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Date:   Wed, 6 Mar 2019 15:50:39 +0800
From:   Chunyan Zhang <zhang.lyra@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Quentin Perret <quentin.perret@....com>,
        Wang, Vincent (王争) 
        <Vincent.Wang@...soc.com>,
        Zhang, Chunyan (张春艳) 
        <Chunyan.Zhang@...soc.com>, Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH] sched/cpufreq: Fix 32bit math overflow

On Tue, Mar 5, 2019 at 4:32 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Mar 04, 2019 at 07:11:01PM +0000, Quentin Perret wrote:
>
> > So yeah, that works for me.
>
> Chunyan, Vincent; can you verify the below cures your ill?

Verified by Vincent, the patch below can fix the problem Vincent found
on our platform before.

Thanks,
Chunyan

>
> ---
> Subject: sched/cpufreq: Fix 32bit math overflow
>
> Vincent Wang reported that get_next_freq() has a mult overflow issue on
> 32bit platforms in the IOWAIT boost case, since in that case {util,max}
> are in freq units instead of capacity units.
>
> Solve this by moving the IOWAIT boost to capacity units. And since this
> means @max is constant; simplify the code.
>
> Cc: Chunyan Zhang <chunyan.zhang@...soc.com>
> Reported-by: Vincent Wang <vincent.wang@...soc.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/sched/cpufreq_schedutil.c | 58 +++++++++++++++++-----------------------
>  1 file changed, 24 insertions(+), 34 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 2efe629425be..72b62ac1c7c2 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -48,10 +48,10 @@ struct sugov_cpu {
>
>         bool                    iowait_boost_pending;
>         unsigned int            iowait_boost;
> -       unsigned int            iowait_boost_max;
>         u64                     last_update;
>
>         unsigned long           bw_dl;
> +       unsigned long           min;
>         unsigned long           max;
>
>         /* The field below is for single-CPU policies only: */
> @@ -303,8 +303,7 @@ static bool sugov_iowait_reset(struct sugov_cpu *sg_cpu, u64 time,
>         if (delta_ns <= TICK_NSEC)
>                 return false;
>
> -       sg_cpu->iowait_boost = set_iowait_boost
> -               ? sg_cpu->sg_policy->policy->min : 0;
> +       sg_cpu->iowait_boost = set_iowait_boost ? sg_cpu->min : 0;
>         sg_cpu->iowait_boost_pending = set_iowait_boost;
>
>         return true;
> @@ -344,14 +343,12 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
>
>         /* Double the boost at each request */
>         if (sg_cpu->iowait_boost) {
> -               sg_cpu->iowait_boost <<= 1;
> -               if (sg_cpu->iowait_boost > sg_cpu->iowait_boost_max)
> -                       sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
> +               sg_cpu->iowait_boost = min(sg_cpu->iowait_boost << 1, SCHED_CAPACITY_SCALE);
>                 return;
>         }
>
>         /* First wakeup after IO: start with minimum boost */
> -       sg_cpu->iowait_boost = sg_cpu->sg_policy->policy->min;
> +       sg_cpu->iowait_boost = sg_cpu->min;
>  }
>
>  /**
> @@ -373,47 +370,38 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
>   * This mechanism is designed to boost high frequently IO waiting tasks, while
>   * being more conservative on tasks which does sporadic IO operations.
>   */
> -static void sugov_iowait_apply(struct sugov_cpu *sg_cpu, u64 time,
> -                              unsigned long *util, unsigned long *max)
> +static unsigned long sugov_iowait_apply(struct sugov_cpu *sg_cpu, u64 time,
> +                                       unsigned long util, unsigned long max)
>  {
> -       unsigned int boost_util, boost_max;
> +       unsigned long boost;
>
>         /* No boost currently required */
>         if (!sg_cpu->iowait_boost)
> -               return;
> +               return util;
>
>         /* Reset boost if the CPU appears to have been idle enough */
>         if (sugov_iowait_reset(sg_cpu, time, false))
> -               return;
> +               return util;
>
> -       /*
> -        * An IO waiting task has just woken up:
> -        * allow to further double the boost value
> -        */
> -       if (sg_cpu->iowait_boost_pending) {
> -               sg_cpu->iowait_boost_pending = false;
> -       } else {
> +       if (!sg_cpu->iowait_boost_pending) {
>                 /*
> -                * Otherwise: reduce the boost value and disable it when we
> -                * reach the minimum.
> +                * No boost pending; reduce the boost value.
>                  */
>                 sg_cpu->iowait_boost >>= 1;
> -               if (sg_cpu->iowait_boost < sg_cpu->sg_policy->policy->min) {
> +               if (sg_cpu->iowait_boost < sg_cpu->min) {
>                         sg_cpu->iowait_boost = 0;
> -                       return;
> +                       return util;
>                 }
>         }
>
> +       sg_cpu->iowait_boost_pending = false;
> +
>         /*
> -        * Apply the current boost value: a CPU is boosted only if its current
> -        * utilization is smaller then the current IO boost level.
> +        * @util is already in capacity scale; convert iowait_boost
> +        * into the same scale so we can compare.
>          */
> -       boost_util = sg_cpu->iowait_boost;
> -       boost_max = sg_cpu->iowait_boost_max;
> -       if (*util * boost_max < *max * boost_util) {
> -               *util = boost_util;
> -               *max = boost_max;
> -       }
> +       boost = (sg_cpu->iowait_boost * max) >> SCHED_CAPACITY_SHIFT;
> +       return max(boost, util);
>  }
>
>  #ifdef CONFIG_NO_HZ_COMMON
> @@ -460,7 +448,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>
>         util = sugov_get_util(sg_cpu);
>         max = sg_cpu->max;
> -       sugov_iowait_apply(sg_cpu, time, &util, &max);
> +       util = sugov_iowait_apply(sg_cpu, time, util, max);
>         next_f = get_next_freq(sg_policy, util, max);
>         /*
>          * Do not reduce the frequency if the CPU has not been idle
> @@ -500,7 +488,7 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
>
>                 j_util = sugov_get_util(j_sg_cpu);
>                 j_max = j_sg_cpu->max;
> -               sugov_iowait_apply(j_sg_cpu, time, &j_util, &j_max);
> +               j_util = sugov_iowait_apply(j_sg_cpu, time, j_util, j_max);
>
>                 if (j_util * max > j_max * util) {
>                         util = j_util;
> @@ -837,7 +825,9 @@ static int sugov_start(struct cpufreq_policy *policy)
>                 memset(sg_cpu, 0, sizeof(*sg_cpu));
>                 sg_cpu->cpu                     = cpu;
>                 sg_cpu->sg_policy               = sg_policy;
> -               sg_cpu->iowait_boost_max        = policy->cpuinfo.max_freq;
> +               sg_cpu->min                     =
> +                       (SCHED_CAPACITY_SCALE * policy->cpuinfo.min_freq) /
> +                       policy->cpuinfo.max_freq;
>         }
>
>         for_each_cpu(cpu, policy->cpus) {

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