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

Waiman Long
2025-04-10T14:21:10Z

On 4/10/25 6:38 AM, Gabriele Monaco wrote:
> On Thu, 2025-04-10 at 10:26 +0200, Thomas Gleixner wrote:
>> On Thu, Apr 10 2025 at 08:54, Gabriele Monaco wrote:
>>>
>>> +/*  cpumask excluded from migration */
>>> +static cpumask_var_t tmigr_unavailable_cpumask;
>> Why is this a negated mask instead of being the obvious and intuitive
>> available mask?
> Well, the way I started writing the patch I found it easier to do the
> double andnot in tmigr_isolated_exclude_cpumask to see what changed.
> I see the way it evolved is just messier..
> I'll apply your solution which seems much neater!
>
>>>   if (firstexp != KTIME_MAX) {
>>> - migrator = cpumask_any_but(cpu_online_mask, cpu);
>>> + migrator = cpumask_nth_andnot(0, cpu_possible_mask,
>>> +       tmigr_unavailable_cpumask);
>> That's exactly what this negated mask causes: incomprehensible code.
>>
>> cpumask_clear_cpu(cpu, available_mask);
>>         ...
>> migrator = cpumask_first(available_mask);
>>
>> is too simple and obvious, right?
>>
>>> + /* Fall back to any online in case all are isolated. */
>> 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).

No, cpuset is not allowed to take all the CPUs from the cgroup root
(housekeeping CPUs). So it can't happen.

>>> + if (migrator >= nr_cpu_ids)
>>> + migrator = cpumask_any_but(cpu_online_mask, cpu);
>>>   work_on_cpu(migrator, tmigr_trigger_active, NULL);
>>>   }
>>>
>>>   return 0;
>>>  }
>>>
>>> -static int tmigr_cpu_online(unsigned int cpu)
>>> +static int tmigr_cpu_available(unsigned int cpu)
>>>  {
>>> - struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
>>> + struct tmigr_cpu *tmc = per_cpu_ptr(&tmigr_cpu, cpu);
>>>
>>>   /* Check whether CPU data was successfully initialized */
>>>   if (WARN_ON_ONCE(!tmc->tmgroup))
>>>   return -EINVAL;
>>>
>>> + /* Silently ignore online requests if isolated */
>> This comment makes no sense.
>>
>>      /* Isolated CPUs are not participating in timer migration */
>>
>> makes it entirely clear what this is about, no?
>>
>> That brings me to the general design decision here. Your changelog
>> explains at great length WHAT the change is doing, but completely
>> fails
>> to explain the consequences and the rationale why this is the right
>> thing to do.
>>
>> By excluding the isolated CPUs from migration completely, any
>> 'global'
>> timer, which is armed on such a CPU, has to be expired on that
>> isolated
>> CPU. That's fundamentaly different from e.g. RCU isolation.
>>
>> It might be the right thing to do and harmless, but without a proper
>> explanation it's a silent side effect of your changes, which leaves
>> people scratching their heads.
> Mmh, essentially the idea is that global timer should not migrate from
> housekeeping to isolated cores. I assumed the opposite never occurs (as
> global timers /should/ not even start on isolated cores on a properly
> isolated system), but you're right, that's not quite true.
>
> Thinking about it now, since global timers /can/ start on isolated
> cores, that makes them quite different from offline ones and probably
> considering them the same is just not the right thing to do..
>
> I'm going to have a deeper thought about this whole approach, perhaps
> something simpler just preventing migration in that one direction would
> suffice.
>
>>> + if (cpu_is_isolated(cpu))
>>> + return 0;
>>>   raw_spin_lock_irq(&tmc->lock);
>>> - trace_tmigr_cpu_online(tmc);
>>> + trace_tmigr_cpu_available(tmc);
>>>   tmc->idle = timer_base_is_idle();
>>>   if (!tmc->idle)
>>>   __tmigr_cpu_activate(tmc);
>>> - tmc->online = true;
>>> + tmc->available = true;
>>> + tmc->idle = true;
>> How so?
>>
>>> + cpumask_clear_cpu(cpu, tmigr_unavailable_cpumask);
>>>   raw_spin_unlock_irq(&tmc->lock);
>>>   return 0;
>>>  }
>>>
>>> +int tmigr_isolated_exclude_cpumask(cpumask_var_t exclude_cpumask)
>> cpumask_var_t is wrong here. 'const struct cpumask *' is what you
>> want.
> You mean in the function argument? That's exactly how it is handled in
> workqueue_unbound_exclude_cpumask. I get cpumask_var_t is not
> necessarily a pointer, perhaps it's worth changing it there too..
> Or am I missing your point?

cpumask_var_t can be interchangeable with "struct cpumask *" as long as
the passed-in cpumask is not being modified.

Cheers,
Longman