linux-kernel - Re: [PATCH -v2] vmx: increase ple

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Message-ID: <4D2FBE16.9070901@intel.com>
Date:	Fri, 14 Jan 2011 11:08:06 +0800
From:	"Zhai, Edwin" <edwin.zhai@...el.com>
To:	Rik van Riel <riel@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"avi@...hat.com" <avi@...hat.com>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>
Subject: Re: [PATCH -v2] vmx: increase ple_gap default to 64

Acked.

Thanks,
edwin

Rik van Riel wrote:
> On some CPUs, a ple_gap of 41 is simply insufficient to ever trigger
> PLE exits, even with the minimalistic PLE test from kvm-unit-tests.
>
> http://git.kernel.org/?p=virt/kvm/kvm-unit-tests.git;a=commitdiff;h=eda71b28fa122203e316483b35f37aaacd42f545
>
> For example, the Xeon X5670 CPU needs a ple_gap of at least 48 in
> order to get pause loop exits:
>
> # modprobe kvm_intel ple_gap=47
> # taskset 1 /usr/local/bin/qemu-system-x86_64 -device testdev,chardev=log -chardev stdio,id=log -kernel x86/vmexit.flat -append ple-round-robin -smp 2
> VNC server running on `::1:5900'
> enabling apic
> enabling apic
> ple-round-robin 58298446
> # rmmod kvm_intel
> # modprobe kvm_intel ple_gap=48
> # taskset 1 /usr/local/bin/qemu-system-x86_64 -device testdev,chardev=log -chardev stdio,id=log -kernel x86/vmexit.flat -append ple-round-robin -smp 2
> VNC server running on `::1:5900'
> enabling apic
> enabling apic
> ple-round-robin 36616
>
> Increase the ple_gap to 128 to be on the safe side.  Is this enough
> for a CPU with HT that has a busy sibling thread, or should it be
> even larger?   On the X5670, loading up the sibling thread with an
> infinite loop does not seem to increase the required ple_gap.
>
> Signed-off-by: Rik van Riel <riel@...hat.com>
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 81fcbe9..c61fcbf 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -90,14 +90,14 @@ module_param(vmm_exclusive, bool, S_IRUGO);
>   * These 2 parameters are used to config the controls for Pause-Loop Exiting:
>   * ple_gap:    upper bound on the amount of time between two successive
>   *             executions of PAUSE in a loop. Also indicate if ple enabled.
> - *             According to test, this time is usually small than 41 cycles.
> + *             According to test, this time is usually smaller than 128 cycles.
>   * ple_window: upper bound on the amount of time a guest is allowed to execute
>   *             in a PAUSE loop. Tests indicate that most spinlocks are held for
>   *             less than 2^12 cycles
>   * Time is measured based on a counter that runs at the same rate as the TSC,
>   * refer SDM volume 3b section 21.6.13 & 22.1.3.
>   */
> -#define KVM_VMX_DEFAULT_PLE_GAP    41
> +#define KVM_VMX_DEFAULT_PLE_GAP    128
>  #define KVM_VMX_DEFAULT_PLE_WINDOW 4096
>  static int ple_gap = KVM_VMX_DEFAULT_PLE_GAP;
>  module_param(ple_gap, int, S_IRUGO);
>
>   
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