On 02/05/2018 10:50 PM, Joel Fernandes wrote:
On Wed, Jan 31, 2018 at 9:50 AM, Rohit Jain rohit.k.jain@oracle.com wrote:
kernel/sched/fair.c | 38 ++++++++++++++++++++++++++++---------- 1 file changed, 28 insertions(+), 10 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 26a71eb..ce5ccf8 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5625,6 +5625,11 @@ static unsigned long capacity_orig_of(int cpu) return cpu_rq(cpu)->cpu_capacity_orig; }
+static inline bool full_capacity(int cpu) +{
return capacity_of(cpu) >= (capacity_orig_of(cpu)*3)/4;
+}
- static unsigned long cpu_avg_load_per_task(int cpu) { struct rq *rq = cpu_rq(cpu);
@@ -6081,7 +6086,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
for_each_cpu(cpu, cpu_smt_mask(core)) { cpumask_clear_cpu(cpu, cpus);
if (!idle_cpu(cpu))
if (!idle_cpu(cpu) || !full_capacity(cpu)) idle = false; }
There's some difference in logic between select_idle_core and select_idle_cpu as far as the full_capacity stuff you're adding goes. In select_idle_core, if all CPUs are !full_capacity, you're returning -1. But in select_idle_cpu you're returning the best idle CPU that's the most cap among the !full_capacity ones. Why there is this different in logic? Did I miss something?
<snip>
Let me re-try :)
For select_idle_core, we are doing a search for a fully idle and full capacity core, the fail-safe is select_idle_cpu because we will re-scan the CPUs. The notion is to select an idle CPU no matter what, because being on an idle CPU is better than waiting on a non-idle one. In select_idle_core you can be slightly picky about the core because select_idle_cpu is a fail safe. I measured the performance impact of choosing the "best among low cap" vs the code changes I have (for select_idle_core) and could not find a statistically significant impact, hence went with the simpler code changes.
That's Ok with me. Just that I remember Peter messing with this path and that it was expensive to scan too much for some systems. The other thing is you're really doing to do a "fail safe" as you call it search here with SIS_PROP set. Do you see a difference in perf when doing the same approach as you took in select_idle_core?
I didn't see any measurable impact by changing select_idle_core from the above logic to be the same logic as select_idle_cpu. I am OK with either if there are concerns.
Thanks, Rohit
Peter, are you with the approach Rohit has adopted to pick best capacity idle CPU in select_idle_cpu? I guess nr--; will bail out early if we have SIS_PROP set, incase the scan cost gets too much but then again we might end scanning too few CPUs.
thanks,
- Joel