linux-kernel - Re: [PATCH RESEND v2 1/4] cpuidle-haltpoll: ensure grow start value is nonzero

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Date:   Fri, 15 Nov 2019 11:26:43 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Zhenzhong Duan <zhenzhong.duan@...cle.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        kvm-devel <kvm@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH RESEND v2 1/4] cpuidle-haltpoll: ensure grow start value
 is nonzero

On Wed, Nov 6, 2019 at 12:56 PM Zhenzhong Duan
<zhenzhong.duan@...cle.com> wrote:
>
> dev->poll_limit_ns could be zeroed in certain cases (e.g. by
> guest_halt_poll_ns = 0). If guest_halt_poll_grow_start is zero,
> dev->poll_limit_ns will never be bigger than zero.

I would rephrase this in the following way:

"If guest_halt_poll_grow_start is zero and dev->poll_limit_ns becomes
zero for any reason, it will never be greater than zero again, so use
..."

The patch itself looks OK to me.

> Use param callback to avoid writing zero to guest_halt_poll_grow_start.
>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...cle.com>
> ---
>  drivers/cpuidle/governors/haltpoll.c | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/cpuidle/governors/haltpoll.c b/drivers/cpuidle/governors/haltpoll.c
> index 7a703d2..660859d 100644
> --- a/drivers/cpuidle/governors/haltpoll.c
> +++ b/drivers/cpuidle/governors/haltpoll.c
> @@ -20,6 +20,26 @@
>  #include <linux/module.h>
>  #include <linux/kvm_para.h>
>
> +static int grow_start_set(const char *val, const struct kernel_param *kp)
> +{
> +       int ret;
> +       unsigned int n;
> +
> +       if (!val)
> +               return -EINVAL;
> +
> +       ret = kstrtouint(val, 0, &n);
> +       if (ret || !n)
> +               return -EINVAL;
> +
> +       return param_set_uint(val, kp);
> +}
> +
> +static const struct kernel_param_ops grow_start_ops = {
> +       .set = grow_start_set,
> +       .get = param_get_uint,
> +};
> +
>  static unsigned int guest_halt_poll_ns __read_mostly = 200000;
>  module_param(guest_halt_poll_ns, uint, 0644);
>
> @@ -33,7 +53,7 @@
>
>  /* value in us to start growing per-cpu halt_poll_ns */
>  static unsigned int guest_halt_poll_grow_start __read_mostly = 50000;
> -module_param(guest_halt_poll_grow_start, uint, 0644);
> +module_param_cb(guest_halt_poll_grow_start, &grow_start_ops, &guest_halt_poll_grow_start, 0644);
>
>  /* allow shrinking guest halt poll */
>  static bool guest_halt_poll_allow_shrink __read_mostly = true;
> --
> 1.8.3.1
>