Re: [PATCH 2/3] sched/fair: Introduce scaled capacity awareness in select_idle_sibling code path

[joelaf]

Hi Rohit,

On Tue, Sep 26, 2017 at 12:48 PM, Rohit Jain <rohit.k.jain@xxxxxxxxxx> wrote:
[...]
>>> +       unsigned int backup_cap = 0;
>>> +
>>> +       rcpu = rcpu_backup = -1;
>>>
>>>          if (!static_branch_likely(&sched_smt_present))
>>>                  return -1;
>>> @@ -6057,10 +6060,20 @@ static int select_idle_core(struct task_struct
>>> *p, struct sched_domain *sd, int
>>>                          cpumask_clear_cpu(cpu, cpus);
>>>                          if (!idle_cpu(cpu))
>>>                                  idle = false;
>>> +
>>> +                       if (full_capacity(cpu)) {
>>> +                               rcpu = cpu;
>>> +                       } else if ((rcpu == -1) && (capacity_of(cpu) >
>>> backup_cap)) {
>>> +                               backup_cap = capacity_of(cpu);
>>> +                               rcpu_backup = cpu;
>>> +                       }
>>
>> Here you comparing capacity of different SMT threads.
>>
>>>                  }
>>>
>>> -               if (idle)
>>> -                       return core;
>>> +               if (idle) {
>>> +                       if (rcpu == -1)
>>> +                               return (rcpu_backup != -1 ? rcpu_backup :
>>> core);
>>> +                       return rcpu;
>>> +               }
>>
>>
>> This didn't make much sense to me, here you are returning either an
>> SMT thread or a core. That doesn't make much of a difference because
>> SMT threads share the same capacity (SD_SHARE_CPUCAPACITY). I think
>> what you want to do is find out the capacity of a 'core', not an SMT
>> thread, and compare the capacity of different cores and consider the
>> one which has least RT/IRQ interference.
>
>
> IIUC the capacities of each strand is scaled by IRQ and 'rt_avg' for that
> 'rq'. Now if the strand is idle now and gets an interrupt in the future,
> the 'core' would look like:
>
>    +----+----+
>    | I  |    |
>    | T  |    |
>    +----+----+
>
> (I -> Interrupt, T-> Thread we are trying to schedule).
>
> whereas if the other strand on the core was taking interrupt the core
> would look like:
>
>    +----+----+
>    | I  | T  |
>    |    |    |
>    +----+----+
>
> With this case, because we know from the past avg, one of the strands is
> running low on capacity, I am trying to return a better strand for the
> thread to start on.
>

I know what you're trying to do but they way you've retrofitted it into the
core looks weird (to me) and makes the code unreadable and ugly IMO.

Why not do something simpler like skip the core if any SMT thread has been
running at lesser capacity? I'm not sure if this works great or if the maintainers
will prefer your or my below approach, but I find the below diff much cleaner
for the select_idle_core bit. It also makes more sense since resources are
shared at SMT level so makes sense to me to skip the core altogether for this:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6ee7242dbe0a..f324a84e29f1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5738,14 +5738,17 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
 
 	for_each_cpu_wrap(core, cpus, target) {
 		bool idle = true;
+		bool full_cap = true;
 
 		for_each_cpu(cpu, cpu_smt_mask(core)) {
 			cpumask_clear_cpu(cpu, cpus);
 			if (!idle_cpu(cpu))
 				idle = false;
+			if (!full_capacity(cpu))
+				full_cap = false;
 		}
 
-		if (idle)
+		if (idle && full_cap)
 			return core;
 	}
 


thanks,

- Joel