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Message-ID: <1b3650c5-822e-4789-81d2-0304573cabd9@oracle.com>
Date: Sun, 28 Jan 2024 23:22:50 +0200
From: Mihai Carabas <mihai.carabas@...cle.com>
To: Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
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ankur.a.arora@...cle.com
Subject: Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with
smp_cond_load_relaxed
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@...radead.org>
>> Signed-off-by: Mihai Carabas <mihai.carabas@...cle.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(¤t_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
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