Re: [PATCH 7/7] CPU controller V1 - (temporary) cpuset interface

Paul Jackson
2006-08-22T18:56:15Z

Mike wrote:
> By cat'ing each task pid
> (including init's) to root (or mikeg) task's file?

I guess you meant:
echo'ing
not:
cat'ing

Lets say for example one has cpusets:
/dev/cpuset
/dev/cpuset/foo

One cannot move the tasks in 'foo' to the top (root) cpuset by doing:

cat < /dev/cpuset/foo/tasks > /dev/cpuset/tasks # fails

That cat fails because the tasks file has to be written one pid at a
time, not in big buffered writes of multiple lines like cat does.

The usual code for doing this move is:

while read i
do
/bin/echo $i > /dev/cpuset/tasks
done < /dev/cpuset/foo/tasks

There is a cute trick that lets you move all the tasks in one cpuset to
another cpuset in a one-liner, by making use of the "sed -u" unbuffered
option:

sed -nu p < /dev/cpuset/foo/tasks > /dev/cpuset/tasks # works

For serious production work, the above is still racey. A task could be
added to the 'foo' cpuset when another task in 'foo' forks while the
copying is being done. The following loop minimizes (doesn't perfectly
solve) this race:

while test -s /dev/cpsuet/foo/tasks
do
sed -nu p < /dev/cpuset/foo/tasks > /dev/cpuset/tasks
done

The above loop is still theoretically racey with fork, but seems to
work in practice.

--
I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <pj@sgi.com> 1.925.600.0401