Re: [PATCH v2] freezer,sched: Use saved_state to reduce some spurious wakeups

[Elliot Berman <quic_eberman@quicinc.com>]

On 9/4/2023 2:23 PM, Peter Zijlstra wrote:
> On Wed, Aug 30, 2023 at 10:42:39AM -0700, Elliot Berman wrote:
> 
>> Avoid the spurious wakeups by saving the state of TASK_FREEZABLE tasks.
>> If the task was running before entering TASK_FROZEN state
>> (__refrigerator()) or if the task received a wake up for the saved
>> state, then the task is woken on thaw. saved_state from PREEMPT_RT locks
>> can be re-used because freezer would not stomp on the rtlock wait flow:
>> TASK_RTLOCK_WAIT isn't considered freezable.
> 
> You don't actually assert that anywhere I think, so the moment someone
> makes that happen you crash and burn.
> 

I can certainly add an assertion on the freezer side.

> Also:
> 
>> -#ifdef CONFIG_PREEMPT_RT
>> +#if IS_ENABLED(CONFIG_PREEMPT_RT) || IS_ENABLED(CONFIG_FREEZER)
> 
> That makes wakeup more horrible for everyone :/

I don't think the hot wakeup path is significantly impacted because the 
added checks come after the hot path is already not taken.

wait_task_inactive() is impacted in the case of contention on pi_lock, 
but I don't think that is part of any hot path.

I'll run some further tests on my end to be sure about the wake up 
latency. Are there any benchmarks/tests you like for measuring the hot 
path? I can run those as well.

Thanks,
Elliot