linux-kernel - Re: [patch 3/3] convert CONFIG tag for extended accounting routines

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Date:	Thu, 03 Aug 2006 14:15:21 -0700
From:	Jay Lan <jlan@....com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Jay Lan <jlan@...r.sgi.com>, linux-kernel@...r.kernel.org,
	nagar@...son.ibm.com, balbir@...ibm.com, jes@....com,
	csturtiv@....com, tee@....com, guillaume.thouvenin@...l.net
Subject: Re: [patch 3/3] convert CONFIG tag for extended accounting routines

Andrew Morton wrote:
> On Wed, 02 Aug 2006 21:23:35 -0700
> Jay Lan <jlan@...r.sgi.com> wrote:
> 
> 
>>+/**
>>+ * acct_update_integrals - update mm integral fields in task_struct
>>+ * @tsk: task_struct for accounting
>>+ */
>>+void acct_update_integrals(struct task_struct *tsk)
>>+{
>>+	if (likely(tsk->mm)) {
>>+		long delta =
>>+			cputime_to_jiffies(tsk->stime) - tsk->acct_stimexpd;
> 
> 
> If a 32 architecture chooses to implement a 64-bit cputime_t, this
> expression might go wrong for very long-running tasks and high HZ.
> 
> Perhaps we should do all this in terms of cputime_t and export everything
> to userspace as u64?

Andrew,

We export to userspace the acct_rss_mem1 and acct_vm_mem1, both as u64.
The above logic is to calculate stime delta since last update. Note that
acc_update_integrals() is invoked at do_execve, do_exit, _AND_ at
account_system_time, which is called every jiffy by timer interrupt
handler.

The tsk->acct_stimexpd is used to save the tsk->stime of last update.
It should be changed to cputime_t as well. I will include the fix in
my next fix patch.

> 
> 
>>+		if (delta == 0)
>>+			return;
>>+		tsk->acct_stimexpd = tsk->stime;
>>+		tsk->acct_rss_mem1 += delta * get_mm_rss(tsk->mm);
>>+		tsk->acct_vm_mem1 += delta * tsk->mm->total_vm;
> 
> 
> It's a bit weird to be multiplying RSS by time.  What unit is a "byte
> second"?
> 
> If this is not a bug then I guess this is an intermediate term for
> additional downstream processing.  There is information loss here and I'd
> have thought that it would be better to simply send `delta' and the rss
> straight to userspace, let userspace work out what math it wants to perform
> on it.  If that makes sense?
> 
> I see that the code has been like this for a long time, so treat this as a
> "please educate me about BSD accounting" email ;)

This is not a BSD accounting thing. It came from UNICOS and IRIX.
I am pinging the person who knows how the real world users use these
two fields...

Regards,
  - jay

> 


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