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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: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1301991265.2225.12.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2011-04-05 10:14:25 +02:00 committed by Ingo Molnar
parent b2a8b4b819
commit 49c022e657
2 changed files with 15 additions and 4 deletions
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3
kernel/sched.c
View File

@ -6331,6 +6331,9 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
break;
#endif
}
update_max_interval();
return NOTIFY_OK;
}

16
kernel/sched_fair.c
View File

@ -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;