How is run queue length computed in linux proc filesystem

[Valdis.Kletnieks at vt.edu]

On Fri, 16 Aug 2013 12:57:23 -0400, Rik van Riel said:
> On 08/16/2013 09:20 AM, emani murali wrote:
> > Hi,
> >
> > I'm trying to obtain number of runnable processes from linux kernel. sar
> > -q gives this information readily. However I'm trying to get this value
> > from /proc filesystem. There is no file in /proc that gives this value
> > directly, then how is runq-sz computed.
>
> Tools like top, vmstat, and sar, simply look at all the processes
> in /proc, and count the number that are in runnable (R) state.

Which is why I asked what Emani is trying to do - scanning /proc is
inherently racy, and the runq-sz value can change and/or be different than
counting the number of runnable processes.  A slight bit of jitter in the
output of top probably doesn't matter.  Feeding the number back into a
scheduler loop or something like that may matter a lot...
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