Re: [PATCH v4] KVM: halt-polling: poll for the upcoming fire timers

From: Christian Borntraeger
Date: Tue May 24 2016 - 13:15:40 EST


On 05/24/2016 09:57 AM, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
> If an emulated lapic timer will fire soon(in the scope of 10us the
> base of dynamic halt-polling, lower-end of message passing workload
> latency TCP_RR's poll time < 10us) we can treat it as a short halt,
> and poll to wait it fire, the fire callback apic_timer_fn() will set
> KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll.
> This can avoid context switch overhead and the latency which we wake
> up vCPU.
>
> This feature is slightly different from current advance expiration
> way. Advance expiration rely on the vCPU is running(do polling before
> vmentry). But in some cases, the timer interrupt may be blocked by
> other thread(i.e., IF bit is clear) and vCPU cannot be scheduled to
> run immediately. So even advance the timer early, vCPU may still see
> the latency. But polling is different, it ensures the vCPU to aware
> the timer expiration before schedule out.
>
> echo HRTICK > /sys/kernel/debug/sched_features in dynticks guests.
>
> Context switching - times in microseconds - smaller is better
> -------------------------------------------------------------------------
> Host OS 2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
> ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw
> --------- ------------- ------ ------ ------ ------ ------ ------- -------
> kernel Linux 4.6.0+ 7.9800 11.0 10.8 14.6 9.4300 13.0 10.2 vanilla
> kernel Linux 4.6.0+ 15.3 13.6 10.7 12.5 9.0000 12.8 7.38000 poll
>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: David Matlack <dmatlack@xxxxxxxxxx>
> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Cc: Yang Zhang <yang.zhang.wz@xxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> v3 -> v4:
> * add module parameter halt_poll_ns_timer
> * rename patch subject since lapic maybe just for x86.
> v2 -> v3:
> * add Yang's statement to patch description
> v1 -> v2:
> * add return statement to non-x86 archs
> * capture never expire case for x86 (hrtimer is not started)
>
> arch/arm/include/asm/kvm_host.h | 4 ++++
> arch/arm64/include/asm/kvm_host.h | 4 ++++
> arch/mips/include/asm/kvm_host.h | 4 ++++
> arch/powerpc/include/asm/kvm_host.h | 4 ++++
> arch/s390/include/asm/kvm_host.h | 4 ++++
> arch/x86/kvm/lapic.c | 11 +++++++++++
> arch/x86/kvm/lapic.h | 1 +
> arch/x86/kvm/x86.c | 5 +++++
> include/linux/kvm_host.h | 1 +
> virt/kvm/kvm_main.c | 15 +++++++++++----
> 10 files changed, 49 insertions(+), 4 deletions(-)
[...]

@@ -1966,7 +1970,7 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
> grow = READ_ONCE(halt_poll_ns_grow);
> /* 10us base */
> if (val == 0 && grow)
> - val = 10000;
> + val = halt_poll_ns_timer;

Drop this hunk and leave this at 10000, so that a user can disable the timer
logic, but keep the old polling?


> else
> val *= grow;
>
>