linux-kernel - Re: [PATCH] sched/nohz: Fix NOHZ imbalance by adding options for ILB CPU

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <xhsmhtt23h0nw.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 19 Aug 2025 16:00:03 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Adam Li <adamli@...amperecomputing.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org
Cc: dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, cl@...ux.com, frederic@...nel.org,
 linux-kernel@...r.kernel.org, patches@...erecomputing.com, Adam Li
 <adamli@...amperecomputing.com>
Subject: Re: [PATCH] sched/nohz: Fix NOHZ imbalance by adding options for
 ILB CPU

On 19/08/25 02:57, Adam Li wrote:
> A qualified CPU to run NOHZ idle load balancing (ILB) has to be:
> 1) housekeeping CPU in housekeeping_cpumask(HK_TYPE_KERNEL_NOISE)
> 2) and in nohz.idle_cpus_mask
> 3) and idle
> 4) and not current CPU
>
> If most CPUs are in nohz_full CPU list there is few housekeeping CPU left.
> In the worst case if all CPUs are in nohz_full only the boot CPU is used
> for housekeeping. And the housekeeping CPU is usually busier so it will
> be unlikely added to nohz.idle_cpus_mask.
>
> Therefore if there is few housekeeping CPUs, find_new_ilb() may likely
> failed to find any CPU to do NOHZ idle load balancing. Some NOHZ CPUs may
> stay idle while other CPUs are busy.
>
> This patch adds fallback options when looking for ILB CPU if there is
> no CPU meeting above requirements. Then it searches in bellow order:
> 1) Try looking for the first idle housekeeping CPU
> 2) Try looking for the first idle CPU in nohz.idle_cpus_mask if no SMT.
> 3) Select the first housekeeping CPU even if it is busy.
>
> With this patch the NOHZ idle balancing happens more frequently.
>

I'm not understanding why, in the scenarios outlined above, more NOHZ idle
balancing is a good thing.

Considering only housekeeping CPUs, they're all covered by wakeup, periodic
and idle balancing (on top of NOHZ idle balancing when relevant). So if
find_new_ilb() never finds a NOHZ-idle CPU, then that means your HK CPUs
are either always busy or never stopping the tick when going idle, IOW they
always have some work to do within a jiffy boundary.

Am I missing something?