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Message-ID: <ceab7277-b95e-477c-9729-d46dc5be54c8@oracle.com>
Date: Mon, 29 Jan 2024 21:20:57 +0200
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v3 7/7] cpuidle/poll_state: replace cpu_relax with
 smp_cond_load_relaxed
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To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
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References: <1706524834-11275-1-git-send-email-mihai.carabas@oracle.com>
 <1706524834-11275-8-git-send-email-mihai.carabas@oracle.com>
 <CAJZ5v0gPoLan0M2A-x5V=ZNCxbYdv5e5DvNZTyo6Bd3e9HThYQ@mail.gmail.com>
From: Mihai Carabas <mihai.carabas@oracle.com>
In-Reply-To: <CAJZ5v0gPoLan0M2A-x5V=ZNCxbYdv5e5DvNZTyo6Bd3e9HThYQ@mail.gmail.com>
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La 29.01.2024 16:52, Rafael J. Wysocki a scris:
> On Mon, Jan 29, 2024 at 12:56 PM Mihai Carabas <mihai.carabas@oracle.com> 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));
> The inner parens are not necessary AFAICS.

Provides better reading. Do you want to remove these?

> Also, doesn't this return a value which can be used for checking if
> _TIF_NEED_RESCHED is set instead of the condition below?

Yes, indeed - should I modify this check? (somehow I wanted to preserve 
the original check)

Thank you,
Mihai

>> +
>> +                       if (loop_count < POLL_IDLE_RELAX_COUNT)
>> +                               break;
>> +
>>                          if (local_clock_noinstr() - time_start > limit) {
>>                                  dev->poll_time_limit = true;
>>                                  break;
>> --