Messages in this thread | | | From | Darren Hart <> | Subject | Re: RT task scheduling | Date | Thu, 6 Apr 2006 10:24:34 -0700 |
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On Wednesday 05 April 2006 21:19, Peter Williams wrote: > Darren Hart wrote: > > My last mail specifically addresses preempt-rt, but I'd like to know > > people's thoughts regarding this issue in the mainline kernel. Please > > see my previous post "realtime-preempt scheduling - rt_overload behavior" > > for a testcase that produces unpredictable scheduling results. > > > > Part of the issue here is to define what we consider "correct behavior" > > for SCHED_FIFO realtime tasks. Do we (A) need to strive for "strict > > realtime priority scheduling" where the NR_CPUS highest priority runnable > > SCHED_FIFO tasks are _always_ running? Or do we (B) take the best effort > > approach with an upper limit RT priority imbalances, where an imbalance > > may occur (say at wakeup or exit) but will be remedied within 1 tick. > > The smpnice patches improve load balancing, but don't provide (A). > > > > More details in the previous mail... > > I'm currently researching some ideas to improve smpnice that may help in > this situation. The basic idea is that as well as trying to equally > distribute the weighted load among the groups/queues we should also try > to achieve equal "average load per task" for each group/queue. (As well > as helping with problems such as yours, this will help to restore the > "equal distribution of nr_running" amongst groups/queues aim that is > implicit without smpnice due to the fact that load is just a smoothed > version of nr_running.)
Can you elaborate on what you mean by "average load per task" ?
Also, since smpnice is (correct me if I am wrong) load_balancing, I don't think it will prevent the problem from happening, but rather fix it when it does. If we want to prevent it from happening, I think we need to do something like the rt_overload code from the RT patchset.
> > In find_busiest_group(), I think that load balancing in the case where > *imbalance is greater than busiest_load_per_task will tend towards this > result and also when *imbalance is less than busiest_load_per_task AND > busiest_load_per_task is less than this_load_per_task. However, in the > case where *imbalance is less than busiest_load_per_task AND > busiest_load_per_task is greater than this_load_per_task this will not > be the case as the amount of load moved from "busiest" to "this" will be > less than or equal to busiest_load_per_task and this will actually > increase the value of busiest_load_per_task. So, although it will > achieve the aim of equally distributing the weighted load, it won't help > the second aim of equal "average load per task" values for groups/queues. > > The obvious way to fix this problem is to alter the code so that more > than busiest_load_per_task is moved from "busiest" to "this" in these > cases while at the same time ensuring that the imbalance between their > loads doesn't get any bigger. I'm working on a patch along these lines. > > Changes to find_idlest_group() and try_to_wake_up() taking into account > the "average load per task" on the candidate queues/groups as well as > their weighted loads may also help and I'll be looking at them as well. > It's not immediately obvious to me how this can be done so any ideas > would be welcome. It will likely involve taking the load weight of the > waking task into account as well. > > Peter - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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