On Thu, 30 Apr 2020 at 12:14, Valentin Schneider <valentin.schneider@xxxxxxx> wrote: > > > On 30/04/20 08:44, Vincent Guittot wrote: > > 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 > > Mph, I had already forgotten your changes there. Is that really always the > case for newidle? In e.g. the busiest->group_type == group_fully_busy case, > I think we can pull more than one task. for newly_idle load balance, detach_tasks stops after finding 1 suitable 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 > > > > IIRC Steve's implementation was to "simply" pull one task from any CPU > within the LLC domain that had > 1 runnable tasks. I quite like this since > picking any one task is almost always better than switching to the idle > task, but it wasn't a complete newidle_balance() replacement just yet. > > > > >> > >> Sadly I don't think anyone has been looking at it any recently.
