Re: [PATCH v1] sched: fix nohz idle load balancer issues

[Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>]

* Suresh Siddha <suresh.b.siddha@intel.com> [2011-09-27 16:49:36]:

> One of the reasons why we saw lib_cpu not idle is probably because that
> info was stale.
> 
> Consider this scenario.
> 
> a. got a tick when the cpu was busy, so idle_at_tick was not set
> b. cpu went idle
> c. same cpu got the kick IPI from other busy cpu
> d. and as it has idle_at_tick not set, it couldn't proceed with the nohz
> idle balance.

Good point ..we chould use idle_cpu() instead there ..

> I think we are mostly likely seeing the above mentioned scenario.
> 
> Also Vatsa, there is a deadlock associated by using
> __smp_call_funciton_single() in the nohz_balancer_kick(). So I am
> planning to remove the IPI that is used to kick the nohz balancer and
> instead use the resched_cpu logic to kick the nohz balancer.
> 
> I will post this patch mostly tomorrow. That patch will not use the
> idle_at_tick check in the nohz_idle_balance(). So that should address
> your issue in some cases if not most.

Ok ..would be glad to test your change ..I am however doubtfull if it
will eliminate rest of the issues I pointed out with nohz load balancer.

- vatsa