Okanovic, Haris <harisokn@xxxxxxxxxx> writes:
> On Thu, 2024-02-15 at 09:41 +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@xxxxxxxxxxxxx>
>> Signed-off-by: Mihai Carabas <mihai.carabas@xxxxxxxxxx>
>> ---
>> drivers/cpuidle/poll_state.c | 15 ++++++++++-----
>> 1 file changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index 9b6d90a72601..1e45be906e72 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -13,6 +13,7 @@
>> static int __cpuidle poll_idle(struct cpuidle_device *dev,
>> struct cpuidle_driver *drv, int index)
>> {
>> + unsigned long ret;
>> u64 time_start;
>>
>> time_start = local_clock_noinstr();
>> @@ -26,12 +27,16 @@ 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;
>> +
>> + ret = smp_cond_load_relaxed(¤t_thread_info()->flags,
>> + VAL & _TIF_NEED_RESCHED ||
>> + loop_count++ >= POLL_IDLE_RELAX_COUNT);
>
> Is it necessary to repeat this 200 times with a wfe poll?
The POLL_IDLE_RELAX_COUNT is there because on x86 each cpu_relax()
iteration is much shorter.
With WFE, it makes less sense.
> Does kvm not implement a timeout period?
Not yet, but it does become more useful after a WFE haltpoll is
available on ARM64.
Haltpoll does have a timeout, which you should be able to tune via
/sys/module/haltpoll/parameters/ but that, of course, won't help here.
> Could you make it configurable? This patch improves certain workloads
> on AWS Graviton instances as well, but blocks up to 6ms in 200 * 30us
> increments before going to wfi, which is a bit excessive.
Yeah, this looks like a problem. We could solve it by making it an
architectural parameter. Though I worry about ARM platforms with
much smaller default timeouts.
The other possibility is using WFET in the primitive, but then we
have that dependency and that's a bigger change.
Will address this in the next version.
Thanks for pointing this out.
--
ankur
