linux-kernel - Re: [PATCH v6 01/10] cpuidle/poll_state: poll via smp_cond_load

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Message-ID: <8734n9s717.fsf@oracle.com>
Date: Mon, 12 Aug 2024 15:36:20 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: "Tomohiro Misono (Fujitsu)" <misono.tomohiro@...itsu.com>
Cc: 'Ankur Arora' <ankur.a.arora@...cle.com>,
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Subject: Re: [PATCH v6 01/10] cpuidle/poll_state: poll via
 smp_cond_load_relaxed()


Tomohiro Misono (Fujitsu) <misono.tomohiro@...itsu.com> writes:

>> Subject: [PATCH v6 01/10] cpuidle/poll_state: poll via smp_cond_load_relaxed()
>>
>> From: Mihai Carabas <mihai.carabas@...cle.com>
>>
>> The inner loop in poll_idle() polls up to POLL_IDLE_RELAX_COUNT times,
>> checking to see if the thread has the TIF_NEED_RESCHED bit set. The
>> loop exits once the condition is met, or if the poll time limit has
>> been exceeded.
>>
>> To minimize the number of instructions executed each iteration, the
>> time check is done only infrequently (once every POLL_IDLE_RELAX_COUNT
>> iterations). In addition, each loop iteration executes cpu_relax()
>> which on certain platforms provides a hint to the pipeline that the
>> loop is busy-waiting, thus allowing the processor to reduce power
>> consumption.
>>
>> However, cpu_relax() is defined optimally only on x86. On arm64, for
>> instance, it is implemented as a YIELD which only serves a hint to the
>> CPU that it prioritize a different hardware thread if one is available.
>> arm64, however, does expose a more optimal polling mechanism via
>> smp_cond_load_relaxed() which uses LDXR, WFE to wait until a store
>> to a specified region.
>>
>> So restructure the loop, folding both checks in smp_cond_load_relaxed().
>> Also, move the time check to the head of the loop allowing it to exit
>> straight-away once TIF_NEED_RESCHED is set.
>>
>> Suggested-by: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Mihai Carabas <mihai.carabas@...cle.com>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>>  drivers/cpuidle/poll_state.c | 10 +++++-----
>>  1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index 9b6d90a72601..532e4ed19e0f 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -21,21 +21,21 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
>>
>>  	raw_local_irq_enable();
>>  	if (!current_set_polling_and_test()) {
>> -		unsigned int loop_count = 0;
>> +		unsigned int loop_count;
>>  		u64 limit;
>>
>>  		limit = cpuidle_poll_time(drv, dev);
>>
>>  		while (!need_resched()) {
>> -			cpu_relax();
>> -			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
>> -				continue;
>> -
>>  			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);
>>  		}
>>  	}
>>  	raw_local_irq_disable();
>> --
>> 2.43.5
>
> Reviewed-by: Misono Tomohiro <misono.tomohiro@...itsu.com>

Thanks!

--
ankur