UPSTREAM: sched: use load_avg for selecting idlest group
find_idlest_group() only compares the runnable_load_avg when looking for
the least loaded group. But on fork intensive use case like hackbench
where tasks blocked quickly after the fork, this can lead to selecting the
same CPU instead of other CPUs, which have similar runnable load but a
lower load_avg.
When the runnable_load_avg of 2 CPUs are close, we now take into account
the amount of blocked load as a 2nd selection factor. There is now 3 zones
for the runnable_load of the rq:
-[0 .. (runnable_load - imbalance)] : Select the new rq which has
significantly less runnable_load
-](runnable_load - imbalance) .. (runnable_load + imbalance)[ : The
runnable loads are close so we use load_avg to chose between the 2 rq
-[(runnable_load + imbalance) .. ULONG_MAX] : Keep the current rq which
has significantly less runnable_load
The scale factor that is currently used for comparing runnable_load,
doesn't work well with small value. As an example, the use of a scaling
factor fails as soon as this_runnable_load == 0 because we always select
local rq even if min_runnable_load is only 1, which doesn't really make
sense because they are just the same. So instead of scaling factor, we use
an absolute margin for runnable_load to detect CPUs with similar
runnable_load and we keep using scaling factor for blocked load.
For use case like hackbench, this enable the scheduler to select different
CPUs during the fork sequence and to spread tasks across the system.
Tests have been done on a Hikey board (ARM based octo cores) for several
kernel. The result below gives min, max, avg and stdev values of 18 runs
with each configuration.
The v4.8+patches configuration also includes the changes below which is
part of the proposal made by Peter to ensure that the clock will be up to
date when the fork task will be attached to the rq.
@@ -2568,6 +2568,7 @@ void wake_up_new_task(struct task_struct *p)
__set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
#endif
rq = __task_rq_lock(p, &rf);
+ update_rq_clock(rq);
post_init_entity_util_avg(&p->se);
activate_task(rq, p, 0);
hackbench -P -g 1
ea86cb4b7621 7dc603c9028e v4.8 v4.8+patches
min 0.049 0.050 0.051 0,048
avg 0.057 0.057(0%) 0.057(0%) 0,055(+5%)
max 0.066 0.068 0.070 0,063
stdev +/-9% +/-9% +/-8% +/-9%
Change-Id: I8fe40bbd106454282b1963db825705f3cf377b9e
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Matt Fleming <matt@codeblueprint.co.uk>
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
1 file changed