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

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Message-ID: <57448C22.2030504@de.ibm.com>
Date:	Tue, 24 May 2016 19:15:14 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	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

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?


>  	else
>  		val *= grow;
> 
>