On Thu, 2007-11-01 at 17:01 +0530, Srivatsa Vaddagiri wrote: > On Wed, Oct 31, 2007 at 10:10:32PM +0100, Peter Zijlstra wrote: > > Currently the ideal slice length does not take group scheduling into account. > > Change it so that it properly takes all the runnable tasks on this cpu into > > account and caluclate the weight according to the grouping hierarchy. > > > > Also fixes a bug in vslice which missed a factor NICE_0_LOAD. > > > > Signed-off-by: Peter Zijlstra <[email protected]> > > CC: Srivatsa Vaddagiri <[email protected]> > > --- > > kernel/sched_fair.c | 42 +++++++++++++++++++++++++++++++----------- > > 1 file changed, 31 insertions(+), 11 deletions(-) > > > > Index: linux-2.6/kernel/sched_fair.c > > =================================================================== > > --- linux-2.6.orig/kernel/sched_fair.c > > +++ linux-2.6/kernel/sched_fair.c > > @@ -331,10 +331,15 @@ static u64 __sched_period(unsigned long > > */ > > static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se) > > { > > - u64 slice = __sched_period(cfs_rq->nr_running); > > + unsigned long nr_running = rq_of(cfs_rq)->nr_running; > > + u64 slice = __sched_period(nr_running); > > > > - slice *= se->load.weight; > > - do_div(slice, cfs_rq->load.weight); > > + for_each_sched_entity(se) { > > + cfs_rq = cfs_rq_of(se); > > + > > + slice *= se->load.weight; > > + do_div(slice, cfs_rq->load.weight); > > + } > > > > return slice; > > > Lets say we have two groups A and B on CPU0, of equal weight (1024). > > Further, > > A has 1 task (A0) > B has 1000 tasks (B0 .. B999) > > Agreed its a extreme case, but illustrates the problem I have in mind > for this patch. > > All tasks of same weight=1024. > > Before this patch > ================= > > sched_slice(grp A) = 20ms * 1/2 = 10ms > sched_slice(A0) = 20ms > > sched_slice(grp B) = 20ms * 1/2 = 10ms > sched_slice(B0) = (20ms * 1000/20) * 1 / 1000 = 1ms > sched_slice(B1) = ... = sched_slice(B99) = 1 ms > > Fairness between groups and tasks would be obtained as below: > > A0 B0-B9 A0 B10-B19 A0 B20-B29 > |--------|--------|--------|--------|--------|--------|-----//--| > 0 10ms 20ms 30ms 40ms 50ms 60ms > > After this patch > ================ > > sched_slice(grp A) = (20ms * 1001/20) * 1/2 ~= 500ms > sched_slice(A0) = 500ms Hmm, right that is indeed not intended > sched_slice(grp B) = 500ms > sched_slice(B0) = 0.5ms This 0.5 is indeed correct, whereas the previous 1ms was not > Fairness between groups and tasks would be obtained as below: > > A0 B0 - B99 A0 > |-----------------------|-----------------------|-----------------------| > 0 500ms 1000ms 1500ms > > Did I get it right? If so, I don't like the fact that group A is allowed to run > for a long time (500ms) before giving chance to group B. Hmm, quite bad indeed. > Can I know what real problem is being addressed by this change to > sched_slice()? sched_slice() is about lantecy, its intended purpose is to ensure each task is ran exactly once during sched_period() - which is sysctl_sched_latency when nr_running <= sysctl_sched_nr_latency, and otherwise linearly scales latency.
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