Subject: [PATCH] arm64: Improve parking of stopped CPUs

From: [email-masked] (Will Deacon)
Date: Wed, 1 Feb 2017 15:07:56 +0000

On Wed, Feb 01, 2017 at 02:31:38PM +0000, Suzuki K Poulose wrote:
> On 01/02/17 14:16, Will Deacon wrote:
> >On Wed, Feb 01, 2017 at 09:48:52AM +0000, Jayachandran C wrote:
> >>The current code puts the stopped cpus in an 'yield' instruction loop.
> >>Using a busy loop here is unnecessary, we can use the cpu_park_loop()
> >>function here to do a wfi/wfe.
> >>
> >>Signed-off-by: Jayachandran C <[email-masked]>
> >>---
> >> arch/arm64/kernel/smp.c | 3 +--
> >> 1 file changed, 1 insertion(+), 2 deletions(-)
> >>
> >>diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> >>index cbaab44..0691d2f 100644
> >>--- a/arch/arm64/kernel/smp.c
> >>+++ b/arch/arm64/kernel/smp.c
> >>@@ -829,8 +829,7 @@ static void ipi_cpu_stop(unsigned int cpu)
> >>
> >> 	local_irq_disable();
> >>
> >>-	while (1)
> >>-		cpu_relax();
> >>+	cpu_park_loop();
> >> }
> >
> >Hmm, so we actually added the yield for QEMU's benefit iirc, where QEMU
> >will trap the yield and schedule a different vCPU. Should we be adding
> >a yield to cpu_park_loop instead?
> 
> Wouldn't wfi/wfe trigger the same ? I don't know how yield affects a physical
> CPU. The cpu_park_loop is also used by CPUs which cannot run due to the missing
> capabilities on the system. As long as yield() doesn't affect the PCPUs, we
> could do that. 

Yes, good point, it looks like WFE should do the same thing.

> Going another step further, we could also include local_irq_disable() in
> cpu_park_loop().

Why?

Will