Re: [PATCH] sched/cpufreq: Fix 32bit math overflow

[Rafael J. Wysocki]

On Tuesday, March 5, 2019 9:32:02 AM CET Peter Zijlstra 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?
> 
> ---
> 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@xxxxxxxxxx>
> Reported-by: Vincent Wang <vincent.wang@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

> ---
>  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) {
>