Re: io_uring NAPI busy poll RCU is causing 50 context switches/second to my sqpoll thread

[Pavel Begunkov <asml.silence@gmail.com>]

On 7/31/24 00:14, Olivier Langlois wrote:
> On Tue, 2024-07-30 at 21:25 +0100, Pavel Begunkov wrote:
>>
>> Removing an entry or two once every minute is definitely not
>> going to take 50% CPU, RCU machinery is running in background
>> regardless of whether io_uring uses it or not, and it's pretty
>> cheap considering ammortisation.
>>
>> If anything it more sounds from your explanation like the
>> scheduler makes a wrong decision and schedules out the sqpoll
>> thread even though it could continue to run, but that's need
>> a confirmation. Does the CPU your SQPOLL is pinned to stays
>> 100% utilised?
> 
> Here are the facts as they are documented in the github issue.
> 
> 1. despite thinking that I was doing NAPI busy polling, I was not
> because my ring was not receiving any sqe after its initial setup.
> 
> This is what the patch developped with your input
> https://lore.kernel.org/io-uring/382791dc97d208d88ee31e5ebb5b661a0453fb79.1722374371.git.olivier@trillion01.com/T/#u
> 
> is addressing
> 
> (BTW, I should check if there is such a thing, but I would love to know
> if the net code is exposing a tracepoint when napi_busy_poll is called
> because it is very tricky to know if it is done for real or not)
> 
> 2. the moment a second ring has been attached to the sqpoll thread that
> was receving a lot of sqe, the NAPI busy loop started to be made for
> real and the sqpoll cpu usage unexplicably dropped from 99% to 55%
> 
> 3. here is my kernel cmdline:
> hugepages=72 isolcpus=0,1,2 nohz_full=0,1,2 rcu_nocbs=0,1,2
> rcu_nocb_poll irqaffinity=3 idle=nomwait processor.max_cstate=1
> intel_idle.max_cstate=1 nmi_watchdog=0
> 
> there is absolutely nothing else on CPU0 where the sqpoll thread
> affinity is set to run.
> 
> 4. I got the idea of doing this:
> echo common_pid == sqpoll_pid > /sys/kernel/tracing/events/sched/filter
> echo 1 > /sys/kernel/tracing/events/sched/sched_switch/enable
> 
> and I have recorded over 1,000 context switches in 23 seconds with RCU
> related kernel threads.
> 
> 5. just for the fun of checking out, I have disabled NAPI polling on my
> io_uring rings and the sqpoll thread magically returned to 99% CPU
> usage from 55%...
> 
> I am open to other explanations for what I have observed but my current
> conclusion is based on what I am able to see... the evidence appears
> very convincing to me...

You're seeing something that doesn't make much sense to me, and we need
to understand what that is. There might be a bug _somewhere_, that's
always a possibility, but before saying that let's get a bit more data.

While the app is working, can you grab a profile and run mpstat for the
CPU on which you have the SQPOLL task?

perf record -g -C <CPU number> --all-kernel &
mpstat -u -P <CPU number> 5 10 &

And then as usual, time it so that you have some activity going on,
mpstat interval may need adjustments, and perf report it as before.

-- 
Pavel Begunkov