On Mon, May 6, 2013 at 8:00 AM, Alex Shi <[email protected]> wrote: >> >> blocked_load_avg is the expected "to wake" contribution from tasks >> already assigned to this rq. >> >> e.g. this could be: >> load = this_rq->cfs.runnable_load_avg + this_rq->cfs.blocked_load_avg; > > Current load balance doesn't consider slept task's load which is > represented by blocked_load_avg. And the slept task is not on_rq, so > consider it in load balance is a little strange.
The load-balancer has a longer time horizon; think of blocked_loag_avg to be a signal for the load, already assigned to this cpu, which is expected to appear (within roughly the next quantum). Consider the following scenario: tasks: A,B (40% busy), C (90% busy) Suppose we have: CPU 0: CPU 1: A C B Then, when C blocks the load balancer ticks. If we considered only runnable_load then A or B would be eligible for migration to CPU 1, which is essentially where we are today. > > But your concern is worth to try. I will change the patchset and give > the testing results. > >> >> Although, in general I have a major concern with the current implementation: >> >> The entire reason for stability with the bottom up averages is that >> when load migrates between cpus we are able to migrate it between the >> tracked sums. >> >> Stuffing observed averages of these into the load_idxs loses that >> mobility; we will have to stall (as we do today for idx > 0) before we >> can recognize that a cpu's load has truly left it; this is a very >> similar problem to the need to stably track this for group shares >> computation. >> >> To that end, I would rather see the load_idx disappear completely: >> (a) We can calculate the imbalance purely from delta (runnable_avg + >> blocked_avg) >> (b) It eliminates a bad tunable. > > I also show the similar concern of load_idx months ago. seems overlooked. :) >> >>> - return cpu_rq(cpu)->load.weight; >>> + return (unsigned long)cpu_rq(cpu)->cfs.runnable_load_avg; >> >> Isn't this going to truncate on the 32-bit case? > > I guess not, the old load.weight is unsigned long, and runnable_load_avg > is smaller than the load.weight. so it should be fine. > > btw, according to above reason, guess move runnable_load_avg to > 'unsigned long' type is ok, do you think so? > Hmm, so long as it's unsigned long and not u32 that should be OK. >From a technical standpoint: We make the argument that we run out of address space before we can overflow load.weight in the 32-bit case, we can make the same argument here. > > -- > Thanks > Alex -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
