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 potentially outdated value to be used if this cpu is able to pull tasks 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. 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. Or maybe I'm all wrong :) -- 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/
