosen as B. However, kernel thread and
> user space thread that does not have mm are not supposed to be
> covered by NUMA balancing because NUMA balancing only considers
> user pages via VMAs.
> 
> According to Peter's suggestion for fixing this issue, we use
> PF_KTHREAD to skip the kernel thread. curr->mm is also checked
> because it is possible that user_mode_thread() might create a
> user thread without an mm. As per Prateek's analysis, after
> adding the PF_KTHREAD check, there is no need to further check
> the PF_IDLE flag:
> "
> - play_idle_precise() already ensures PF_KTHREAD is set before adding
>   PF_IDLE
> 
> - cpu_startup_entry() is only called from the startup thread which
>   should be marked with PF_KTHREAD (based on my understanding looking at
>   commit cff9b2332ab7 ("kernel/sched: Modify initial boot task idle
>   setup"))
> "
> 
> In summary, the check in task_numa_compare() now aligns with
> task_tick_numa().
> 
> Suggested-by: Michal Koutny <mkoutny@...e.com>
> Tested-by: Ayush Jain <Ayush.jain3@....com>
> Signed-off-by: Libo Chen <libo.chen@...cle.com>
> Tested-by: Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>

Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>