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SubjectRe: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in select_idle_sibling
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On 02/09/2018 04:53 AM, Peter Zijlstra wrote:
> On Mon, Jan 29, 2018 at 03:27:09PM -0800, Rohit Jain wrote:
>> 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;
>> +}
> I don't like that name; >.75 != 1.
>
> Maybe invert things and do something like:
>
> static inline bool reduced_capacity(int cpu)
> {
> return capacity_of(cpu) < (3*capacity_orig_of(cpu))/4;
> }

OK, I will change the name and invert the logic.

>> @@ -6110,11 +6116,13 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
>> for_each_cpu(cpu, cpu_smt_mask(target)) {
>> if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
>> continue;
>> + if (idle_cpu(cpu) && (capacity_of(cpu) > max_cap)) {
>> + max_cap = capacity_of(cpu);
>> + rcpu = cpu;
>> + }
> if (idle_cpu(cpu)) {
> if (!reduced_capacity(cpu))
> return cpu;
>
> if (capacity_cpu(cpu) > max_cap) {
> max_cap = capacity_cpu(cpu);
> rcpu = cpu;
> }
> }
>
> Would be more consistent, I think.

OK

>
>> }
>>
>> - return -1;
>> + return rcpu;
>> }
>
>
>> @@ -6143,6 +6151,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>> u64 time, cost;
>> s64 delta;
>> int cpu, nr = INT_MAX;
>> + int best_cpu = -1;
>> + unsigned int best_cap = 0;
> Randomly different names for the same thing as in select_idle_smt().
> Thinking up two different names for the same thing is more work; be more
> lazy.

OK, will be more consistent in v1

>
>> this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>> if (!this_sd)
>> @@ -6173,8 +6183,15 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>> return -1;
>> if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
>> continue;
>> + if (idle_cpu(cpu)) {
>> + if (full_capacity(cpu)) {
>> + best_cpu = cpu;
>> + break;
>> + } else if (capacity_of(cpu) > best_cap) {
>> + best_cap = capacity_of(cpu);
>> + best_cpu = cpu;
>> + }
>> + }
> No need for the else. And you'll note you're once again inconsistent
> with your previous self.
>
> But here I worry about big.little a wee bit. I think we're allowed big
> and little cores on the same L3 these days, and you can't directly
> compare capacity between them.
>
> Morten / Dietmar, any comments?
>
>> @@ -6193,13 +6210,14 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>> struct sched_domain *sd;
>> int i;
>>
>> - if (idle_cpu(target))
>> + if (idle_cpu(target) && full_capacity(target))
>> return target;
>>
>> /*
>> * If the previous cpu is cache affine and idle, don't be stupid.
>> */
>> - if (prev != target && cpus_share_cache(prev, target) && idle_cpu(prev))
>> + if (prev != target && cpus_share_cache(prev, target) && idle_cpu(prev) &&
>> + full_capacity(prev))
>> return prev;
> split before idle_cpu() for a better balance.


OK

Thanks,
Rohit

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