linux-kernel - Re: [patch 1/3] add basic accounting fields to taskstats

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Message-ID: <44D7AF34.10301@sgi.com>
Date:	Mon, 07 Aug 2006 14:23:00 -0700
From:	Jay Lan <jlan@....com>
To:	Jay Lan <jlan@....com>
Cc:	balbir@...ibm.com, Andrew Morton <akpm@...l.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Shailabh Nagar <nagar@...son.ibm.com>,
	Jes Sorensen <jes@....com>,
	Chris Sturtivant <csturtiv@....com>, Tony Ernst <tee@....com>
Subject: Re: [patch 1/3] add basic accounting fields to taskstats

Jay Lan wrote:

[snip]

>>
>>
>>> +    /* Each process gets a minimum of a half tick cpu time */
>>> +    if ((stats->ac_utime == 0) && (stats->ac_stime == 0)) {
>>> +        stats->ac_stime = USEC_PER_TICK/2;
>>> +    }
>>> +
>>
>>
>>
>> This is confusing. Half tick does not make any sense from the
>> scheduler view point (or am I missing something?), so why
>> return half a tick to the user.
> 
> 
> It must be inherited from old code dated back to Cray UNICOS.
> I do not know if bad thing can happen if both utime and stime
> are less than 1 usec...  I guess not. But i agree that
> half a tick does not make sense. To play safe, we can change
> it to 1 usec if both utime and stime are sub microsecond.
> What do you think?

Hi Balbir,

I figured this out. The tsk->stime (and utime as well) are
charged by 1 tick (or cputime) from the timer interrupt handler
through update_process_times->account_{user,system}_time.

The clock resolution is a tick. Any short process less than
1 tick will the counter being 0. It can be from 0 to 0.99999...
tick. A half tick is the average value.

I think it makes more sense to assign a half tick than assign
1 usec to the stime. What do you think? Certainly the code need
better explanation.

Regards,
  - jay


[snip]
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