linux-kernel - Re: [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update

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Date:	Sat, 30 Jan 2016 15:44:04 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
	luto@...capital.net, peterz@...radead.org, clark@...hat.com
Subject: Re: [PATCH 1/4] sched,time: remove non-power-of-two divides from
 __acct_update_integrals

On Fri, Jan 29, 2016 at 10:36:02PM -0500, riel@...hat.com wrote:
> From: Rik van Riel <riel@...hat.com>
> 
> When running a microbenchmark calling an invalid syscall number
> in a loop, on a nohz_full CPU, we spend a full 9% of our CPU
> time in __acct_update_integrals.
> 
> This function converts cputime_t to jiffies, to a timeval, only to
> convert the timeval back to microseconds before discarding it.
> 
> This patch leaves __acct_update_integrals functionally equivalent,
> but speeds things up by about 12%, with 10 million calls to an
> invalid syscall number dropping from 3.7 to 3.25 seconds.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  kernel/tsacct.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/tsacct.c b/kernel/tsacct.c
> index 975cb49e32bf..41667b23dbd0 100644
> --- a/kernel/tsacct.c
> +++ b/kernel/tsacct.c
> @@ -93,9 +93,9 @@ void xacct_add_tsk(struct taskstats *stats, struct task_struct *p)
>  {
>  	struct mm_struct *mm;
>  
> -	/* convert pages-usec to Mbyte-usec */
> -	stats->coremem = p->acct_rss_mem1 * PAGE_SIZE / MB;
> -	stats->virtmem = p->acct_vm_mem1 * PAGE_SIZE / MB;
> +	/* convert pages-nsec/KB to Mbyte-usec, see __acct_update_integrals */
> +	stats->coremem = p->acct_rss_mem1 * PAGE_SIZE / (1000 * KB);
> +	stats->virtmem = p->acct_vm_mem1 * PAGE_SIZE / (1000 * KB);
>  	mm = get_task_mm(p);
>  	if (mm) {
>  		/* adjust to KB unit */
> @@ -125,22 +125,21 @@ static void __acct_update_integrals(struct task_struct *tsk,
>  {
>  	if (likely(tsk->mm)) {
>  		cputime_t time, dtime;
> -		struct timeval value;
>  		unsigned long flags;
>  		u64 delta;
>  
>  		local_irq_save(flags);
>  		time = stime + utime;
>  		dtime = time - tsk->acct_timexpd;
> -		jiffies_to_timeval(cputime_to_jiffies(dtime), &value);
> -		delta = value.tv_sec;
> -		delta = delta * USEC_PER_SEC + value.tv_usec;
> +		delta = cputime_to_nsecs(dtime);

You might want to add a comment specifying why we don't call cputime_to_usecs()
directly (because we optimize if delta < TICK_NSEC).

Although this has a good impact on nohz_full, it might have a tiny bad one on !nohz_full
because now we first convert jiffies to nsecs (which implies a multiplication by 1000)
that we later divide again by 1000. Now this is ok because I plan to convert tsk->utime/stime
to nsecs and thus remove most of the cputime_t use and conversions everywhere.

>  
> -		if (delta == 0)
> +		if (delta < TICK_NSEC)
>  			goto out;


> +
>  		tsk->acct_timexpd = time;
> -		tsk->acct_rss_mem1 += delta * get_mm_rss(tsk->mm);
> -		tsk->acct_vm_mem1 += delta * tsk->mm->total_vm;
> +		/* The final unit will be Mbyte-usecs, see xacct_add_tsk */
> +		tsk->acct_rss_mem1 += delta * get_mm_rss(tsk->mm) / 1024;
> +		tsk->acct_vm_mem1 += delta * tsk->mm->total_vm / 1024;

The use of 1024 and the change on MB above are confusing me. Why are we doing that?

Thanks.

>  	out:
>  		local_irq_restore(flags);
>  	}
> -- 
> 2.5.0
>