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Date: Tue, 14 Apr 2009 11:48:04 +0200 From: Peter Zijlstra <a.p.zijlstra@...llo.nl> To: Gautham R Shenoy <ego@...ibm.com> Cc: Ingo Molnar <mingo@...e.hu>, Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>, linux-kernel@...r.kernel.org, Balbir Singh <balbir@...ibm.com>, Suresh Siddha <suresh.b.siddha@...el.com>, Andi Kleen <andi@...stfloor.org>, Randy Dunlap <randy.dunlap@...cle.com> Subject: Re: [RFC PATCH v2 0/2] sched: Nominate a power-efficient ILB On Tue, 2009-04-14 at 10:25 +0530, Gautham R Shenoy wrote: > Hi, > > This is the second iteration of the patchset which aims at improving > the idle-load balancer nomination logic, by taking the system topology > into consideration. > > Changes from v1 (found here: http://lkml.org/lkml/2009/4/2/246) > o Fixed the kernel-doc style comments. > o Renamed a variable to better reflect it's usage. > > Background > ====================================== > An idle-load balancer is an idle-cpu which does not turn off it's sched_ticks > and performs load-balancing on behalf of the other idle CPUs. Currently, > this idle load balancer is nominated as the first_cpu(nohz.cpu_mask) > > The drawback of the current method is that the CPU numbering in the > cores/packages need not necessarily be sequential. For example, on a > two-socket, Quad core system, the CPU numbering can be as follows: > > |-------------------------------| |-------------------------------| > | | | | | | > | 0 | 2 | | 1 | 3 | > |-------------------------------| |-------------------------------| > | | | | | | > | 4 | 6 | | 5 | 7 | > |-------------------------------| |-------------------------------| > > Now, the other power-savings settings such as the sched_mc/smt_power_savings > and the power-aware IRQ balancer try to balance tasks/IRQs by taking > the system topology into consideration, with the intention of keeping > as many "power-domains" (cores/packages) in the low-power state. > > The current idle-load-balancer nomination does not necessarily align towards > this policy. For eg, we could be having tasks and interrupts largely running > on the first package with the intention of keeping the second package idle. > Hence, CPU 0 may be busy. The first_cpu in the nohz.cpu_mask happens to be CPU1, > which in-turn becomes nominated as the idle-load balancer. CPU1 being from > the 2nd package, would in turn prevent the 2nd package from going into a > deeper sleep state. > > Instead the role of the idle-load balancer could have been assumed by an > idle CPU from the first package, thereby helping the second package go > completely idle. > > This patchset has been tested with 2.6.30-rc1 on a Two-Socket > Quad core system with the topology as mentioned above. > > |----------------------------------------------------------------------------| > | With Patchset + sched_mc_power_savings = 1 | > |----------------------------------------------------------------------------| > |make -j2 options| time taken | LOC timer interrupts | LOC timer interrupts| > | | | on Package 0 | on Package 1 | > |----------------------------------------------------------------------------| > |taskset -c 0,2 | | CPU0 | CPU2 | CPU1 | CPU3 | > | | 227.234s | 56969 | 57080 | 1003 | 588 | > | | |----------------------------------------------| > | | | CPU4 | CPU6 | CPU5 | CPU7 | > | | | 55995 | 703 | 583 | 600 | > |----------------------------------------------------------------------------| > |taskset -c 1,3 | | CPU0 | CPU2 | CPU1 | CPU3 | > | | 227.136s | 1109 | 611 | 57074 | 57091 | > | | |----------------------------------------------| > | | | CPU4 | CPU6 | CPU5 | CPU7 | > | | | 709 | 637 | 56133 | 587 | > |----------------------------------------------------------------------------| > > We see here that the idle load balancer is chosen from the package which is > busy. In the first case, it's CPU4 and in the second case it's CPU5. > > |----------------------------------------------------------------------------| > | With Patchset + sched_mc_power_savings = 1 | > |----------------------------------------------------------------------------| > |make -j2 options| time taken | LOC timer interrupts | LOC timer interrupts| > | | | on Package 0 | on Package 1 | > |----------------------------------------------------------------------------| > |taskset -c 0,2 | | CPU0 | CPU2 | CPU1 | CPU3 | > | | 228.786s | 59094 | 61994 | 13984 | 43652 | > | | |----------------------------------------------| > | | | CPU4 | CPU6 | CPU5 | CPU7 | > | | | 1827 | 734 | 748 | 760 | > |----------------------------------------------------------------------------| > |taskset -c 1,3 | | CPU0 | CPU2 | CPU1 | CPU3 | > | | 228.435s | 57013 | 876 | 58596 | 61633 | > | | |----------------------------------------------| > | | | CPU4 | CPU6 | CPU5 | CPU7 | > | | | 772 | 1133 | 850 | 910 | > |----------------------------------------------------------------------------| > > Here, we see that the idle load balancer is chosen from the other package, > despite choosing sched_mc_power_savings = 1. In the first case, we have > CPU1 and CPU3 sharing the responsibility among themselves. In the second case, > it's CPU0 and CPU6, which assume that role. Both tables above claim to be _with_ the pathes :-), from the accompanying text one can deduce its the bottom one that is without. Patches look straight-forward enough, seems good stuff. Thanks! -- 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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