On Thu, 30 Apr 2020 at 01:13, Valentin Schneider <valentin.schneider@xxxxxxx> wrote: > > > On 28/04/20 06:02, Scott Wood 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. > > > > 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? > > > > Random thought that just occurred to me; in the grand scheme of things, > with something in the same spirit as task-stealing (i.e. don't bother with > a full fledged balance at newidle, just pick one spare task somewhere), > none of this would be required. newly idle load balance already stops after picking 1 task Now if your proposal is to pick one random task on one random cpu, I'm clearly not sure that's a good idea > > Sadly I don't think anyone has been looking at it any recently.
