linux-kernel - Re: [PATCH v2 04/11] sched: Simplify wake_up

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Date:   Tue, 19 Oct 2021 11:10:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qian Cai <quic_qiancai@...cinc.com>
Cc:     gor@...ux.ibm.com, jpoimboe@...hat.com, jikos@...nel.org,
        mbenes@...e.cz, pmladek@...e.com, mingo@...nel.org,
        linux-kernel@...r.kernel.org, joe.lawrence@...hat.com,
        fweisbec@...il.com, tglx@...utronix.de, hca@...ux.ibm.com,
        svens@...ux.ibm.com, sumanthk@...ux.ibm.com,
        live-patching@...r.kernel.org, paulmck@...nel.org,
        rostedt@...dmis.org, x86@...nel.org
Subject: Re: [PATCH v2 04/11] sched: Simplify wake_up_*idle*()

On Tue, Oct 19, 2021 at 10:56:48AM +0200, Peter Zijlstra wrote:
> On Mon, Oct 18, 2021 at 11:47:32PM -0400, Qian Cai wrote:
> > Peter, any thoughts? I did confirm that reverting the commit fixed the issue.
> > 
> > On 10/13/2021 10:32 AM, Qian Cai wrote:
> > > 
> > > 
> > > On 9/29/2021 11:17 AM, Peter Zijlstra wrote:
> > >> --- a/kernel/smp.c
> > >> +++ b/kernel/smp.c
> > >> @@ -1170,14 +1170,14 @@ void wake_up_all_idle_cpus(void)
> > >>  {
> > >>  	int cpu;
> > >>  
> > >> -	preempt_disable();
> > >> +	cpus_read_lock();
> > >>  	for_each_online_cpu(cpu) {
> > >> -		if (cpu == smp_processor_id())
> > >> +		if (cpu == raw_smp_processor_id())
> > >>  			continue;
> > >>  
> > >>  		wake_up_if_idle(cpu);
> > >>  	}
> > >> -	preempt_enable();
> > >> +	cpus_read_unlock();
> 
> Right, so yesterday I thought: YW2KGrvvv/vSA+97@...ez.programming.kicks-ass.net
> but today I might have another idea, lemme go prod at this a bit more.

Here, hows this then?

---
diff --git a/kernel/smp.c b/kernel/smp.c
index ad0b68a3a3d3..61ddc7a3bafa 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -1170,14 +1170,12 @@ void wake_up_all_idle_cpus(void)
 {
 	int cpu;
 
-	cpus_read_lock();
-	for_each_online_cpu(cpu) {
-		if (cpu == raw_smp_processor_id())
-			continue;
-
-		wake_up_if_idle(cpu);
+	for_each_cpu(cpu) {
+		preempt_disable();
+		if (cpu != smp_processor_id() && cpu_online(cpu))
+			wake_up_if_idle(cpu);
+		preempt_enable();
 	}
-	cpus_read_unlock();
 }
 EXPORT_SYMBOL_GPL(wake_up_all_idle_cpus);