Re: [PATCH] procfs: export context switch counts in /proc/*/stat

["Albert Cahalan" <acahalan@gmail.com>]

David Wragg writes:
> Benjamin LaHaise <bcrl@kvack.org> writes:
>> On Mon, Dec 18, 2006 at 11:50:08PM +0000, David Wragg wrote:

>>> This patch (against 2.6.19/2.6.19.1) adds the four context
>>> switch values (voluntary context switches, involuntary
>>> context switches, and the same values accumulated from
>>> terminated child processes) to the end of /proc/*/stat,
>>> similarly to min_flt, maj_flt and the time used values.

Hmmm, OK, do people have a use for these values?

>> Please put these into new files, as the stat files in /proc are
>> horribly overloaded and have always been somewhat problematic
>> when it comes to changing how things are reported due to internal
>> changes to the kernel.  Cheers,

No thanks. Yours truly, the maintainer of "ps", "top", "vmstat", etc.

> The delay accounting value was added to the end of /proc/pid/stat back
> in July without discussion, so I assumed this approach was still
> considered satisfactory.

/proc/*/stat is the very best place in /proc for any per-process
data that will be commonly needed. Unlike /proc/*/status, few
people are tempted to screw with the formatting and/or spelling.
Unlike the /sys crap, it doesn't take 3 syscalls PER VALUE to
get at the data.

The things to ask are of course: will this really be used, and
does it really belong in /proc at all?

> Putting just these four values into a new file would seem a little
> odd, since they have a lot in common with the other getrusage values
> that are already in /proc/pid/stat.  One possibility is to add
> /proc/pid/rusage, mirroring the full struct rusage in text form, since
> struct rusage is already part of the kernel ABI (though Linux doesn't
> fill in half of the values).

Since we already have a struct defined and all...

sys_get_rusage(int pid)

> Or perhaps it makes sense to reorganize all the values from
> /proc/pid/stat and its siblings into a sysfs-like one-value-per-file
> structure, though that might introduce atomicity and efficiency issues
> (calculating some of the values involves iterating over the threads in
> the process; with everything in one file, these loops are folded
> together).

Yeah, big time. Things are quite bad in /proc, but /sys is a joke.