On Tue, 28 Apr 2020 at 07:02, Scott Wood <swood@xxxxxxxxxx> wrote: > > These patches mitigate latency caused by newidle_balance() on large > systems, by enabling interrupts when the lock is dropped, and exiting > early at various points if an RT task is runnable on the current CPU. > > When applied to an RT kernel on a 72-core machine (2 threads per core), I > saw significant reductions in latency as reported by rteval -- from > over 500us to around 160us with hyperthreading disabled, and from > over 1400us to around 380us with hyperthreading enabled. Do you know how each patch contributes to the decrease ? Because patch 3 not only impacts newly idle lb but each and every lb. So most of the decrease might come from aborting the busy or idle lb at the highest sched_domai level which scan all cpus and moving newly idle load balance is not a major part. > > This isn't the first time something like this has been tried: > https://lore.kernel.org/lkml/20121222003019.433916240@xxxxxxxxxxx/ > That attempt ended up being reverted: > https://lore.kernel.org/lkml/5122CD9C.9070702@xxxxxxxxxx/ > > The problem in that case was the failure to keep BH disabled, and the > difficulty of fixing that when called from the post_schedule() hook. > This patchset uses finish_task_switch() to call newidle_balance(), which > enters in non-atomic context so we have full control over what we disable > and when. > > There was a note at the end about wanting further discussion on the matter -- > does anyone remember if that ever happened and what the conclusion was? > Are there any other issues with enabling interrupts here and/or moving > the newidle_balance() call? > > Rik van Riel (1): > sched,rt: break out of load balancing if an RT task appears > > Scott Wood (2): > sched/fair: Call newidle_balance() from finish_task_switch() > sched/fair: Enable interrupts when dropping lock in newidle_balance() > > kernel/sched/core.c | 7 +++-- > kernel/sched/fair.c | 72 +++++++++++++++++++++++--------------------- > kernel/sched/sched.h | 12 +++++--- > 3 files changed, 49 insertions(+), 42 deletions(-) > > -- > 2.18.2 >
