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

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Message-ID: <df61af06-a43e-05c5-66e8-5a68b08ff14b@quicinc.com>
Date:   Mon, 4 Sep 2023 20:59:03 -0700
From:   Elliot Berman <quic_eberman@...cinc.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Ingo Molnar <mingo@...hat.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>,
        Thomas Gleixner <tglx@...utronix.de>, <kernel@...cinc.com>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-pm@...r.kernel.org>,
        "Prakash Viswalingam" <quic_prakashv@...cinc.com>
Subject: Re: [PATCH v2] freezer,sched: Use saved_state to reduce some spurious
 wakeups



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