Re: [PATCH 4/4] sched/fair: Add more core cookie check in wake up fast path

Fernand Sieber
2025-09-23T09:31:03Z

Hi Prateek,

On 9/23/2025 2:25 PM, K Prateek Nayak wrote:
> So with Patch 1, you already check for cookie matching while entering
> select_idle_smt() and now, each pass of the loop again does a
> sched_core_cookie_match() which internally loops through the smt mask
> again! Seems wasteful.

Right. The change in select_idle_smt() is unnecessary.

> On an SMT-8 system, all the looping over smt mask per wakeup will add
> up. Is that not a concern? A single task with core cookie enabled will
> add massive overhead for all wakeup in the system.

In such a scenario there should generally be no looping because I introduced an
early return in patch 3 in __sched_core_cookie_match(). Perhaps it's worth
extracting this early return as standalone optimization patch? Something like
this:

@@ -1404,10 +1404,12 @@ static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
if (!sched_core_enabled(rq))
return true;

+ if (rq->core->core_cookie == p->core_cookie)
+ return true;
+
for_each_cpu(cpu, cpu_smt_mask(cpu_of(rq))) {
if (!available_idle_cpu(cpu)) {
- idle_core = false;
- break;
+ return false;
}
}

@@ -1415,7 +1417,7 @@ static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
* A CPU in an idle core is always the best choice for tasks with
* cookies.
*/
- return idle_core || rq->core->core_cookie == p->core_cookie;
+ return true;
}

Thanks,
Fernand