On Fri, Jan 12, 2018 at 04:15:04PM +0100, Mike Galbraith wrote: > On Fri, 2018-01-12 at 15:58 +0100, Frederic Weisbecker wrote: > > On Fri, Jan 12, 2018 at 06:23:08AM +0100, Mike Galbraith wrote: > > > On Thu, 2018-01-11 at 12:22 -0800, Linus Torvalds wrote: > > > > On Thu, Jan 11, 2018 at 12:16 PM, Eric Dumazet <eduma...@google.com> > > > > wrote: > > > > > > > > > > Note that when I implemented TCP Small queues, I did experiments > > > > > between > > > > > using a work queue or a tasklet, and workqueues added unacceptable P99 > > > > > latencies, when many user threads are competing with kernel threads. > > > > > > > > Yes. > > > > > > > > So I think one solution might be to have a hybrid system, where we do > > > > the softirq's synchronously normally (which is what you really want > > > > for good latency). > > > > > > > > But then fall down on a threaded model - but that fallback case should > > > > be per-softirq, not global. So if one softirq uses a lot of CPU time, > > > > that shouldn't affect the latency of other softirqs. > > > > > > > > So maybe we could get rid of the per-cpu ksoftirqd entirely, and > > > > replace it with with per-cpu and per-softirq workqueues? > > > > > > How would that be better than what RT used to do, and I still do for my > > > RT kernels via boot option, namely split ksoftirqd into per-softirq > > > threads. > > > > Workqueue are probably more simple. Unless you need to set specific prios > > to your ksoftirqds? Not sure if that's tunable on workqueues. > > No, you can't prioritize workqueues, and they spawn threads whenever > they bloody well feel like. > > I carry a hack to give users minimal control over kthread/workqueue > priority. Very handy thing to have, especially if you're doing high > utilization stuff, and would prefer your box actually survive it.
How useful system_highpri_wq can be in this regard?
