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SubjectRe: [PATCH v4] KVM: halt-polling: poll for the upcoming fire timers
From
2016-05-25 1:15 GMT+08:00 Christian Borntraeger <borntraeger@de.ibm.com>:
> On 05/24/2016 09:57 AM, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@hotmail.com>
>>
>> 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@redhat.com>
>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>> Cc: David Matlack <dmatlack@google.com>
>> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
>> Cc: Yang Zhang <yang.zhang.wz@gmail.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
>> ---
>> 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?

I see, sorry for misunderstand you in v3.

Regards,
Wanpeng Li

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