linux-kernel - Re: [PATCH v2 2/2] cpuacct: split usage into user_usage and sys

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Date:	Thu, 10 Mar 2016 14:27:28 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Zhao Lei <zhaolei@...fujitsu.com>
Cc:	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com, tj@...nel.org,
	Yang Dongsheng <yangds.fnst@...fujitsu.com>
Subject: Re: [PATCH v2 2/2] cpuacct: split usage into user_usage and
 sys_usage.

On Fri, Mar 04, 2016 at 05:47:06PM +0800, Zhao Lei wrote:
> +static u64 cpuacct_cpuusage_read(struct cpuacct *ca, int cpu,
> +				 enum cpuacct_usage_index index)
>  {
> +	struct cpuacct_usage *cpuusage = per_cpu_ptr(ca->cpuusage, cpu);
> +	u64 data = 0;
> +	int i = 0;
> +
> +	/*
> +	 * We allow index == CPUACCT_USAGE_NRUSAGE here to read
> +	 * the sum of suages.
> +	 */
> +	BUG_ON(index > CPUACCT_USAGE_NRUSAGE);
> +
> +	if (index == CPUACCT_USAGE_NRUSAGE) {
> +		raw_spin_lock_irq(&cpu_rq(cpu)->lock);
> +		for (i = 0; i < CPUACCT_USAGE_NRUSAGE; i++)
> +			data += cpuusage->usages[i];
> +		raw_spin_unlock_irq(&cpu_rq(cpu)->lock);

Why do you unconditionally take the lock here? You really don't need it
on 64 bit.

> +
> +		goto out;
> +	}
>  
>  #ifndef CONFIG_64BIT
>  	/*
>  	 * Take rq->lock to make 64-bit read safe on 32-bit platforms.
>  	 */
>  	raw_spin_lock_irq(&cpu_rq(cpu)->lock);
> +	data = cpuusage->usages[index];
>  	raw_spin_unlock_irq(&cpu_rq(cpu)->lock);
>  #else
> +	data = cpuusage->usages[index];
>  #endif
>  
> +out:
>  	return data;
>  }
>  
> +static void cpuacct_cpuusage_write(struct cpuacct *ca, int cpu,
> +				   enum cpuacct_usage_index index, u64 val)
>  {
> +	struct cpuacct_usage *cpuusage = per_cpu_ptr(ca->cpuusage, cpu);
> +	int i = 0;
> +
> +	/*
> +	 * We allow index == CPUACCT_USAGE_NRUSAGE here to write
> +	 * val to each index of usages.
> +	 */
> +	BUG_ON(index > CPUACCT_USAGE_NRUSAGE);
> +
> +	if (index == CPUACCT_USAGE_NRUSAGE) {
> +		raw_spin_lock_irq(&cpu_rq(cpu)->lock);
> +		for (i = 0; i < CPUACCT_USAGE_NRUSAGE; i++)
> +			cpuusage->usages[i] = val;
> +		raw_spin_unlock_irq(&cpu_rq(cpu)->lock);
> +
> +		return;
> +	}

Same for the above, and the below is dead code, you only ever call this
with NRUSAGE.

>  #ifndef CONFIG_64BIT
>  	/*
>  	 * Take rq->lock to make 64-bit write safe on 32-bit platforms.
>  	 */
>  	raw_spin_lock_irq(&cpu_rq(cpu)->lock);
> +	cpuusage->usages[index] = val;
>  	raw_spin_unlock_irq(&cpu_rq(cpu)->lock);
>  #else
> +	cpuusage->usages[index] = val;
>  #endif
>  }
>  

> @@ -246,9 +344,15 @@ void cpuacct_charge(struct task_struct *tsk, u64 cputime)
>  
>  	ca = task_ca(tsk);
>  
> +	user_time = user_mode(task_pt_regs(tsk));
> +
>  	while (true) {
> -		u64 *cpuusage = per_cpu_ptr(ca->cpuusage, cpu);
> -		*cpuusage += cputime;
> +		struct cpuacct_usage *cpuusage = per_cpu_ptr(ca->cpuusage, cpu);
> +
> +		if (user_time)
> +			cpuusage->usages[CPUACCT_USAGE_USER] += cputime;
> +		else
> +			cpuusage->usages[CPUACCT_USAGE_SYSTEM] += cputime;
>  
>  		ca = parent_ca(ca);
>  		if (!ca)

Have you tried to measure the performance impact of this?

Also, that code seems particularly silly for not using this_cpu_ptr().
After all, we only ever call this on current.

Also that ca iteration looks daft, should we fix that to read:

	for (ca = task_ca(tsk); ca; ca = parent_ca(ca))