On 1/8/2018 5:18 PM, Peter Zijlstra wrote:
> On Mon, Jan 08, 2018 at 02:12:37PM -0800, subhra mazumdar wrote:
>> @@ -2751,6 +2763,31 @@ context_switch(struct rq *rq, struct task_struct
>> *prev,
>> struct task_struct *next, struct rq_flags *rf)
>> {
>> struct mm_struct *mm, *oldmm;
>> + int this_cpu = rq->cpu;
>> + struct sched_domain *sd;
>> + int prev_busy, next_busy;
>> +
>> + if (rq->curr_util == UTIL_UNINITIALIZED)
>> + prev_busy = 0;
>> + else
>> + prev_busy = (prev != rq->idle);
>> + next_busy = (next != rq->idle);
>> +
>> + /*
>> + * From sd_llc downward update the SMT utilization.
>> + * Skip the lowest level 0.
>> + */
>> + sd = rcu_dereference_sched(per_cpu(sd_llc, this_cpu));
>> + if (next_busy != prev_busy) {
>> + for_each_lower_domain(sd) {
>> + if (sd->level == 0)
>> + break;
>> + sd_context_switch(sd, rq, next_busy - prev_busy);
>> + }
>> + }
>> +
>
> No, we're not going to be adding atomic ops here. We've been arguing
> over adding a single memory barrier to this path, atomic are just not
> going to happen.
>
> Also this is entirely the wrong way to do this, we already have code
> paths that _know_ if they're going into or coming out of idle.
Yes, it would be more efficient to adjust the busy-cpu count of each level
of the hierarchy in pick_next_task_idle and put_prev_task_idle.
- Steve
