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Date: Mon, 09 Sep 2013 14:07:53 -0700 From: Jason Low <jason.low2@...com> To: Peter Zijlstra <peterz@...radead.org> Cc: mingo@...hat.com, linux-kernel@...r.kernel.org, efault@....de, pjt@...gle.com, preeti@...ux.vnet.ibm.com, akpm@...ux-foundation.org, mgorman@...e.de, riel@...hat.com, aswin@...com, scott.norton@...com, srikar@...ux.vnet.ibm.com Subject: Re: [RFC][PATCH v4 3/3] sched: Periodically decay max cost of idle balance On Mon, 2013-09-09 at 13:49 +0200, Peter Zijlstra wrote: > On Wed, Sep 04, 2013 at 12:10:01AM -0700, Jason Low wrote: > > On Fri, 2013-08-30 at 12:18 +0200, Peter Zijlstra wrote: > > > On Thu, Aug 29, 2013 at 01:05:36PM -0700, Jason Low wrote: > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > > index 58b0514..bba5a07 100644 > > > > --- a/kernel/sched/core.c > > > > +++ b/kernel/sched/core.c > > > > @@ -1345,7 +1345,7 @@ ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags) > > > > > > > > if (rq->idle_stamp) { > > > > u64 delta = rq_clock(rq) - rq->idle_stamp; > > > > - u64 max = 2*rq->max_idle_balance_cost; > > > > + u64 max = 2*(sysctl_sched_migration_cost + rq->max_idle_balance_cost); > > > > > > You re-introduce sched_migration_cost here because max_idle_balance_cost > > > can now drop down to 0 again? > > > > Yes it was so that max_idle_balance_cost would be at least sched_migration_cost > > and that we would still skip idle_balance if avg_idle < sched_migration_cost. > > > > I also initially thought that adding sched_migration_cost would also account for > > the extra "costs" of idle balancing that are not accounted for in the time spent > > on each newidle load balance. Come to think of it though, sched_migration_cost > > might be too large when used in that context considering we're already using the > > max cost. > > Right, so shall we do as Srikar suggests and drop that initial check? I agree that we can delete the check between avg_idle and max_idle_balance_cost so that large costs in higher domains don't cause balancing to be skipped in lower domains as Srikar suggested. Should we keep the old "if (this_rq->avg_idle < sysctl_sched_migration_cost)" check? Also, are the other costs of idle balancing, specifically the cost of cache refreshes, not considered in the max_newidle_lb_cost? I was wondering if there is a non expensive way we can also take those into account. For example, can we multiply max_newidle_lb_cost by a factor of 1.2x to 2x, but also increase the decay factor (4% to 10% per second)? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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