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Date: Sun, 28 Jan 2024 23:22:50 +0200
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Subject: Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with
 smp_cond_load_relaxed
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To: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
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References: <1700488898-12431-1-git-send-email-mihai.carabas@oracle.com>
 <1700488898-12431-8-git-send-email-mihai.carabas@oracle.com>
 <20231211114642.GB24899@willie-the-truck>
From: Mihai Carabas <mihai.carabas@oracle.com>
In-Reply-To: <20231211114642.GB24899@willie-the-truck>
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La 11.12.2023 13:46, Will Deacon a scris:
> On Mon, Nov 20, 2023 at 04:01:38PM +0200, Mihai Carabas wrote:
>> cpu_relax on ARM64 does a simple "yield". Thus we replace it with
>> smp_cond_load_relaxed which basically does a "wfe".
>>
>> Suggested-by: Peter Zijlstra <peterz@infradead.org>
>> Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
>> ---
>>   drivers/cpuidle/poll_state.c | 14 +++++++++-----
>>   1 file changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index 9b6d90a72601..440cd713e39a 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -26,12 +26,16 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
>>   
>>   		limit = cpuidle_poll_time(drv, dev);
>>   
>> -		while (!need_resched()) {
>> -			cpu_relax();
>> -			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
>> -				continue;
>> -
>> +		for (;;) {
>>   			loop_count = 0;
>> +
>> +			smp_cond_load_relaxed(&current_thread_info()->flags,
>> +					      (VAL & _TIF_NEED_RESCHED) ||
>> +					      (loop_count++ >= POLL_IDLE_RELAX_COUNT));
>> +
>> +			if (loop_count < POLL_IDLE_RELAX_COUNT)
>> +				break;
>> +
>>   			if (local_clock_noinstr() - time_start > limit) {
>>   				dev->poll_time_limit = true;
>>   				break;
> Doesn't this make ARCH_HAS_CPU_RELAX a complete misnomer?

This controls the build of poll_state.c and the generic definition of 
smp_cond_load_relaxed (used by x86) is using cpu_relax(). Do you propose 
other approach here?


>
> Will