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Commit f6d3093d authored by Rafael J. Wysocki's avatar Rafael J. Wysocki Committed by Todd Kjos
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BACKPORT: sched: idle: Do not stop the tick upfront in the idle loop 

Push the decision whether or not to stop the tick somewhat deeper
into the idle loop.

Stopping the tick upfront leads to unpleasant outcomes in case the
idle governor doesn't agree with the nohz code on the duration of the
upcoming idle period.  Specifically, if the tick has been stopped and
the idle governor predicts short idle, the situation is bad regardless
of whether or not the prediction is accurate.  If it is accurate, the
tick has been stopped unnecessarily which means excessive overhead.
If it is not accurate, the CPU is likely to spend too much time in
the (shallow, because short idle has been predicted) idle state
selected by the governor [1].

As the first step towards addressing this problem, change the code
to make the tick stopping decision inside of the loop in do_idle().
In particular, do not stop the tick in the cpu_idle_poll() code path.
Also don't do that in tick_nohz_irq_exit() which doesn't really have
enough information on whether or not to stop the tick.

Cherry-picked from 2aaf709a518d26563b80fd7a42379d7aa7ffed4a

 - Fixed conflict with tick_nohz_stopped_cpu not appearing in the
   chunk context. Trivial fix.

Change-Id: Ie316e210971f28aae80dd0f35c5564cb17a81901
Link: https://marc.info/?l=linux-pm&m=150116085925208&w=2 # [1]
Link: https://tu-dresden.de/zih/forschung/ressourcen/dateien/projekte/haec/powernightmares.pdf


Suggested-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Todd Kjos <tkjos@google.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Paven Kondati <pkondeti@qti.qualcomm.com>
Signed-off-by: default avatarDaniel Lezcano <daniel.lezcano@linaro.org>
parent 55e591cc

include/linux/tick.h

+2 −0
Original line number Diff line number Diff line
@@ -115,6 +115,7 @@ enum tick_dep_bits { 
extern bool tick_nohz_enabled;

extern int tick_nohz_tick_stopped(void);

extern void tick_nohz_idle_stop_tick(void);

extern void tick_nohz_idle_restart_tick(void);

extern void tick_nohz_idle_enter(void);

extern void tick_nohz_idle_exit(void);

extern void tick_nohz_irq_exit(void);
@@ -135,6 +136,7 @@ static inline void tick_nohz_idle_stop_tick_protected(void) 
#define tick_nohz_enabled (0)

static inline int tick_nohz_tick_stopped(void) { return 0; }

static inline void tick_nohz_idle_stop_tick(void) { }

static inline void tick_nohz_idle_restart_tick(void) { }

static inline void tick_nohz_idle_enter(void) { }

static inline void tick_nohz_idle_exit(void) { }

kernel/sched/idle.c

+6 −3
Original line number Diff line number Diff line
@@ -222,13 +222,13 @@ static void do_idle(void) 
	__current_set_polling();

	quiet_vmstat();

	tick_nohz_idle_enter();

	tick_nohz_idle_stop_tick_protected();



	while (!need_resched()) {

		check_pgt_cache();

		rmb();



		if (cpu_is_offline(smp_processor_id())) {

			tick_nohz_idle_stop_tick_protected();

			cpuhp_report_idle_dead();

			arch_cpu_idle_dead();

		}
@@ -242,10 +242,13 @@ static void do_idle(void) 
		 * broadcast device expired for us, we don't want to go deep

		 * idle as we know that the IPI is going to arrive right away.

		 */

		if (cpu_idle_force_poll || tick_check_broadcast_expired())

		if (cpu_idle_force_poll || tick_check_broadcast_expired()) {

			tick_nohz_idle_restart_tick();

			cpu_idle_poll();

		else

		} else {

			tick_nohz_idle_stop_tick();

			cpuidle_idle_call();

		}

		arch_cpu_idle_exit();

	}

kernel/time/tick-sched.c

+18 −8
Original line number Diff line number Diff line
@@ -1009,13 +1009,11 @@ void tick_nohz_irq_exit(void) 
{

	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);



	if (ts->inidle) {

	if (ts->inidle)

		tick_nohz_start_idle(ts);

		__tick_nohz_idle_stop_tick(ts);

	} else {

	else

		tick_nohz_full_update_tick(ts);

}

}



/**

 * tick_nohz_get_sleep_length - return the length of the current sleep
@@ -1075,6 +1073,20 @@ static void tick_nohz_account_idle_ticks(struct tick_sched *ts) 
#endif

}



static void __tick_nohz_idle_restart_tick(struct tick_sched *ts, ktime_t now)

{

	tick_nohz_restart_sched_tick(ts, now);

	tick_nohz_account_idle_ticks(ts);

}



void tick_nohz_idle_restart_tick(void)

{

	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);



	if (ts->tick_stopped)

		__tick_nohz_idle_restart_tick(ts, ktime_get());

}



/**

 * tick_nohz_idle_exit - restart the idle tick from the idle task

 *
@@ -1099,10 +1111,8 @@ void tick_nohz_idle_exit(void) 
	if (ts->idle_active)

		tick_nohz_stop_idle(ts, now);



	if (ts->tick_stopped) {

		tick_nohz_restart_sched_tick(ts, now);

		tick_nohz_account_idle_ticks(ts);

	}

	if (ts->tick_stopped)

		__tick_nohz_idle_restart_tick(ts, now);



	local_irq_enable();

}