Hi Peter, Thank you for the clarification. On 08/19/2013 04:00 PM, Peter Zijlstra wrote: > On Mon, Aug 19, 2013 at 09:47:47AM +0530, Preeti U Murthy wrote: >> Hi Peter, >> >> On 08/16/2013 03:42 PM, Peter Zijlstra wrote: >> >> I have a few comments and clarification to seek. >> >> 1. How are you ensuring from this patch that sgs->group_power does not >> change over the course of load balancing? > > Well, we only set it the one time when creating the sgs data. > >> The only path to update_group_power() where sg->sgp->power gets >> updated, is from update_sg_lb_stats(). You are updating sgs->group_power >> in update_sg_lb_stats(). Any change to group->sgp->power will get >> reflected in sgs->group_power as well right? > > Nope, we set it to whatever value group->sgp->power is at the time of > sgs 'creation' and live with that value from then on. We do this after > the possible update_group_power() call. > > That said, it is very rare to have group->sgp->power change during the > load-balance pass, we would have to trigger the time_after case for > NEWLY_IDLE and then get a concurrent !NEWLY_IDLE load-balance pass. > > This patch takes away that possibility and uses a consistent group power > reading for the entire load-balance invocation as well as does away with > that double dereference all the time.
Fair enough. I overlooked the fact that "group" can be manipulated by multiple load balancing passes whereas "sgs" is created during every load balance pass. > >> 2. This point is aside from your patch. In the current implementation, >> each time the cpu power gets updated in update_cpu_power(), should not >> the power of the sched_groups comprising of that cpu also get updated? >> Why wait till the load balancing is done at the sched_domain level of >> that group, to update its group power? > > What would be the advantage of doing so? We also take snapshots of > cpu/group/domain load, we don't update those either. Having all that > dynamically update during the load-balance pass would make it an > impossible situation. > > You'd fail to meet progress guarantees that way since you'd never be > able to pin-point a 'busiest' group/cpu because by the time you've made > any decision you have to go back to make it again because things might > have changed again. > > So instead what we do is we take a snapshot and live with that state. If > the values change so fast that our load-balance pass is invalid by the > time we're finished, too bad, better luck next time. Agree. Thank you for the clarifications. Regards Preeti U Murthy > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
