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Date: Fri, 27 Mar 2015 11:09:07 +0530 From: Srikar Dronamraju <srikar@...ux.vnet.ibm.com> To: Jason Low <jason.low2@...com> Cc: Wanpeng Li <wanpeng.li@...ux.intel.com>, Preeti U Murthy <preeti@...ux.vnet.ibm.com>, peterz@...radead.org, mingo@...nel.org, riel@...hat.com, daniel.lezcano@...aro.org, vincent.guittot@...aro.org, pjt@...gle.com, benh@...nel.crashing.org, efault@....de, linux-kernel@...r.kernel.org, iamjoonsoo.kim@....com, svaidy@...ux.vnet.ibm.com, tim.c.chen@...ux.intel.com, morten.rasmussen@....com Subject: Re: [PATCH V2] sched: Improve load balancing in the presence of idle CPUs * Jason Low <jason.low2@...com> [2015-03-26 22:07:21]: > On Fri, 2015-03-27 at 10:12 +0800, Wanpeng Li wrote: > > Hi Preeti, > > On Thu, Mar 26, 2015 at 06:32:44PM +0530, Preeti U Murthy wrote: > > > > > >1. An ILB CPU was chosen from the first numa domain to trigger nohz idle > > >load balancing [Given the experiment, upto 6 CPUs per core could be > > >potentially idle in this domain.] > > > > > >2. However the ILB CPU would call load_balance() on itself before > > >initiating nohz idle load balancing. > > > > > >3. Given cores are SMT8, the ILB CPU had enough opportunities to pull > > >tasks from its sibling cores to even out load. > > > > > >4. Now that the ILB CPU was no longer idle, it would abort nohz idle > > >load balancing > > > > I don't see abort nohz idle load balancing when ILB CPU was no longer idle > > in nohz_idle_balance(), could you explain more in details? > > Hi Wanpeng, > > In nohz_idle_balance(), there is a check for need_resched() so if the > cpu has something to run, it should exit nohz_idle_balance(), which may > cause it to not do the idle balancing on the other CPUs. > Yes, the need_resched() in nohz_idle_balance() would exit the nohz_idle_balance if it has something to run. However I wonder if we should move the need_resched check out of the for loop. i.e the need_resched check should probably be there with the idle check. With the current code when the ilb cpus are not free: - We would be updating the nohz.next_balance even through we havent done any load balance. - We might iterate thro the nohz.idle_cpus_mask()s to find balance_cpus. -- Thanks and Regards Srikar Dronamraju -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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