On 05/09/2014 11:24 AM, Mike Galbraith wrote:
On Fri, 2014-05-09 at 10:22 -0400, Rik van Riel wrote:
On 05/09/2014 03:34 AM, Mike Galbraith wrote:
On Fri, 2014-05-09 at 01:27 -0400, Rik van Riel wrote:
On Thu, 08 May 2014 22:20:25 -0400
Rik van Riel <r...@redhat.com> wrote:
Looks like SD_BALANCE_WAKE is not gotten from the sd flags at
all, but passed into select_task_rq by try_to_wake_up, as a
hard coded sd_flags argument.
Should we do that, if SD_WAKE_BALANCE is not set for any sched domain?
I answered my own question. The sd_flag SD_WAKE_BALANCE simply means
"this is a wakeup of a previously existing task, please place it
properly".
However, it appears that the current code will fall back to the large
loop with select_idlest_group and friends, if prev_cpu and cpu are not
part of the same SD_WAKE_AFFINE sched domain. That is a bug...
ttwu(): cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
We pass SD_BALANCE_WAKE for a normal wakeup, so sd will only be set if
we encounter a domain during traversal where Joe User has told us to do
(expensive) wake balancing before we hit a domain shared by waker/wakee.
The user can turn SD_WAKE_AFFINE off beyond socket, and we'll not pull
cross node on wakeup.
Or, you could create an override button to say despite SD_WAKE_AFFINE
perhaps having been set during domain construction (because of some
pseudo-random numbers), don't do that if we have a preferred node, or
just make that automatically part of having numa scheduling enabled, and
don't bother wasting cycles if preferred && this != preferred.
That's not the problem.
The problem is that if we do not do an affine wakeup, due to
SD_WAKE_AFFINE not being set on a top level domain, we will
not try to run p on prev_cpu, but we will fall through into
the loop with find_idlest_group, etc...
If no ->flags & SD_BALANCE_WAKE is encountered during traversal, sd
remains NULL, we fall through to return prev_cpu.
We do fall through, but into this loop:
while (sd) {
struct sched_group *group;
int weight;
if (!(sd->flags & sd_flag)) {
sd = sd->child;
continue;
}
group = find_idlest_group(sd, p, cpu, sd_flag);
if (!group) {
sd = sd->child;
continue;
}
new_cpu = find_idlest_cpu(group, p, cpu);
if (new_cpu == -1 || new_cpu == cpu) {
/* Now try balancing at a lower domain level of
cpu */
sd = sd->child;
continue;
}
/* Now try balancing at a lower domain level of new_cpu */
cpu = new_cpu;
weight = sd->span_weight;
sd = NULL;
for_each_domain(cpu, tmp) {
if (weight <= tmp->span_weight)
break;
if (tmp->flags & sd_flag)
sd = tmp;
}
/* while loop will break here if sd == NULL */
}
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