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

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Date:	Wed, 25 May 2016 07:16:49 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	David Matlack <dmatlack@...gle.com>,
	Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [PATCH v4] KVM: halt-polling: poll for the upcoming fire timers

2016-05-25 1:15 GMT+08:00 Christian Borntraeger <borntraeger@...ibm.com>:
> On 05/24/2016 09:57 AM, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@...mail.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@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Cc: David Matlack <dmatlack@...gle.com>
>> Cc: Christian Borntraeger <borntraeger@...ibm.com>
>> Cc: Yang Zhang <yang.zhang.wz@...il.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.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