On Tue, Jan 19, 2016 at 07:48:14PM +0100, Andrew Jones wrote: > On Tue, Jan 19, 2016 at 01:43:07PM +0000, Marc Zyngier wrote: > > >> On Tue, Jan 19, 2016 at 01:37:16PM +0100, Andrew Jones wrote: > > > OK, CCing him. One thing I see is that without this change we're > > > currently setting the clock feature CLOCK_EVT_FEAT_C3STOP, even though > > > it's not true. Having that set may disable the oneshot capabilityj > > > necessary to switch to nohz mode? I'll just stop there with my > > > speculation though, so Marc won't have to correct too much... > > > > You're spot on. See 82a5619 in the kernel tree. When I did a similar > > change in kvmtool, I saw a massive reduction in the number of timer > > interrupts injected (specially when the number of vcpu is relatively high). > > > > This also have interesting benefits when running on a model, where > > you're trying to squeeze the last bits of "performance" from the monster... > > > > Hmm, I'm probably testing this wrong, but I don't see any difference in > the number of injected timer interrupts. My guest, which I boot with > UEFI, has > > CONFIG_ARM_ARCH_TIMER=y > CONFIG_ARM_ARCH_TIMER_EVTSTREAM=y > CONFIG_ARM_TIMER_SP804=y > CONFIG_HIGH_RES_TIMERS=y > CONFIG_TICK_ONESHOT=y > CONFIG_NO_HZ_COMMON=y > # CONFIG_HZ_PERIODIC is not set > CONFIG_NO_HZ_IDLE=y > # CONFIG_NO_HZ_FULL is not set > CONFIG_NO_HZ=y > CONFIG_HZ_1000=y > CONFIG_HZ=1000 > > I've boot a guest using DT with and without this patch > > ---WITHOUT--- > > # ls /proc/device-tree/timer > compatible interrupts name > # cat /proc/interrupts > CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 > CPU7 > 3: 6958 5766 5166 5187 5576 5129 4695 > 4398 GIC 27 Edge arch_timer > # sleep 120 && cat /proc/interrupts > CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 > CPU7 > 3: 7557 5986 5487 5265 6232 5868 5464 > 4438 GIC 27 Edge arch_timer > > ---WITH--- > > # ls /proc/device-tree/timer > always-on compatible interrupts name > # cat /proc/interrupts > CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 > CPU7 > 3: 7005 6080 4996 5391 5165 5257 4930 > 4844 GIC 27 Edge arch_timer > # sleep 120 && cat /proc/interrupts > CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 > CPU7 > 3: 7523 6505 5264 6717 5273 5391 5526 > 4901 GIC 27 Edge arch_timer > > > > And kvm trace data has > > ---WITHOUT--- > $ grep kvm_timer_update_irq trace.out | wc -l > 94336 > ---WITH--- > $ grep kvm_timer_update_irq trace.out | wc -l > 95838 > >
Must be how I'm looking, because I just tried kvmtool with/without Marc's patch that adds always-on, but don't see any reduction of interrupts there either. I used a defconfig guest kernel. Also, not that I think it should matter, but my host kernel is 4.4-rc4 based. I'd like to be able to see a difference with/without this always-on patch, not because I don't think we should take it anyway, but because I need a test case for the ACPI counterpart. Thanks, drew
