Re: [RFC] sched: Limit idle_balance() when it is being used too frequently

[Jason Low <jason.low2@hp.com>]

On Tue, 2013-07-16 at 22:20 +0200, Peter Zijlstra wrote:
> On Tue, Jul 16, 2013 at 12:21:03PM -0700, Jason Low wrote:
> > When running benchmarks on an 8 socket 80 core machine with a 3.10 kernel,
> > there can be a lot of contention in idle_balance() and related functions.
> > On many AIM7 workloads in which CPUs go idle very often and idle balance
> > gets called a lot, it is actually lowering performance.
> > 
> > Since idle balance often helps performance (when it is not overused), I
> > looked into trying to avoid attempting idle balance only when it is
> > occurring too frequently.
> > 
> > This RFC patch attempts to keep track of the approximate "average" time between
> > idle balance attempts per CPU. Each time the idle_balance() function is
> > invoked, it will compute the duration since the last idle_balance() for
> > the current CPU. The avg time between idle balance attempts is then updated
> > using a very similar method as how rq->avg_idle is computed. 
> > 
> > Once the average time between idle balance attempts drops below a certain
> > value (which in this patch is sysctl_sched_idle_balance_limit), idle_balance
> > for that CPU will be skipped. The average time between idle balances will
> > continue to be updated, even if it ends up getting skipped. The
> > initial/maximum average is set a lot higher though to make sure that the
> > avg doesn't fall below the threshold until the sample size is large and to
> > prevent the avg from being overestimated.
> 
> One of the things I've been talking about for a while now is how I'd
> like to use the idle guestimator used for cpuidle for newidle balance.
> 
> Basically based on the estimated idle time limit how far/wide you'll
> search for tasks to run.
> 
> You can remove the sysctl and auto-tune by measuring how long it takes
> on avg to do a newidle balance.

Hi Peter,

When you say how long it takes on avg to do a newidle balance, are you
referring to the avg time it takes for each call to CPU_NEWLY_IDLE
load_balance() to complete, or the avg time it takes for newidle balance
attempts within a domain to eventually successfully pull/move a task(s)?

Thanks,
Jason