linux-kernel - Re: [PATCH v4 3/3] sched/cputime: Add steal time support to full dynticks CPU time accounting

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Date:	Tue, 7 Jun 2016 12:47:53 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <wanpeng.li@...mail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v4 3/3] sched/cputime: Add steal time support to full
 dynticks CPU time accounting



On 07/06/2016 10:00, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> This patch adds guest steal-time support to full dynticks CPU
> time accounting. After the following commit:
> 
> ff9a9b4c4334 ("sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity")
> 
> ... time sampling became jiffy based, even if it's still listened
> to ring boundaries, so steal_account_process_tick() is reused
> to account how many 'ticks' are stolen-time, after the last accumulation.

I still have no idea how to parse this.  What are "ring boundaries"?
Rik, can you suggest a better commit message?

> Suggested-and-Reviewed-by: Rik van Riel <riel@...hat.com>

Please split Suggested-by and Reviewed-by.

> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 75f98c5..9ff036b 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -257,7 +257,7 @@ void account_idle_time(cputime_t cputime)
>  		cpustat[CPUTIME_IDLE] += (__force u64) cputime;
>  }
>  
> -static __always_inline bool steal_account_process_tick(void)
> +static __always_inline unsigned long steal_account_process_tick(void)
>  {
>  #ifdef CONFIG_PARAVIRT
>  	if (static_key_false(&paravirt_steal_enabled)) {
> @@ -279,7 +279,7 @@ static __always_inline bool steal_account_process_tick(void)
>  		return steal_jiffies;
>  	}
>  #endif
> -	return false;
> +	return 0;
>  }
>  
>  /*
> @@ -691,9 +691,13 @@ static cputime_t get_vtime_delta(struct task_struct *tsk)
>  
>  static void __vtime_account_system(struct task_struct *tsk)
>  {
> -	cputime_t delta_cpu = get_vtime_delta(tsk);
> +	cputime_t delta_time = get_vtime_delta(tsk);
> +	cputime_t steal_time = jiffies_to_cputime(steal_account_process_tick());
>  
> -	account_system_time(tsk, irq_count(), delta_cpu, cputime_to_scaled(delta_cpu));
> +	if (steal_time < delta_time) {
> +		delta_time -= steal_time;
> +		account_system_time(tsk, irq_count(), delta_time, cputime_to_scaled(delta_time));
> +	}
>  }
>  
>  void vtime_account_system(struct task_struct *tsk)
> @@ -718,13 +722,18 @@ void vtime_gen_account_irq_exit(struct task_struct *tsk)
>  
>  void vtime_account_user(struct task_struct *tsk)
>  {
> -	cputime_t delta_cpu;
> +	cputime_t delta_time, steal_time;
>  
>  	write_seqcount_begin(&tsk->vtime_seqcount);
>  	tsk->vtime_snap_whence = VTIME_SYS;
>  	if (vtime_delta(tsk)) {
> -		delta_cpu = get_vtime_delta(tsk);
> -		account_user_time(tsk, delta_cpu, cputime_to_scaled(delta_cpu));
> +		delta_time = get_vtime_delta(tsk);
> +		steal_time = jiffies_to_cputime(steal_account_process_tick());
> +
> +		if (steal_time < delta_time) {
> +			delta_time -= steal_time;
> +			account_user_time(tsk, delta_time, cputime_to_scaled(delta_time));
> +		}
>  	}
>  	write_seqcount_end(&tsk->vtime_seqcount);
>  }
> 

You're adding almost the same code to two callers of get_vtime_delta out
of three.  I don't know the vtime accounting code very well, but why
doesn't the same apply to account_idle_time?

If it does, you should instead change get_vtime_delta to process steal
time and subtract it from the result.

Secondarily, when can it happen that steal_time > delta_time?

Thanks,

Paolo