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Wysocki" , Dietmar Eggemann , Daniel Bristot de Oliveira , kernel-team@android.com, Oleg Nesterov Subject: Re: [RFC][PATCH] freezer,sched: Rewrite core freezer logic Message-ID: References: <20210525151432.16875-1-will@kernel.org> <20210525151432.16875-17-will@kernel.org> <20210602125452.GG30593@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20210602125452.GG30593@willie-the-truck> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Jun 02, 2021 at 01:54:53PM +0100, Will Deacon wrote: > There's also Documentation/power/freezing-of-tasks.rst to update. I'm not Since it's .rst, the only update I'm willing to do is delete it outright. > sure if fs/proc/array.c should be updated to display frozen tasks; I > couldn't see how that was useful, but thought I'd mention it anyway. Yeah, I considered it too, but I figured that if we're all frozen there's noone left to observe us being frozen, so I didn't bother. > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > index 2982cfab1ae9..bfadc1dbcf24 100644 > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -95,7 +95,12 @@ struct task_group; > > #define TASK_WAKING 0x0200 > > #define TASK_NOLOAD 0x0400 > > #define TASK_NEW 0x0800 > > -#define TASK_STATE_MAX 0x1000 > > +#define TASK_FREEZABLE 0x1000 > > +#define __TASK_FREEZABLE_UNSAFE 0x2000 > > Give that this is only needed to avoid lockdep checks, maybe we should avoid > allocating the bit if lockdep is not enabled? Otherwise, people might start > to use it for other things. Something like #define __TASK_FREEZABLE_UNSAFE (0x2000 * IS_ENABLED(CONFIG_LOCKDEP)) ? > > +#define TASK_FROZEN 0x4000 > > +#define TASK_STATE_MAX 0x8000 > > + > > +#define TASK_FREEZABLE_UNSAFE (TASK_FREEZABLE | __TASK_FREEZABLE_UNSAFE) > > We probably want to preserve the "DO NOT ADD ANY NEW CALLERS OF THIS STATE" > comment for the unsafe stuff. Done. > > +/* Recursion relies on tail-call optimization to not blow away the stack */ > > +static bool __frozen(struct task_struct *p) > > +{ > > + if (p->state == TASK_FROZEN) > > + return true; > > READ_ONCE()? task_struct::state is volatile -- for now. I've got other patches to deal with that. > > + > > + /* > > + * If stuck in TRACED, and the ptracer is FROZEN, we're frozen too. > > + */ > > + if (task_is_traced(p)) > > + return frozen(rcu_dereference(p->parent)); > > + > > + /* > > + * If stuck in STOPPED and the parent is FROZEN, we're frozen too. > > + */ > > + if (task_is_stopped(p)) > > + return frozen(rcu_dereference(p->real_parent)); > > This looks convincing, but I really can't tell if we're missing anything. Yeah, Oleg would be the one to tell us I suppose. > > +static bool __freeze_task(struct task_struct *p) > > +{ > > + unsigned long flags; > > + unsigned int state; > > + bool frozen = false; > > + > > + raw_spin_lock_irqsave(&p->pi_lock, flags); > > + state = READ_ONCE(p->state); > > + if (state & TASK_FREEZABLE) { > > + /* > > + * Only TASK_NORMAL can be augmented with TASK_FREEZABLE, > > + * since they can suffer spurious wakeups. > > + */ > > + WARN_ON_ONCE(!(state & TASK_NORMAL)); > > + > > +#ifdef CONFIG_LOCKDEP > > + /* > > + * It's dangerous to freeze with locks held; there be dragons there. > > + */ > > + if (!(state & __TASK_FREEZABLE_UNSAFE)) > > + WARN_ON_ONCE(debug_locks && p->lockdep_depth); > > +#endif > > + > > + p->state = TASK_FROZEN; > > + frozen = true; > > + } > > + raw_spin_unlock_irqrestore(&p->pi_lock, flags); > > + > > + return frozen; > > +} > > + > > /** > > * freeze_task - send a freeze request to given task > > * @p: task to send the request to > > @@ -116,20 +173,8 @@ bool freeze_task(struct task_struct *p) > > { > > unsigned long flags; > > > > spin_lock_irqsave(&freezer_lock, flags); > > + if (!freezing(p) || frozen(p) || __freeze_task(p)) { > > spin_unlock_irqrestore(&freezer_lock, flags); > > return false; > > } > > I've been trying to figure out how this serialises with ttwu(), given that > frozen(p) will go and read p->state. I suppose it works out because only the > freezer can wake up tasks from the FROZEN state, but it feels a bit brittle. p->pi_lock; both ttwu() and __freeze_task() (which is essentially a variant of set_special_state()) take ->pi_lock. I'll put in a comment. > > @@ -137,7 +182,7 @@ bool freeze_task(struct task_struct *p) > > if (!(p->flags & PF_KTHREAD)) > > fake_signal_wake_up(p); > > else > > - wake_up_state(p, TASK_INTERRUPTIBLE); > > + wake_up_state(p, TASK_INTERRUPTIBLE); // TASK_NORMAL ?!? > > > > spin_unlock_irqrestore(&freezer_lock, flags); > > return true; > > @@ -148,8 +193,8 @@ void __thaw_task(struct task_struct *p) > > unsigned long flags; > > > > spin_lock_irqsave(&freezer_lock, flags); > > - if (frozen(p)) > > - wake_up_process(p); > > + WARN_ON_ONCE(freezing(p)); > > + wake_up_state(p, TASK_FROZEN | TASK_NORMAL); > > Why do we need TASK_NORMAL here? It's a left-over from hacking, but I left it in because anything TASK_NORMAL should be able to deal with spuriuos wakeups, something try_to_freeze() now also relies on. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel