On 3 November 2014 03:12, Wanpeng Li <kernel...@gmail.com> wrote: > Hi Vincent, > On 14/10/31 下午4:47, Vincent Guittot wrote: >> >> This patchset consolidates several changes in the capacity and the usage >> tracking of the CPU. It provides a frequency invariant metric of the usage >> of >> CPUs and generally improves the accuracy of load/usage tracking in the >> scheduler. The frequency invariant metric is the foundation required for >> the >> consolidation of cpufreq and implementation of a fully invariant load >> tracking. >> These are currently WIP and require several changes to the load balancer >> (including how it will use and interprets load and capacity metrics) and >> extensive validation. The frequency invariance is done with >> arch_scale_freq_capacity and this patchset doesn't provide the backends of >> the function which are architecture dependent. >> >> As discussed at LPC14, Morten and I have consolidated our changes into a >> single >> patchset to make it easier to review and merge. >> >> During load balance, the scheduler evaluates the number of tasks that a >> group >> of CPUs can handle. The current method assumes that tasks have a fix load >> of >> SCHED_LOAD_SCALE and CPUs have a default capacity of SCHED_CAPACITY_SCALE. >> This assumption generates wrong decision by creating ghost cores or by > > > I don't know the history, could you explain what's the meaning of 'ghost > cores' ?
The capacity_factor gives the number of tasks that can be handled by a group of CPUs by dividing the group's capacity by SCHED_CAPACITY_SCALE For a system with SMT, the default capacity of a core is 1178 so the capacity of each CPU for a dual threads per core is 589. At CPU level we have a capacity_factor of 1 = div_round_closest(589, 1024) At core level we still have a capacity_factor of 1 = div_round_closest(1178, 1024). This is a intended behavior to promote 1 task per core Then, if we have 4 cores in a node, the capacity_factor is 5 = div_round_closest(4712, 1024) whereas we should have 4. So a 5th ghost core has appeared in the group and the load balancer will not considered the group as overloaded if there is 5 tasks whereas it should in order to try to move this 5th task on an idle core (if there is one) Patch [0] solves some use cases by ensuring that we will not have more cores than possible so we can't have more than 4 core for the previous example. Now, if some RT tasks are running and using almost 1 core (1024 as an example), the capacity_factor is still 4 = div_round_closest(3688, 1024) whereas a core is nearly fully used and the capacity_factor should be 3 [0] https://lkml.org/lkml/2013/8/28/194 Regards, Vincent > > Regards, > Wanpeng Li > > >> removing real ones when the original capacity of CPUs is different from >> the >> default SCHED_CAPACITY_SCALE. With this patch set, we don't try anymore to >> evaluate the number of available cores based on the group_capacity but >> instead >> we evaluate the usage of a group and compare it with its capacity. >> >> This patchset mainly replaces the old capacity_factor method by a new one >> and >> keeps the general policy almost unchanged. These new metrics will be also >> used >> in later patches. >> [snip] -- 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/
