linux-kernel - Re: [PATCH] timers: Exclude isolated cpus from timer migation

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Message-ID: <a2d6f50e-8f65-4586-b7df-20609bdc111e@redhat.com>
Date: Thu, 10 Apr 2025 10:50:39 -0400
From: Waiman Long <llong@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>, Waiman Long <llong@...hat.com>
Cc: Gabriele Monaco <gmonaco@...hat.com>, Thomas Gleixner
 <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] timers: Exclude isolated cpus from timer migation


On 4/10/25 10:43 AM, Frederic Weisbecker wrote:
> Le Thu, Apr 10, 2025 at 10:35:55AM -0400, Waiman Long a écrit :
>> On 4/10/25 9:03 AM, Frederic Weisbecker wrote:
>>> Le Thu, Apr 10, 2025 at 12:38:25PM +0200, Gabriele Monaco a écrit :
>>>> On Thu, 2025-04-10 at 10:26 +0200, Thomas Gleixner wrote:
>>>>> How can that happen? There is always at least _ONE_ housekeeping,
>>>>> non-isolated, CPU online, no?
>>>>>
>>>> In my understanding it shouldn't, but I'm not sure there's anything
>>>> preventing the user from isolating everything via cpuset.
>>>> Anyway that's something no one in their mind should do, so I guess I'd
>>>> just opt for the cpumask_first (or actually cpumask_any, like before
>>>> the change).
>>> With "nohz_full=..." or "isolcpus=nohz,..." there is always at least one
>>> housekeeping CPU. But with isolcpus=[domain] or cpusets equivalents
>>> (v1 cpuset.sched_load_balance, v2 isolated partion) there is nothing that
>>> prevents all CPUs from being isolated.
>> Actually v2 won't allow users to isolate all the CPUs. Users can probably do
>> that with v1's cpuset.sched_load_balance.
> Perhaps, and I think isolcpus= can too.

No, I don't think so. The following code is in kernel/sched/isolation.c:

first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging); 
if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) { 
__cpumask_set_cpu(smp_processor_id(), housekeeping_staging); 
__cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask); if 
(!housekeeping.flags) { pr_warn("Housekeeping: must include one present 
CPU, " "using boot CPU:%d\n", smp_processor_id()); } }

Cheers, Longman