Re: [PATCH v8 01/11] cpuidle/poll_state: poll via smp_cond_load_relaxed()

[Ankur Arora]

Catalin Marinas <catalin.marinas@xxxxxxx> writes:

> On Wed, Sep 25, 2024 at 04:24:15PM -0700, Ankur Arora wrote:
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index 9b6d90a72601..fc1204426158 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -21,21 +21,20 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
>>
>>  	raw_local_irq_enable();
>>  	if (!current_set_polling_and_test()) {
>> -		unsigned int loop_count = 0;
>>  		u64 limit;
>>
>>  		limit = cpuidle_poll_time(drv, dev);
>>
>>  		while (!need_resched()) {
>> -			cpu_relax();
>> -			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
>> -				continue;
>> -
>> -			loop_count = 0;
>> +			unsigned int loop_count = 0;
>>  			if (local_clock_noinstr() - time_start > limit) {
>>  				dev->poll_time_limit = true;
>>  				break;
>>  			}
>> +
>> +			smp_cond_load_relaxed(&current_thread_info()->flags,
>> +					      VAL & _TIF_NEED_RESCHED ||
>> +					      loop_count++ >= POLL_IDLE_RELAX_COUNT);
>
> The above is not guaranteed to make progress if _TIF_NEED_RESCHED is
> never set. With the event stream enabled on arm64, the WFE will
> eventually be woken up, loop_count incremented and the condition would
> become true.

That makes sense.

> However, the smp_cond_load_relaxed() semantics require that
> a different agent updates the variable being waited on, not the waiting
> CPU updating it itself.

Right. And, that seems to work well with the semantics of WFE. And,
the event stream (if enabled) has a side effect that allows the exit
from the loop.

> Also note that the event stream can be disabled
> on arm64 on the kernel command line.

Yes, that's a good point. In patch-11 I tried to address that aspect
by only allowing haltpoll to be force loaded.

But, I guess your point is that its not just haltpoll that has a problem,
but also regular polling -- and maybe the right thing to do would be to
disable polling if the event stream is disabled.

> Does the code above break any other architecture?

Me (and others) have so far tested x86, ARM64 (with/without the
event stream), and I believe riscv. I haven't seen any obvious
breakage. But, that's probably because most of the time somebody would
be set TIF_NEED_RESCHED.

> I'd say if you want
> something like this, better introduce a new smp_cond_load_timeout()
> API. The above looks like a hack that may only work on arm64 when the
> event stream is enabled.

I had a preliminary version of smp_cond_load_relaxed_timeout() here:
 https://lore.kernel.org/lkml/87edae3a1x.fsf@xxxxxxxxxx/

Even with an smp_cond_load_timeout(), we would need to fallback to
something like the above for uarchs without WFxT.

> A generic option is udelay() (on arm64 it would use WFE/WFET by
> default). Not sure how important it is for poll_idle() but the downside
> of udelay() that it won't be able to also poll need_resched() while
> waiting for the timeout. If this matters, you could instead make smaller
> udelay() calls. Yet another problem, I don't know how energy efficient
> udelay() is on x86 vs cpu_relax().
>
> So maybe an smp_cond_load_timeout() would be better, implemented with
> cpu_relax() generically and the arm64 would use LDXR, WFE and rely on
> the event stream (or fall back to cpu_relax() if the event stream is
> disabled).

Yeah, something like that might work.

--
ankur