linux-kernel - Re: [PATCH v2] perfcounters: record time running and time enabled for each counter

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Date:	Sun, 22 Mar 2009 10:13:35 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] perfcounters: record time running and time enabled
 for each counter

Andrew Morton writes:

> Perhaps one of the reasons why this code is confusing is the blurring
> between the "time" at which an event occured and the "time" between the
> occurrence of two events.  A weakness in English, I guess.  Using the term
> "interval" in the latter case will help a lot.

Except that we aren't measuring an "interval", we're measuring the
combined length of a whole series of intervals.  What's a good word
for that?

> > +	atomic64_t			child_time_enabled;
> > +	atomic64_t			child_time_running;
> 
> These read like booleans, but why are they atomic64_t's?

OK so this file could use more comments, but I did answer that
question in the patch description.

> > -	return put_user(cntval, (u64 __user *) buf) ? -EFAULT : sizeof(cntval);
> > +	if (count != n * sizeof(u64))
> > +		return -EINVAL;
> > +
> > +	if (!access_ok(VERIFY_WRITE, buf, count))
> > +		return -EFAULT;
> > +	
> 
> <panics>
> 
> Oh.
> 
> It would be a lot more reassuring to verify `uptr', rather than `buf' here.
> 
> The patch adds new trailing whitespace.  checkpatch helps.
> 
> > +	for (i = 0; i < n; ++i)
> > +		if (__put_user(values[i], uptr + i))
> > +			return -EFAULT;
> 
> And here we iterate across `n', whereas we verified `count'.

And the fact that we just verified count == n * 8, four lines above,
doesn't give you any comfort?

Paul.
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