On Sun, May 08, 2016 at 10:08:55AM +0200, Mike Galbraith wrote: > > Maybe give the criteria a bit margin, not just wakees tend to equal > > llc_size, > > but the numbers are so wild to easily break the fragile condition, like: > > Seems lockless traversal and averages just lets multiple CPUs select > the same spot. An atomic reservation (feature) when looking for an > idle spot (also for fork) might fix it up. Run the thing as RT, > push/pull ensures that it reaches box saturation regardless of the > number of messaging threads, whereas with fair class, any number > 1 > will certainly stack tasks before the box is saturated.
Yes, good idea, bringing order to the race to grab idle CPU is absolutely helpful. In addition, I would argue maybe beefing up idle balancing is a more productive way to spread load, as work-stealing just does what needs to be done. And seems it has been (sub-unconsciously) neglected in this case, :) Regarding wake_wide(), it seems the M:N is 1:24, not 6:6*24, if so, the slave will be 0 forever (as last_wakee is never flipped). Basically whenever a waker has more than 1 wakee, the wakee_flips will comfortably grow very large (with last_wakee alternating), whereas when a waker has 0 or 1 wakee, the wakee_flips will just be 0. So recording only the last_wakee seems not right unless you have other good reason. If not the latter, counting waking wakee times should be better, and then allow the statistics to happily play.
