Messages in this thread | | | From | Jason Xing <> | Date | Fri, 21 Apr 2023 10:48:51 +0800 | Subject | Re: [PATCH 0/3] softirq: uncontroversial change |
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On Fri, Apr 21, 2023 at 1:34 AM Paolo Abeni <pabeni@redhat.com> wrote: > > Hi all, > On Thu, 2022-12-22 at 14:12 -0800, Jakub Kicinski wrote: > > Catching up on LWN I run across the article about softirq > > changes, and then I noticed fresh patches in Peter's tree. > > So probably wise for me to throw these out there. > > > > My (can I say Meta's?) problem is the opposite to what the RT > > sensitive people complain about. In the current scheme once > > ksoftirqd is woken no network processing happens until it runs. > > > > When networking gets overloaded - that's probably fair, the problem > > is that we confuse latency tweaks with overload protection. We have > > a needs_resched() in the loop condition (which is a latency tweak) > > Most often we defer to ksoftirqd because we're trying to be nice > > and let user space respond quickly, not because there is an > > overload. But the user space may not be nice, and sit on the CPU > > for 10ms+. Also the sirq's "work allowance" is 2ms, which is > > uncomfortably close to the timer tick, but that's another story. > > > > We have a sirq latency tracker in our prod kernel which catches > > 8ms+ stalls of net Tx (packets queued to the NIC but there is > > no NAPI cleanup within 8ms) and with these patches applied > > on 5.19 fully loaded web machine sees a drop in stalls from > > 1.8 stalls/sec to 0.16/sec. I also see a 50% drop in outgoing > > TCP retransmissions and ~10% drop in non-TLP incoming ones. > > This is not a network-heavy workload so most of the rtx are > > due to scheduling artifacts. > > > > The network latency in a datacenter is somewhere around neat > > 1000x lower than scheduling granularity (around 10us). > > > > These patches (patch 2 is "the meat") change what we recognize > > as overload. Instead of just checking if "ksoftirqd is woken" > > it also caps how long we consider ourselves to be in overload, > > a time limit which is different based on whether we yield due > > to real resource exhaustion vs just hitting that needs_resched(). > > > > I hope the core concept is not entirely idiotic. It'd be great > > if we could get this in or fold an equivalent concept into ongoing > > work from others, because due to various "scheduler improvements" > > every time we upgrade the production kernel this problem is getting > > worse :( > [...] > Please allow me to revive this old thread.
Hi Paolo,
So good to hear this :)
> > My understanding is that we want to avoid adding more heuristics here, > preferring a consistent refactor. > > I would like to propose a revert of: > > 4cd13c21b207 softirq: Let ksoftirqd do its job > > the its follow-ups: > > 3c53776e29f8 Mark HI and TASKLET softirq synchronous > 0f50524789fc softirq: Don't skip softirq execution when softirq thread is parking
More than this, I list some related patches mentioned in the above commit 3c53776e29f8: 1ff688209e2e ("watchdog: core: make sure the watchdog_worker is not deferred") 8d5755b3f77b ("watchdog: softdog: fire watchdog even if softirqs do not get to run") 217f69743681 ("net: busy-poll: allow preemption in sk_busy_loop()")
> > The problem originally addressed by 4cd13c21b207 can now be tackled > with the threaded napi, available since: > > 29863d41bb6e net: implement threaded-able napi poll loop support > > Reverting the mentioned commit should address the latency issues > mentioned by Jakub - I verified it solves a somewhat related problem in > my setup - and reduces the layering of heuristics in this area.
Sure, it is. I also can verify its usefulness in the real workload. Some days ago I also sent a heuristics patch [1] that can bypass the ksoftirqd if the user chooses to mask some type of softirq. Let the user decide it.
But I observed that if we mask some softirqs, or we can say, completely revert the commit 4cd13c21b207, the load would go higher and the kernel itself may occupy/consume more time than before. They were tested under the similar workload launched by our applications.
[1]: https://lore.kernel.org/all/20230410023041.49857-1-kerneljasonxing@gmail.com/
> > A refactor introducing uniform overload detection and proper resource > control will be better, but I admit it's beyond me and anyway it could > still land afterwards.
+1
Thanks, Jason > > Any opinion more then welcome! > > Thanks, > > Paolo >
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