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Commit 8cb75e0c authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

sched/preempt: Fix up missed PREEMPT_NEED_RESCHED folding 



With various drivers wanting to inject idle time; we get people
calling idle routines outside of the idle loop proper.

Therefore we need to be extra careful about not missing
TIF_NEED_RESCHED -> PREEMPT_NEED_RESCHED propagations.

While looking at this, I also realized there's a small window in the
existing idle loop where we can miss TIF_NEED_RESCHED; when it hits
right after the tif_need_resched() test at the end of the loop but
right before the need_resched() test at the start of the loop.

So move preempt_fold_need_resched() out of the loop where we're
guaranteed to have TIF_NEED_RESCHED set.

Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-x9jgh45oeayzajz2mjt0y7d6@git.kernel.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent c9c89868

arch/x86/include/asm/mwait.h

+1 −1
Original line number Diff line number Diff line
@@ -53,7 +53,7 @@ static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx) 
		if (!need_resched())

			__mwait(eax, ecx);

	}

	__current_clr_polling();

	current_clr_polling();

}



#endif /* _ASM_X86_MWAIT_H */

include/linux/preempt.h

+15 −0
Original line number Diff line number Diff line
@@ -134,6 +134,21 @@ do { \ 
#undef preempt_check_resched

#endif



#ifdef CONFIG_PREEMPT

#define preempt_set_need_resched() \

do { \

	set_preempt_need_resched(); \

} while (0)

#define preempt_fold_need_resched() \

do { \

	if (tif_need_resched()) \

		set_preempt_need_resched(); \

} while (0)

#else

#define preempt_set_need_resched() do { } while (0)

#define preempt_fold_need_resched() do { } while (0)

#endif



#ifdef CONFIG_PREEMPT_NOTIFIERS



struct preempt_notifier;

include/linux/sched.h

+15 −0
Original line number Diff line number Diff line
@@ -2745,6 +2745,21 @@ static inline bool __must_check current_clr_polling_and_test(void) 
}

#endif



static inline void current_clr_polling(void)

{

	__current_clr_polling();



	/*

	 * Ensure we check TIF_NEED_RESCHED after we clear the polling bit.

	 * Once the bit is cleared, we'll get IPIs with every new

	 * TIF_NEED_RESCHED and the IPI handler, scheduler_ipi(), will also

	 * fold.

	 */

	smp_mb(); /* paired with resched_task() */



	preempt_fold_need_resched();

}



static __always_inline bool need_resched(void)

{

	return unlikely(tif_need_resched());

kernel/cpu/idle.c

+10 −7
Original line number Diff line number Diff line
@@ -105,14 +105,17 @@ static void cpu_idle_loop(void) 
				__current_set_polling();

			}

			arch_cpu_idle_exit();

		}



		/*

			 * We need to test and propagate the TIF_NEED_RESCHED

			 * bit here because we might not have send the

			 * reschedule IPI to idle tasks.

		 * Since we fell out of the loop above, we know

		 * TIF_NEED_RESCHED must be set, propagate it into

		 * PREEMPT_NEED_RESCHED.

		 *

		 * This is required because for polling idle loops we will

		 * not have had an IPI to fold the state for us.

		 */

			if (tif_need_resched())

				set_preempt_need_resched();

		}

		preempt_set_need_resched();

		tick_nohz_idle_exit();

		schedule_preempt_disabled();

	}

kernel/sched/core.c

+1 −2
Original line number Diff line number Diff line
@@ -1510,8 +1510,7 @@ void scheduler_ipi(void) 
	 * TIF_NEED_RESCHED remotely (for the first time) will also send

	 * this IPI.

	 */

	if (tif_need_resched())

		set_preempt_need_resched();

	preempt_fold_need_resched();



	if (llist_empty(&this_rq()->wake_list)

			&& !tick_nohz_full_cpu(smp_processor_id())