Commit 49c022e6 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

sched: Clean up rebalance_domains() load-balance interval calculation 



Instead of the possible multiple-evaluation of num_online_cpus()
in rebalance_domains() that Linus reported, avoid it altogether
in the normal case since it's implemented with a Hamming weight
function over a cpu bitmask which can be darn expensive for those
with big iron.

This also makes it cleaner, smaller and documents the code.

Reported-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1301991265.2225.12.camel@twins>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent b2a8b4b8

kernel/sched.c

+3 −0
Original line number Diff line number Diff line
@@ -6331,6 +6331,9 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu) 
		break;

#endif

	}



	update_max_interval();



	return NOTIFY_OK;

}

kernel/sched_fair.c

+12 −4
Original line number Diff line number Diff line
@@ -3820,6 +3820,17 @@ void select_nohz_load_balancer(int stop_tick) 


static DEFINE_SPINLOCK(balancing);



static unsigned long __read_mostly max_load_balance_interval = HZ/10;



/*

 * Scale the max load_balance interval with the number of CPUs in the system.

 * This trades load-balance latency on larger machines for less cross talk.

 */

static void update_max_interval(void)

{

	max_load_balance_interval = HZ*num_online_cpus()/10;

}



/*

 * It checks each scheduling domain to see if it is due to be balanced,

 * and initiates a balancing operation if so.
@@ -3849,10 +3860,7 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle) 


		/* scale ms to jiffies */

		interval = msecs_to_jiffies(interval);

		if (unlikely(!interval))

			interval = 1;

		if (interval > HZ*num_online_cpus()/10)

			interval = HZ*num_online_cpus()/10;

		interval = clamp(interval, 1UL, max_load_balance_interval);



		need_serialize = sd->flags & SD_SERIALIZE;

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