From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755199AbeEANTP (ORCPT ); Tue, 1 May 2018 09:19:15 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:47884 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753800AbeEANTN (ORCPT ); Tue, 1 May 2018 09:19:13 -0400 Date: Tue, 1 May 2018 15:19:04 +0200 From: Peter Zijlstra To: "Kohli, Gaurav" Cc: tglx@linutronix.de, mpe@ellerman.id.au, mingo@kernel.org, bigeasy@linutronix.de, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, Neeraj Upadhyay , Will Deacon , Oleg Nesterov Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup Message-ID: <20180501131904.GG12217@hirez.programming.kicks-ass.net> References: <20180425200917.GZ4082@hirez.programming.kicks-ass.net> <20180426084131.GV4129@hirez.programming.kicks-ass.net> <20180426085719.GW4129@hirez.programming.kicks-ass.net> <4d3f68f8-e599-6b27-a2e8-9e96b401d57a@codeaurora.org> <20180430111744.GE4082@hirez.programming.kicks-ass.net> <3af3365b-4e3f-e388-8e90-45a3bd4120fd@codeaurora.org> <20180501101845.GE12217@hirez.programming.kicks-ass.net> <20180501113132.GF12217@hirez.programming.kicks-ass.net> <745d762d-9ab3-0749-9b87-9bb03d913071@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <745d762d-9ab3-0749-9b87-9bb03d913071@codeaurora.org> User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On 5/1/2018 5:01 PM, Peter Zijlstra wrote: > > Let me ponder what the best solution is, it's a bit of a mess. So there's: - TASK_PARKED, which we can make a special state; this solves the problem because then wait_task_inactive() is guaranteed to see TASK_PARKED and DTRT. - complete(&kthread->parked), which we can do inside schedule(); this solves the problem because then kthread_park() will not return early and the task really is blocked. and I'm fairly sure I thought of a 3rd way to cure things, but now that I'm writing things down I cannot seem to remember :/ -- could be we muck with wait_task_inactive(). In any case, I hate all of them, but I think the completion one is the least horrible because it gives the strongest guarantees and cleans up most. But promoting TASK_PARKED to special certainly is the earier patch. The below boots, but that's about all I did with it. Opinions? --- --- a/include/linux/kthread.h +++ b/include/linux/kthread.h @@ -62,6 +62,7 @@ void *kthread_probe_data(struct task_str int kthread_park(struct task_struct *k); void kthread_unpark(struct task_struct *k); void kthread_parkme(void); +void kthread_park_complete(struct task_struct *k); int kthreadd(void *unused); extern struct task_struct *kthreadd_task; --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -55,7 +55,6 @@ enum KTHREAD_BITS { KTHREAD_IS_PER_CPU = 0, KTHREAD_SHOULD_STOP, KTHREAD_SHOULD_PARK, - KTHREAD_IS_PARKED, }; static inline void set_kthread_struct(void *kthread) @@ -177,14 +176,12 @@ void *kthread_probe_data(struct task_str static void __kthread_parkme(struct kthread *self) { - __set_current_state(TASK_PARKED); - while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) { - if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags)) - complete(&self->parked); + for (;;) { + set_current_state(TASK_PARKED); + if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags)) + break; schedule(); - __set_current_state(TASK_PARKED); } - clear_bit(KTHREAD_IS_PARKED, &self->flags); __set_current_state(TASK_RUNNING); } @@ -194,6 +191,11 @@ void kthread_parkme(void) } EXPORT_SYMBOL_GPL(kthread_parkme); +void kthread_park_complete(struct task_struct *k) +{ + complete(&to_kthread(k)->parked); +} + static int kthread(void *_create) { /* Copy data: it's on kthread's stack */ @@ -450,22 +452,15 @@ void kthread_unpark(struct task_struct * { struct kthread *kthread = to_kthread(k); - clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags); /* - * We clear the IS_PARKED bit here as we don't wait - * until the task has left the park code. So if we'd - * park before that happens we'd see the IS_PARKED bit - * which might be about to be cleared. + * Newly created kthread was parked when the CPU was offline. + * The binding was lost and we need to set it again. */ - if (test_and_clear_bit(KTHREAD_IS_PARKED, &kthread->flags)) { - /* - * Newly created kthread was parked when the CPU was offline. - * The binding was lost and we need to set it again. - */ - if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags)) - __kthread_bind(k, kthread->cpu, TASK_PARKED); - wake_up_state(k, TASK_PARKED); - } + if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags)) + __kthread_bind(k, kthread->cpu, TASK_PARKED); + + clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags); + wake_up_state(k, TASK_PARKED); } EXPORT_SYMBOL_GPL(kthread_unpark); @@ -488,12 +483,13 @@ int kthread_park(struct task_struct *k) if (WARN_ON(k->flags & PF_EXITING)) return -ENOSYS; - if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) { - set_bit(KTHREAD_SHOULD_PARK, &kthread->flags); - if (k != current) { - wake_up_process(k); - wait_for_completion(&kthread->parked); - } + if (WARN_ON_ONCE(test_bit(KTHREAD_SHOULD_PARK, &kthread->flags))) + return -EBUSY; + + set_bit(KTHREAD_SHOULD_PARK, &kthread->flags); + if (k != current) { + wake_up_process(k); + wait_for_completion(&kthread->parked); } return 0; --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7,6 +7,8 @@ */ #include "sched.h" +#include + #include #include @@ -2718,20 +2720,28 @@ static struct rq *finish_task_switch(str membarrier_mm_sync_core_before_usermode(mm); mmdrop(mm); } - if (unlikely(prev_state == TASK_DEAD)) { - if (prev->sched_class->task_dead) - prev->sched_class->task_dead(prev); + if (unlikely(prev_state & (TASK_DEAD|TASK_PARKED))) { + switch (prev_state) { + case TASK_DEAD: + if (prev->sched_class->task_dead) + prev->sched_class->task_dead(prev); + + /* + * Remove function-return probe instances associated with this + * task and put them back on the free list. + */ + kprobe_flush_task(prev); - /* - * Remove function-return probe instances associated with this - * task and put them back on the free list. - */ - kprobe_flush_task(prev); + /* Task is done with its stack. */ + put_task_stack(prev); - /* Task is done with its stack. */ - put_task_stack(prev); + put_task_struct(prev); + break; - put_task_struct(prev); + case TASK_PARKED: + kthread_park_complete(prev); + break; + } } tick_nohz_task_switch();