getting 'load average' info from inside a kernel module
Robert Bonomi
bonomi at mail.r-bonomi.com
Wed Dec 17 20:14:22 UTC 2014
originally sent to -questions, where it was suggested that -hackers was more
likely to produce some useful info.
> Subject: getting 'load averages' (or something similar) from inside a _kernel_
>
> I'm trying to get the current 'runnable processes' count from inside a
> kernel loadable module for BSD 8.4, so I can tweak it's behavior depending
> on the current activity level. All I've found so far is what's in 'man 9
> runqueue', and it is *badly* out-of-sync with the 8.4 kernel code. <snarl>
> <wry grin> e.g., the external arrays at the beginning of the synopisis
> are shown as being of type 'struct rq' -- but there *ISN"T* any defined
> struct 'rq'; it seems to be named 'runq', at least in /usr/include/sys/runq.h
>
> Then the cr*p gets deeper -- trying to make heads or tails out of how to
> get a count of runnable processes from the _arrays_ (of unknown size)
> described on the 'man 9 runqueue' page info has me defeated.
> Looking at '/usr/include/sys/runq.h', it appears that 'struct runq' is
> a single item with an array of queues (by 'nice' level), and a bitmap of
> which array elements have non-zero length queues.
>
> I'm perfectly willing to brute-force the data out of the queue lists, *IF*
> there's some reasonable, _current_, descriptive info of the format/usage
> of those kernel structures.
if 'man 9 runqueue' is 'mostly' correct -- reality just being 'runq' instead
of 'rq' -- then 'how to find out' how mamy elements are in those 'unknown size'
arrays is the missing element. *OR* if those four items are _not_ arrays,
but simple structs -- given the array of queueheads in the 'runq' struct,
I can probably decipher the rest.
>
> Alternatively, is there a way to directly access the 'sysctl' MIB data in
> kernel memory. sysctl(3) is out -- it's in 'libc', and not available to
> kernel modules. Pointers to descriptions of the kernel in-memory data for
> that MIB would be a big help. Or 'where to ask', if there's a better place.
I'm willing to 'count processes' in the run queues each time I query -- an
'instantenous' value is fine for my purposes, I can derive whatever 'avrage
over time' I need. :))
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