Re: [PATCH v4 0/5] barrier: Add smp_cond_load_*_timewait()

[Ankur Arora <ankur.a.arora@oracle.com>]

Catalin Marinas <catalin.marinas@arm.com> writes:

> On Tue, Sep 02, 2025 at 03:46:52PM -0700, Ankur Arora wrote:
>> Catalin Marinas <catalin.marinas@arm.com> writes:
>> > Can you have a go at poll_idle() to see how it would look like using
>> > this API? It doesn't necessarily mean we have to merge them all at once
>> > but it gives us a better idea of the suitability of the interface.
>>
>> So, I've been testing with some version of the following:
>>
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index 9b6d90a72601..361879396d0c 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -8,35 +8,25 @@
>>  #include <linux/sched/clock.h>
>>  #include <linux/sched/idle.h>
>>
>> -#define POLL_IDLE_RELAX_COUNT	200
>> -
>>  static int __cpuidle poll_idle(struct cpuidle_device *dev,
>>  			       struct cpuidle_driver *drv, int index)
>>  {
>> -	u64 time_start;
>> -
>> -	time_start = local_clock_noinstr();
>> +	unsigned long flags;
>>
>>  	dev->poll_time_limit = false;
>>
>>  	raw_local_irq_enable();
>>  	if (!current_set_polling_and_test()) {
>> -		unsigned int loop_count = 0;
>> -		u64 limit;
>> +		u64 limit, time_end;
>>
>>  		limit = cpuidle_poll_time(drv, dev);
>> +		time_end = local_clock_noinstr() + limit;
>>
>> -		while (!need_resched()) {
>> -			cpu_relax();
>> -			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
>> -				continue;
>> +		flags = smp_cond_load_relaxed_timewait(&current_thread_info()->flags,
>> +						       VAL & _TIF_NEED_RESCHED,
>> +						       (local_clock_noinstr() >= time_end));
>
> It makes sense to have the non-strict comparison, though it changes the
> original behaviour slightly. Just mention it in the commit log.
>
>>
>> -			loop_count = 0;
>> -			if (local_clock_noinstr() - time_start > limit) {
>> -				dev->poll_time_limit = true;
>> -				break;
>> -			}
>> -		}
>> +		dev->poll_time_limit = (local_clock_noinstr() >= time_end);
>
> Could we do this instead and avoid another clock read:
>
> 		dev->poll_time_limit = !(flags & _TIF_NEED_RESCHED);
>
> In the original code, it made sense since it had to check the clock
> anyway and break the loop.
>
> When you repost, please include the rqspinlock and poll_idle changes as
> well to show how the interface is used.

Sure.

--
ankur