linux-kernel - Jiffies wraparound is not treated in the schedstats

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Date:	Wed, 8 Nov 2006 14:05:10 -0400
From:	"Mauricio Lin" <mauriciolin@...il.com>
To:	balbir@...ibm.com
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Jiffies wraparound is not treated in the schedstats

Hi Balbir,

Do you know why in the sched_info_arrive() and sched_info_depart()
functions the calculation of delta_jiffies does not use the time_after
or time_before macro to prevent  the miscalculation when jiffies
overflow?

For instance the delta_jiffies variable is simply calculated as:

delta_jiffies = now - t->sched_info.last_queued;

Do not you think the more logical way should be

if (time_after(now, t->sched_info.last_queued))
   delta_jiffies = now - t->sched_info.last_queued;
else
   delta_jiffies = (MAX_JIFFIES - t->sched_info.last_queued) + now

I have included more variables to measure some issues of schedule in
the kernel (following schedstat idea) and I noticed that jiffies
wraparound has led to wrong values, since the user space tool when
collecting the values is producing negative values.

Any comments?

Can I provide a patch for that?

BR,

Mauricio Lin.
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