linux-kernel - Re: [v6] kvm/fpu: Enable fully eager restore kvm FPU

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Date:	Thu, 23 Apr 2015 13:25:02 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Liang Li <liang.z.li@...el.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
CC:	gleb@...nel.org, Marcelo Tosatti <mtosatti@...hat.com>,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, joro@...tes.org, yang.z.zhang@...el.com,
	Xudong Hao <xudong.hao@...el.com>
Subject: Re: [v6] kvm/fpu: Enable fully eager restore kvm FPU

On 2015-04-23 12:40, Paolo Bonzini wrote:
> 
> 
> On 23/04/2015 23:13, Liang Li wrote:
>> Romove lazy FPU logic and use eager FPU entirely. Eager FPU does
>> not have performance regression, and it can simplify the code.
>>
>> When compiling kernel on westmere, the performance of eager FPU
>> is about 0.4% faster than lazy FPU.
>>
>> Signed-off-by: Liang Li <liang.z.li@...el.com>
>> Signed-off-by: Xudong Hao <xudong.hao@...el.com>
> 
> A patch like this requires much more benchmarking than what you have done.
> 
> First, what guest did you use?  A modern Linux guest will hardly ever exit
> to userspace: the scheduler uses the TSC deadline timer, which is handled
> in the kernel; the clocksource uses the TSC; virtio-blk devices are kicked
> via ioeventfd.
> 
> What happens if you time a Windows guest (without any Hyper-V enlightenments),
> or if you use clocksource=acpi_pm?
> 
> Second, "0.4%" by itself may not be statistically significant.  How did
> you gather the result?  How many times did you run the benchmark?  Did
> the guest report any stolen time?
> 
> 
> And finally, even if the patch was indeed a performance improvement,
> there is much more that you can remove.  fpu_active is always 1, 
> vmx_fpu_activate only has one call site that can be simplified just to
> 
>         vcpu->arch.cr0_guest_owned_bits = X86_CR0_TS;
>         vmcs_writel(CR0_GUEST_HOST_MASK, ~vcpu->arch.cr0_guest_owned_bits);
> 
> and so on.

And it would be good to know how the benchmarks look like on other CPUs
than the chosen Intel model. Including older ones.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux
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