Commit 5aaa0b7a authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

sched/nohz: Fix rq->cpu_load calculations some more 



Follow up on commit 556061b0 ("sched/nohz: Fix rq->cpu_load[]
calculations") since while that fixed the busy case it regressed the
mostly idle case.

Add a callback from the nohz exit to also age the rq->cpu_load[]
array. This closes the hole where either there was no nohz load
balance pass during the nohz, or there was a 'significant' amount of
idle time between the last nohz balance and the nohz exit.

So we'll update unconditionally from the tick to not insert any
accidental 0 load periods while busy, and we try and catch up from
nohz idle balance and nohz exit. Both these are still prone to missing
a jiffy, but that has always been the case.

Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: pjt@google.com
Cc: Venkatesh Pallipadi <venki@google.com>
Link: http://lkml.kernel.org/n/tip-kt0trz0apodbf84ucjfdbr1a@git.kernel.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 9f646389

include/linux/sched.h

+1 −0
Original line number Diff line number Diff line
@@ -145,6 +145,7 @@ extern unsigned long this_cpu_load(void); 




extern void calc_global_load(unsigned long ticks);

extern void update_cpu_load_nohz(void);



extern unsigned long get_parent_ip(unsigned long addr);

kernel/sched/core.c

+43 −10
Original line number Diff line number Diff line
@@ -2517,25 +2517,32 @@ static void __update_cpu_load(struct rq *this_rq, unsigned long this_load, 
	sched_avg_update(this_rq);

}



#ifdef CONFIG_NO_HZ

/*

 * There is no sane way to deal with nohz on smp when using jiffies because the

 * cpu doing the jiffies update might drift wrt the cpu doing the jiffy reading

 * causing off-by-one errors in observed deltas; {0,2} instead of {1,1}.

 *

 * Therefore we cannot use the delta approach from the regular tick since that

 * would seriously skew the load calculation. However we'll make do for those

 * updates happening while idle (nohz_idle_balance) or coming out of idle

 * (tick_nohz_idle_exit).

 *

 * This means we might still be one tick off for nohz periods.

 */



/*

 * Called from nohz_idle_balance() to update the load ratings before doing the

 * idle balance.

 */

void update_idle_cpu_load(struct rq *this_rq)

{

	unsigned long curr_jiffies = jiffies;

	unsigned long curr_jiffies = ACCESS_ONCE(jiffies);

	unsigned long load = this_rq->load.weight;

	unsigned long pending_updates;



	/*

	 * Bloody broken means of dealing with nohz, but better than nothing..

	 * jiffies is updated by one cpu, another cpu can drift wrt the jiffy

	 * update and see 0 difference the one time and 2 the next, even though

	 * we ticked at roughtly the same rate.

	 *

	 * Hence we only use this from nohz_idle_balance() and skip this

	 * nonsense when called from the scheduler_tick() since that's

	 * guaranteed a stable rate.

	 * bail if there's load or we're actually up-to-date.

	 */

	if (load || curr_jiffies == this_rq->last_load_update_tick)

		return;
@@ -2546,13 +2553,39 @@ void update_idle_cpu_load(struct rq *this_rq) 
	__update_cpu_load(this_rq, load, pending_updates);

}



/*

 * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed.

 */

void update_cpu_load_nohz(void)

{

	struct rq *this_rq = this_rq();

	unsigned long curr_jiffies = ACCESS_ONCE(jiffies);

	unsigned long pending_updates;



	if (curr_jiffies == this_rq->last_load_update_tick)

		return;



	raw_spin_lock(&this_rq->lock);

	pending_updates = curr_jiffies - this_rq->last_load_update_tick;

	if (pending_updates) {

		this_rq->last_load_update_tick = curr_jiffies;

		/*

		 * We were idle, this means load 0, the current load might be

		 * !0 due to remote wakeups and the sort.

		 */

		__update_cpu_load(this_rq, 0, pending_updates);

	}

	raw_spin_unlock(&this_rq->lock);

}

#endif /* CONFIG_NO_HZ */



/*

 * Called from scheduler_tick()

 */

static void update_cpu_load_active(struct rq *this_rq)

{

	/*

	 * See the mess in update_idle_cpu_load().

	 * See the mess around update_idle_cpu_load() / update_cpu_load_nohz().

	 */

	this_rq->last_load_update_tick = jiffies;

	__update_cpu_load(this_rq, this_rq->load.weight, 1);

kernel/time/tick-sched.c

+1 −0
Original line number Diff line number Diff line
@@ -576,6 +576,7 @@ void tick_nohz_idle_exit(void) 
	/* Update jiffies first */

	select_nohz_load_balancer(0);

	tick_do_update_jiffies64(now);

	update_cpu_load_nohz();



#ifndef CONFIG_VIRT_CPU_ACCOUNTING

	/*

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