linux-kernel - Re: single_task_running() vs. preemption warnings (was Re: [PATCH] kvm: fix preemption warnings in kvm_vcpu

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Date:	Fri, 18 Sep 2015 09:52:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Dominik Dingel <dingel@...ux.vnet.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-crypto@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	herbert@...dor.apana.org.au, Ingo Molnar <mingo@...hat.com>
Subject: Re: single_task_running() vs. preemption warnings (was Re: [PATCH]
 kvm: fix preemption warnings in kvm_vcpu_block)

On Thu, Sep 17, 2015 at 01:32:55PM -0700, Tim Chen wrote:
> I have no objection to change single_task_running to use
> raw_smp_processor_id.  The worker in mcryptd is bound to
> the cpu so it has no migration/preemption issue.  So it shouldn't care
> which smp_processor_id version is being used.  Yes, please add a comment
> to alert the user of this caveat should you change single_task_running.


We actually have raw_rq() for that, and the whole if thing looks rather
superfluous. So something like the below, except with a suitable comment
on and tested etc.. ;-)

---
 kernel/sched/core.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6ab415aa15c4..f39c0498e284 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2666,10 +2666,7 @@ unsigned long nr_running(void)
  */
 bool single_task_running(void)
 {
-	if (cpu_rq(smp_processor_id())->nr_running == 1)
-		return true;
-	else
-		return false;
+	return raw_rq()->nr_running == 1;
 }
 EXPORT_SYMBOL(single_task_running);
 
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