Re: [patch 14/26] sched: fix next_interval determination in idle_balance()

Paul E. McKenney
2007-07-31T15:02:49Z

On Mon, Jul 30, 2007 at 09:32:54PM -0700, Greg KH wrote:
> -stable review patch. If anyone has any objections, please let us know.

Hello, Greg,

No objections here -- on the contrary, very nice not to have to manually
"taskset" my rcutorture runs. ;-)

Thanx, Paul

> ------------------
>
> From: Christoph Lameter <clameter@sgi.com>
>
> Fix massive SMP imbalance on NUMA nodes observed on 2.6.21.5 with CFS.
> (and later on reproduced without CFS as well).
>
> The intervals of domains that do not have SD_BALANCE_NEWIDLE must be
> considered for the calculation of the time of the next balance.
> Otherwise we may defer rebalancing forever and nodes might stay idle for
> very long times.
>
> Siddha also spotted that the conversion of the balance interval to
> jiffies is missing. Fix that to.
>
> From: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
>
> also continue the loop if !(sd->flags & SD_LOAD_BALANCE).
>
> Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> It did in fact trigger under all three of mainline, CFS, and -rt
> including CFS -- see below for a couple of emails from last Friday
> giving results for these three on the AMD box (where it happened) and on
> a single-quad NUMA-Q system (where it did not, at least not with such
> severity).
>
> Signed-off-by: Christoph Lameter <clameter@sgi.com>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> Signed-off-by: Chris Wright <chrisw@sous-sol.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
> ---
> kernel/sched.c | 22 +++++++++++++---------
> 1 file changed, 13 insertions(+), 9 deletions(-)
>
> --- linux-2.6.21.6.orig/kernel/sched.c
> +++ linux-2.6.21.6/kernel/sched.c
> @@ -2831,17 +2831,21 @@ static void idle_balance(int this_cpu, s
> unsigned long next_balance = jiffies + 60 * HZ;
>
> for_each_domain(this_cpu, sd) {
> - if (sd->flags & SD_BALANCE_NEWIDLE) {
> + unsigned long interval;
> +
> + if (!(sd->flags & SD_LOAD_BALANCE))
> + continue;
> +
> + if (sd->flags & SD_BALANCE_NEWIDLE)
> /* If we've pulled tasks over stop searching: */
> pulled_task = load_balance_newidle(this_cpu,
> - this_rq, sd);
> - if (time_after(next_balance,
> - sd->last_balance + sd->balance_interval))
> - next_balance = sd->last_balance
> - + sd->balance_interval;
> - if (pulled_task)
> - break;
> - }
> + this_rq, sd);
> +
> + interval = msecs_to_jiffies(sd->balance_interval);
> + if (time_after(next_balance, sd->last_balance + interval))
> + next_balance = sd->last_balance + interval;
> + if (pulled_task)
> + break;
> }
> if (!pulled_task)
> /*
>
> --