On Fri, Feb 02, 2024 at 05:15:39PM -0800, Jakub Kicinski wrote: > On Fri, 2 Feb 2024 12:23:44 -0800 Joe Damato wrote: > > > Did you see SO_PREFER_BUSY_POLL by any chance? (In combination with > > > gro_flush_timeout IIRC). We added it a while back with Bjorn, it seems > > > like a great idea to me at the time but I'm unclear if anyone uses it > > > in production.. > > > > I have seen it while reading the code, yes. I think maybe I missed > > something about its interaction with gro_flush_timeout. In my use case, > > the machine has no traffic until after the app is started. > > > > In this case, I haven't needed to worry about regular NAPI monopolizing the > > CPU and preventing busy poll from working. > > > > Maybe I am missing something more nuanced, though? I'll have another look > > at the code, just incase. > > We reused the gro_flush_timeout as an existing "user doesn't care if > packets get delayed by this much in worst case" value. If you set > SO_PREFER_BUSY_POLL the next time you busy pool the NAPI will be marked > as "already scheduled" and a timer is set (to gro_flush_timeout). > If NIC IRQ fires before gro_flush_timeout it gets ignored, because NAPI > is already marked as scheduled. > If you busy poll again the timer gets postponed for another > gro_flush_timeout nsec. > If timer fires we go back to normal NAPI processing. Ah, I see. From my reading of the code in busy_poll_stop (which could be wrong), defer_hard_irqs_count must also be non-zero to postpone the timer. Is that right? If so, I think the tricky thing with this is that these settings are system-wide, so they'd affect non-busy poll apps, too. I think in the ideal case being able to set these on a per-NAPI basis would be very helpful. Maybe something for me to try working on next. > The idea is that you set gro_flush_timeout to some high value, like > 10 msec, and expect your app to poll more often than every 10 msec. Yea, that makes sense. > Then the normal NAPI processing will never kick in, and there will > be only 1 NIC IRQ after which the HW IRQ remains masked. > With high coalescing timer you technically still get an IRQ every > so often and interrupt the app. Worst case (UDP flood) you may even > get into an overload where the app gets starved out completely.. Yup, this is true. I had been using a modified version of a patch from a research paper to avoid enabling NIC IRQs [1][2], but I think making defer_hard_irqs_count and gro_flush_timeout per NAPI parameters would make more sense. [1]: https://gitlab.uwaterloo.ca/p5cai/netstack-exp/-/raw/master/kernel-polling-5.15.79-base.patch?ref_type=heads [2]: https://dl.acm.org/doi/pdf/10.1145/3626780
