Date:   Tue, 29 Aug 2023 17:33:37 -0700
From:   Elliot Berman <quic_eberman@...cinc.com>
To:     Pavan Kondeti <quic_pkondeti@...cinc.com>
CC:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "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] freezer,sched: Use saved_state to reduce some spurious
 wakeups



On 8/28/2023 10:22 PM, Pavan Kondeti wrote:
> On Mon, Aug 28, 2023 at 10:33:04AM -0700, Elliot Berman wrote:
>> After commit f5d39b020809 ("freezer,sched: Rewrite core freezer logic"),
>> tasks that are in TASK_FREEZABLE state and end up getting frozen are
> 
> TASK_FREEZABLE state and what? Pls check once.
> 
>> always woken up. Prior to that commit, tasks could ask freezer to
>> consider them "frozen enough" via freezer_do_not_conut(). As described
>> in Peter's commit, the reason for this change is to prevent these tasks
>> from being woken before SMP is back. The commit introduced a
>> TASK_FREEZABLE state which allows freezer to immediately mark the task
>> as TASK_FROZEN without waking up the task. On the thaw path, the task is
>> woken up even if the task didn't need to wake up and goes back to its
>> TASK_(UN)INTERRUPTIBLE state. Although these tasks are capable of
>> handling of the wakeup, we can observe a power/perf impact from the
>> extra wakeup.
>>
>> We observed on Android many tasks wait in the TASK_FREEZABLE state
>> (particularly due to many of them being binder clients). We observed
>> nearly 4x the number of tasks and a corresponding (almost) linear increase in
>> latency and power consumption when thawing the system. The latency
>> increased from ~15ms to ~50ms.
>>
>> Save the state of TASK_FREEZABLE tasks and restore it after thawing the
>> task without waking the task up. If the task received a wake up for the
>> saved_state before thawing, then the task is still woken upon thawing.
>>
>> Re-use saved_state from RT sleeping spinlocks because freezer doesn't
>> consider TASK_RTLOCK_WAIT freezable.
>>
>> Reported-by: Prakash Viswalingam <quic_prakashv@...cinc.com>
>> Signed-off-by: Elliot Berman <quic_eberman@...cinc.com>
>> ---
>> For testing purposes, I use these commands can help see how many tasks were
>> woken during thawing:
>>
>> 1. Setup:
>>     mkdir /sys/kernel/tracing/instances/freezer
>>     cd /sys/kernel/tracing/instances/freezer
>>     echo 0 > tracing_on ; echo > trace
>>     echo power:suspend_resume > set_event
>>     echo 'enable_event:sched:sched_wakeup if action == \"thaw_processes\" && start == 1' > events/power/suspend_resume/trigger
>>     echo 'traceoff if action == \"thaw_processes\" && start == 0' > events/power/suspend_resume/trigger
>>     echo 1 > tracing_on
>>
>> 2. Let kernel go to suspend
>>
>> 3. After kernel's back up:
>>     cat /sys/kernel/tracing/instances/freezer/trace | grep sched_wakeup | grep -o "pid=[0-9]*" | sort -u | wc -l
>> ---
>>   include/linux/sched.h |  4 ++--
>>   kernel/freezer.c      | 15 +++++++++++++--
>>   kernel/sched/core.c   | 21 +++++++++++++--------
>>   3 files changed, 28 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index eed5d65b8d1f..e4ade5a18df2 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -746,8 +746,8 @@ struct task_struct {
>>   #endif
>>   	unsigned int			__state;
>>   
>> -#ifdef CONFIG_PREEMPT_RT
>> -	/* saved state for "spinlock sleepers" */
>> +#if IS_ENABLED(CONFIG_PREEMPT_RT) || IS_ENABLED(CONFIG_FREEZER)
>> +	/* saved state for "spinlock sleepers" and freezer */
>>   	unsigned int			saved_state;
>>   #endif
>>   
>> diff --git a/kernel/freezer.c b/kernel/freezer.c
>> index 4fad0e6fca64..6222cbfd97ab 100644
>> --- a/kernel/freezer.c
>> +++ b/kernel/freezer.c
>> @@ -71,7 +71,11 @@ bool __refrigerator(bool check_kthr_stop)
>>   	for (;;) {
>>   		bool freeze;
>>   
>> +		raw_spin_lock_irq(&current->pi_lock);
>>   		set_current_state(TASK_FROZEN);
>> +		/* unstale saved_state so that __thaw_task() will wake us up */
>> +		current->saved_state = TASK_RUNNING;
>> +		raw_spin_unlock_irq(&current->pi_lock);
>>   
>>   		spin_lock_irq(&freezer_lock);
>>   		freeze = freezing(current) && !(check_kthr_stop && kthread_should_stop());
>> @@ -129,6 +133,7 @@ static int __set_task_frozen(struct task_struct *p, void *arg)
>>   		WARN_ON_ONCE(debug_locks && p->lockdep_depth);
>>   #endif
>>   
>> +	p->saved_state = p->__state;
>>   	WRITE_ONCE(p->__state, TASK_FROZEN);
>>   	return TASK_FROZEN;
>>   }
>> @@ -174,10 +179,16 @@ bool freeze_task(struct task_struct *p)
>>    * state in p->jobctl. If either of them got a wakeup that was missed because
>>    * TASK_FROZEN, then their canonical state reflects that and the below will
>>    * refuse to restore the special state and instead issue the wakeup.
>> + *
>> + * Otherwise, restore the saved_state before the task entered freezer. For
>> + * typical tasks in the __refrigerator(), saved_state == 0 so nothing happens
>> + * here. For tasks which were TASK_NORMAL | TASK_FREEZABLE, their initial state
>> + * is returned unless they got an expected wakeup. Then they will be woken up as
>> + * TASK_FROZEN back in __thaw_task().
>>    */
> 
> Thanks for the detailed comment. The change looks good to me.
> 
>>   static int __set_task_special(struct task_struct *p, void *arg)
>>   {
>> -	unsigned int state = 0;
>> +	unsigned int state = p->saved_state;
>>   
>>   	if (p->jobctl & JOBCTL_TRACED)
>>   		state = TASK_TRACED;
>> @@ -188,7 +199,7 @@ static int __set_task_special(struct task_struct *p, void *arg)
>>   	if (state)
>>   		WRITE_ONCE(p->__state, state);
>>   
>> -	return state;
>> +	return state & ~TASK_FROZEN;
>>   }
> 
> void __thaw_task(struct task_struct *p)
> {
> 
> ...
> 
> 	if (lock_task_sighand(p, &flags2)) {
> 		/* TASK_FROZEN -> TASK_{STOPPED,TRACED} */
> 		bool ret = task_call_func(p, __set_task_special, NULL);
> 		unlock_task_sighand(p, &flags2);
> 		if (ret)
> 			goto unlock;
> 	}
> 
> 	wake_up_state(p, TASK_FROZEN);
> unlock:
> 	spin_unlock_irqrestore(&freezer_lock, flags);
> }
> 
> 
> The comment there about task change needs update. I feel the "ret"
> should be renamed approriately to indicate whether wakeup is needed
> or not.
> 
> Now that we have saved_state capturing the previous and any state change
> while task is frozen, can that be used and remove the job control and
> associated locking here? for ex: if saved_state is running, we need to
> wakeup otherwise, simply restore the __state from saved_state.
> 
>>   
>>   void __thaw_task(struct task_struct *p)
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index a68d1276bab0..815d955764a5 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -3992,13 +3992,17 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
>>    * The caller holds p::pi_lock if p != current or has preemption
>>    * disabled when p == current.
>>    *
>> - * The rules of PREEMPT_RT saved_state:
>> + * The rules of saved_state:
>>    *
>>    *   The related locking code always holds p::pi_lock when updating
>>    *   p::saved_state, which means the code is fully serialized in both cases.
>>    *
>> - *   The lock wait and lock wakeups happen via TASK_RTLOCK_WAIT. No other
>> - *   bits set. This allows to distinguish all wakeup scenarios.
>> + *   For PREEMPT_RT, the lock wait and lock wakeups happen via TASK_RTLOCK_WAIT.
>> + *   No other bits set. This allows to distinguish all wakeup scenarios.
>> + *
>> + *   For FREEZER, the wakeup happens via TASK_FROZEN. No other bits set. This
>> + *   allows us to prevent early wakeup of tasks before they can be run on
>> + *   asymmetric ISA architectures (eg ARMv9).
>>    */
>>   static __always_inline
>>   bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
>> @@ -4013,13 +4017,14 @@ bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
>>   		return true;
>>   	}
>>   
>> -#ifdef CONFIG_PREEMPT_RT
>> +#if IS_ENABLED(CONFIG_PREEMPT_RT) || IS_ENABLED(CONFIG_FREEZER)
>>   	/*
>>   	 * Saved state preserves the task state across blocking on
>> -	 * an RT lock.  If the state matches, set p::saved_state to
>> -	 * TASK_RUNNING, but do not wake the task because it waits
>> -	 * for a lock wakeup. Also indicate success because from
>> -	 * the regular waker's point of view this has succeeded.
>> +	 * an RT lock or TASK_FREEZABLE tasks.  If the state matches,
>> +	 * set p::saved_state to TASK_RUNNING, but do not wake the task
>> +	 * because it waits for a lock wakeup or __thaw_task(). Also
>> +	 * indicate success because from the regular waker's point of
>> +	 * view this has succeeded.
>>   	 *
>>   	 * After acquiring the lock the task will restore p::__state
>>   	 * from p::saved_state which ensures that the regular
>>
>> ---
>> base-commit: 6995e2de6891c724bfeb2db33d7b87775f913ad1
>> change-id: 20230817-avoid-spurious-freezer-wakeups-9f8619680b3a
> 
> Your patch seems based on v6.4. You might want to resend the patch on
> v6.5 to take the below commit into account.
> 
> 1c06918788e8a ("sched: Consider task_struct::saved_state in
> wait_task_inactive()")

Thanks for pointing this out! I am pretty sure that with that change, we 
can remove the checks for the jobctl and also remove the 
lock_task_sighand(). I'll run some tests.

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