Hello Preeti. On Fri, Mar 29, 2013 at 05:05:37PM +0530, Preeti U Murthy wrote: > Hi Joonsoo > > On 03/28/2013 01:28 PM, Joonsoo Kim wrote: > > sched_slice() compute ideal runtime slice. If there are many tasks > > in cfs_rq, period for this cfs_rq is extended to guarantee that each task > > has time slice at least, sched_min_granularity. And then each task get > > a portion of this period for it. If there is a task which have much larger > > load weight than others, a portion of period can exceed far more than > > sysctl_sched_latency. > > Correct. But that does not matter, the length of the scheduling latency > period is determined by the return value of ___sched_period(), not the > value of sysctl_sched_latency. You would not extend the period,if you > wanted all tasks to have a slice within the sysctl_sched_latency, right? > > So since the value of the length of the scheduling latency period, is > dynamic depending on the number of the processes running, the > sysctl_sched_latency which is the default latency period length is not > mesed with, but is only used as a base to determine the actual > scheduling period. > > > > > For exampple, you can simply imagine that one task with nice -20 and > > 9 tasks with nice 0 on one cfs_rq. In this case, load weight sum for > > this cfs_rq is 88761 + 9 * 1024, 97977. So a portion of slice for the > > task with nice -20 is sysctl_sched_min_granularity * 10 * (88761 / 97977), > > that is, approximately, sysctl_sched_min_granularity * 9. This aspect > > can be much larger if there is more tasks with nice 0. > > Yeah so the __sched_period says that within 40ms, all tasks need to be > scheduled ateast once, and the highest priority task gets nearly 36ms of > it, while the rest is distributed among the others. > > > > > So we should limit this possible weird situation. > > > > Signed-off-by: Joonsoo Kim <iamjoonsoo....@lge.com> > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index e232421..6ceffbc 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -645,6 +645,9 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct > > sched_entity *se) > > } > > slice = calc_delta_mine(slice, se->load.weight, load); > > > > + if (unlikely(slice > sysctl_sched_latency)) > > + slice = sysctl_sched_latency; > > Then in this case the highest priority thread would get > 20ms(sysctl_sched_latency), and the rest would get > sysctl_sched_min_granularity * 10 * (1024/97977) which would be 0.4ms. > Then all tasks would get scheduled ateast once within 20ms + (0.4*9) ms > = 23.7ms, while your scheduling latency period was extended to 40ms,just > so that each of these tasks don't have their sched_slices shrunk due to > large number of tasks.
I don't know I understand your question correctly. I will do my best to answer your comment. :) With this patch, I just limit maximum slice at one time. Scheduling is controlled through the vruntime. So, in this case, the task with nice -20 will be scheduled twice. 20 + (0.4 * 9) + 20 = 43.9 ms And after 43.9 ms, this process is repeated. So I can tell you that scheduling period is preserved as before. If we give a long period to a task at one go, it can cause a latency problem. So IMHO, limiting this is meaningful. Thanks. > > > + > > return slice; > > } > > > > Regards > Preeti U Murthy > > -- > 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/ -- 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/
