Re: [PATCH v2 3/5] change number_of_cpusets to an atomic

On Tue, 24 Apr 2012, Glauber Costa wrote:

quoted

quoted

It doesn't seem to be the case here.

How did you figure that? number_of_cpusets was introduced exactly because
the functions are used in places where we do not pay the cost of calling
__cpuset_node_allowed_soft/hardwall. Have a look at these. They may take
locks etc etc in critical allocation paths

I am not arguing that.

You want to avoid the cost of processing a function, that's fair.
(Note that by "function call cost" I don't mean the cost of processing a
function, but the cost of a (potentially empty) function call.)
The real question is: Are you okay with the cost of a branch + a global
variable (which is almost read only) fetch?

No and that is why the static branching comes in. It takes away the global
read of the number_of_cpusets variable in the critical paths.

The test of a global variable can - and do as of right now - avoid all the
expensive operations like locking, sleeping, etc, and if you don't need to
squeeze every nanosecond you can, they are often simpler - and therefore
better - than static branching.

Better than static branching? This is in critical VM functions and
reducing the cache footprint there is good for everyone.

Just to mention one point I am coming across these days - that initiated all
this: static patching holds the cpu_hotplug.lock. So it can't be called if you
hold any lock that has been already held under the cpu_hotplug.lock. This will
probably mean any lock the cpuset cgroup needs to take, because it is called -
and to do a lot of things - from the cpu hotplug handler, that holds the
cpu_hotplug.lock.

Transitions from one to two cpusets are rare and are only done when a
cpuset is created in the /dev/cpuset hierachy). You could move the
code modification outside of locks or defer action into an event
thread if there are locks in the way.