On Thu, 22 Aug 2019 at 04:18, Rik van Riel <[email protected]> wrote: > > The way the time slice length is currently calculated, not only do high > priority tasks get longer time slices than low priority tasks, but due > to fixed point math, low priority tasks could end up with a zero length > time slice. This can lead to cache thrashing and other inefficiencies.
Have you got more details of those UCs ? > > Cap the minimum time slice length to sysctl_sched_min_granularity. > > Tasks that end up getting a time slice length too long for their relative > priority will simply end up having their vruntime advanced much faster than > other tasks, resulting in them receiving time slices less frequently. In fact that already happen as we wait for the tick to preempt a task (unless you enable sched_feat(HRTICK)) so it sounds reasonable > > Signed-off-by: Rik van Riel <[email protected]> > --- > kernel/sched/fair.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 31a26737a873..8f8c85c6da9b 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -732,6 +732,13 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct > sched_entity *se) > } > slice = __calc_delta(slice, se->load.weight, load); > } > + > + /* > + * To avoid cache thrashing, run at least > sysctl_sched_min_granularity. > + * The vruntime of a low priority task advances faster; those tasks > + * will simply get time slices less frequently. > + */ > + slice = max_t(u64, slice, sysctl_sched_min_granularity); > return slice; > } > > -- > 2.20.1 >
