Re: [PATCH v3 6/7] KVM: VMX: runtime knobs for dynamic PLE window

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 08/21/2014 09:38 PM, Radim Krčmář wrote:
> ple_window is updated on every vmentry, so there is no reason to have it
> read-only anymore.
> ple_window* weren't writable to prevent runtime overflow races;
> they are prevented by a seqlock.
>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
>   arch/x86/kvm/vmx.c | 46 +++++++++++++++++++++++++++++++++++++---------
>   1 file changed, 37 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 273cbd5..1318232 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -132,24 +132,29 @@ module_param(nested, bool, S_IRUGO);
>   #define KVM_VMX_DEFAULT_PLE_WINDOW_MAX    \
>   		INT_MAX / KVM_VMX_DEFAULT_PLE_WINDOW_GROW
>
> +static struct kernel_param_ops param_ops_ple_int;
> +#define param_check_ple_int(name, p) __param_check(name, p, int)
> +
> +static DEFINE_SEQLOCK(ple_window_seqlock);
> +
>   static int ple_gap = KVM_VMX_DEFAULT_PLE_GAP;
>   module_param(ple_gap, int, S_IRUGO);
>
>   static int ple_window = KVM_VMX_DEFAULT_PLE_WINDOW;
> -module_param(ple_window, int, S_IRUGO);
> +module_param(ple_window, ple_int, S_IRUGO | S_IWUSR);
>
>   /* Default doubles per-vcpu window every exit. */
>   static int ple_window_grow = KVM_VMX_DEFAULT_PLE_WINDOW_GROW;
> -module_param(ple_window_grow, int, S_IRUGO);
> +module_param(ple_window_grow, ple_int, S_IRUGO | S_IWUSR);
>
>   /* Default resets per-vcpu window every exit to ple_window. */
>   static int ple_window_shrink = KVM_VMX_DEFAULT_PLE_WINDOW_SHRINK;
> -module_param(ple_window_shrink, int, S_IRUGO);
> +module_param(ple_window_shrink, int, S_IRUGO | S_IWUSR);
>
>   /* Default is to compute the maximum so we can never overflow. */
>   static int ple_window_actual_max = KVM_VMX_DEFAULT_PLE_WINDOW_MAX;
>   static int ple_window_max        = KVM_VMX_DEFAULT_PLE_WINDOW_MAX;
> -module_param(ple_window_max, int, S_IRUGO);
> +module_param(ple_window_max, ple_int, S_IRUGO | S_IWUSR);
>
>

Positive thing about able to change default grow/shrink value is the
flexibility of tuning ple window to different workloads and different
number of cpus.

But is it that a value of shrink = 1 and grow > 1 is problematic ?
(running a undercommit workload and then overcommit workload)