From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Wed, 1 Feb 2017 15:07:56 +0000 Subject: [PATCH] arm64: Improve parking of stopped CPUs In-Reply-To: <3a9f0857-9274-738c-7a65-f24f337e6799@arm.com> References: <1485942532-2643-1-git-send-email-jnair@caviumnetworks.com> <20170201141656.GB8177@arm.com> <3a9f0857-9274-738c-7a65-f24f337e6799@arm.com> Message-ID: <20170201150756.GD8177@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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 > >>--- > >> 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