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

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Message-ID: <20250923093001.1218488-1-sieberf@amazon.com>
Date: Tue, 23 Sep 2025 11:30:00 +0200
From: Fernand Sieber <sieberf@...zon.com>
To: K Prateek Nayak <kprateek.nayak@....com>
CC: <mingo@...hat.com>, <peterz@...radead.org>,
	<linux-kernel@...r.kernel.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<bristot@...hat.com>, <vschneid@...hat.com>, <dwmw@...zon.co.uk>,
	<jschoenh@...zon.de>, <liuyuxua@...zon.com>
Subject: Re: [PATCH 4/4] sched/fair: Add more core cookie check in wake up fast path

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



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