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@xxxxxxxxxxx>
> Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
> ---
> 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(¤t->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(¤t->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,
Pavan
