Hi Tejun,

On 26 November 2012 22:45, Tejun Heo [off-list ref] wrote:

On Tue, Nov 06, 2012 at 04:08:45PM +0530, Viresh Kumar wrote:

I'm pretty skeptical about this.  queue_work() w/o explicit CPU
assignment has always guaranteed that the work item will be executed
on the same CPU.  I don't think there are too many users depending on
that but am not sure at all that there are none.  I asked you last
time that you would at the very least need to audit most users but it
doesn't seem like there has been any effort there.

My bad. I completely missed/forgot that comment from your earlier mails.
Will do it.

That said, if the obtained benefit is big enough, sure, we can
definitely change the behavior, which isn't all that great to begin
with, and try to shake out the bugs quicky by e.g. forcing all work
items to execute on different CPUs, but you're presenting a few
percent of work items being migrated to a different CPU from an
already active CPU, which doesn't seem like such a big benefit to me
even if the migration target CPU is somewhat more efficient.  How much
powersaving are we talking about?

Hmm.. I actually implemented the problem discussed here:
(I know you have seen this earlier :) )

http://www.linuxplumbersconf.org/2012/wp-content/uploads/2012/08/lpc2012-sched-timer-workqueue.pdf

Specifically slides: 12 & 19.

I haven't done much power calculations with it and have tested it more from
functionality point of view.

@Vincent: Can you add some comments here?

--
viresh