On Wed, 2015-04-01 at 18:04 +0100, Morten Rasmussen wrote: > On Wed, Apr 01, 2015 at 07:49:56AM +0100, Preeti U Murthy wrote: > > > > On 04/01/2015 12:24 AM, Jason Low wrote: > > > On Tue, 2015-03-31 at 14:07 +0530, Preeti U Murthy wrote: > > >> Hi Jason, > > >> > > >> On 03/31/2015 12:25 AM, Jason Low wrote: > > >>> Hi Preeti, > > >>> > > >>> I noticed that another commit 4a725627f21d converted the check in > > >>> nohz_kick_needed() from idle_cpu() to rq->idle_balance, causing a > > >>> potentially outdated value to be used if this cpu is able to pull tasks > > >>> using rebalance_domains(), and nohz_kick_needed() directly returning > > >>> false. > > >> > > >> I see that rebalance_domains() will be run at the end of the scheduler > > >> tick interrupt handling. trigger_load_balance() only sets the softirq, > > >> it does not call rebalance_domains() immediately. So the call graph > > >> would be: > > > > > > Oh right, since that only sets the softirq, this wouldn't be the issue, > > > though we would need these changes if we were to incorporate any sort of > > > nohz_kick_needed() logic into the nohz_idle_balance() code path correct? > > > > I am sorry I don't quite get this. Can you please elaborate? > > I think the scenario is that we are in nohz_idle_balance() and decide to > bail out because we have pulled some tasks, but before leaving > nohz_idle_balance() we want to check if more balancing is necessary > using nohz_kick_needed() and potentially kick somebody to continue.
> Note that the balance cpu is currently skipped in nohz_idle_balance(), > but if it wasn't the scenario would be possible. This scenario would also be possible if we call rebalance_domains() first again. I'm wondering if adding the nohz_kick_needed(), ect... in nohz_idle_balance() can address the 10 second latency issue while still calling rebalance_domains() first, since it seems more ideal to try balancing on the current awake CPU first, as you also have mentioned > In that case, we can't rely on rq->idle_balance as it would not be > up-to-date. Also, we may even want to use nohz_kick_needed(rq) where rq > != this_rq, in which case we probably also want an updated status. It > seems that rq->idle_balance is only updated at each tick. Yup, that's about what I was describing. -- 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/
