Re: [PATCH v6] kni: allow configuring the kni thread granularity

[Thomas Monjalon <thomas@monjalon.net>]

20/01/2022 13:41, Tudor Cornea:
> The Kni kthreads seem to be re-scheduled at a granularity of roughly
> 1 millisecond right now, which seems to be insufficient for performing
> tests involving a lot of control plane traffic.
> 
> Even if KNI_KTHREAD_RESCHEDULE_INTERVAL is set to 5 microseconds, it
> seems that the existing code cannot reschedule at the desired granularily,
> due to precision constraints of schedule_timeout_interruptible().
> 
> In our use case, we leverage the Linux Kernel for control plane, and
> it is not uncommon to have 60K - 100K pps for some signaling protocols.
> 
> Since we are not in atomic context, the usleep_range() function seems to be
> more appropriate for being able to introduce smaller controlled delays,
> in the range of 5-10 microseconds. Upon reading the existing code, it would
> seem that this was the original intent. Adding sub-millisecond delays,
> seems unfeasible with a call to schedule_timeout_interruptible().
> 
> KNI_KTHREAD_RESCHEDULE_INTERVAL 5 /* us */
> schedule_timeout_interruptible(
>         usecs_to_jiffies(KNI_KTHREAD_RESCHEDULE_INTERVAL));
> 
> Below, we attempted a brief comparison between the existing implementation,
> which uses schedule_timeout_interruptible() and usleep_range().
> 
> We attempt to measure the CPU usage, and RTT between two Kni interfaces,
> which are created on top of vmxnet3 adapters, connected by a vSwitch.
> 
> insmod rte_kni.ko kthread_mode=single carrier=on
> 
> schedule_timeout_interruptible(usecs_to_jiffies(5))
> kni_single CPU Usage: 2-4 %
> [root@localhost ~]# ping 1.1.1.2 -I eth1
> PING 1.1.1.2 (1.1.1.2) from 1.1.1.1 eth1: 56(84) bytes of data.
> 64 bytes from 1.1.1.2: icmp_seq=1 ttl=64 time=2.70 ms
> 64 bytes from 1.1.1.2: icmp_seq=2 ttl=64 time=1.00 ms
> 64 bytes from 1.1.1.2: icmp_seq=3 ttl=64 time=1.99 ms
> 64 bytes from 1.1.1.2: icmp_seq=4 ttl=64 time=0.985 ms
> 64 bytes from 1.1.1.2: icmp_seq=5 ttl=64 time=1.00 ms
> 
> usleep_range(5, 10)
> kni_single CPU usage: 50%
> 64 bytes from 1.1.1.2: icmp_seq=1 ttl=64 time=0.338 ms
> 64 bytes from 1.1.1.2: icmp_seq=2 ttl=64 time=0.150 ms
> 64 bytes from 1.1.1.2: icmp_seq=3 ttl=64 time=0.123 ms
> 64 bytes from 1.1.1.2: icmp_seq=4 ttl=64 time=0.139 ms
> 64 bytes from 1.1.1.2: icmp_seq=5 ttl=64 time=0.159 ms
> 
> usleep_range(20, 50)
> kni_single CPU usage: 24%
> 64 bytes from 1.1.1.2: icmp_seq=1 ttl=64 time=0.202 ms
> 64 bytes from 1.1.1.2: icmp_seq=2 ttl=64 time=0.170 ms
> 64 bytes from 1.1.1.2: icmp_seq=3 ttl=64 time=0.171 ms
> 64 bytes from 1.1.1.2: icmp_seq=4 ttl=64 time=0.248 ms
> 64 bytes from 1.1.1.2: icmp_seq=5 ttl=64 time=0.185 ms
> 
> usleep_range(50, 100)
> kni_single CPU usage: 13%
> 64 bytes from 1.1.1.2: icmp_seq=1 ttl=64 time=0.537 ms
> 64 bytes from 1.1.1.2: icmp_seq=2 ttl=64 time=0.257 ms
> 64 bytes from 1.1.1.2: icmp_seq=3 ttl=64 time=0.231 ms
> 64 bytes from 1.1.1.2: icmp_seq=4 ttl=64 time=0.143 ms
> 64 bytes from 1.1.1.2: icmp_seq=5 ttl=64 time=0.200 ms
> 
> usleep_range(100, 200)
> kni_single CPU usage: 7%
> 64 bytes from 1.1.1.2: icmp_seq=1 ttl=64 time=0.716 ms
> 64 bytes from 1.1.1.2: icmp_seq=2 ttl=64 time=0.167 ms
> 64 bytes from 1.1.1.2: icmp_seq=3 ttl=64 time=0.459 ms
> 64 bytes from 1.1.1.2: icmp_seq=4 ttl=64 time=0.455 ms
> 64 bytes from 1.1.1.2: icmp_seq=5 ttl=64 time=0.252 ms
> 
> usleep_range(1000, 1100)
> kni_single CPU usage: 2%
> 64 bytes from 1.1.1.2: icmp_seq=1 ttl=64 time=2.22 ms
> 64 bytes from 1.1.1.2: icmp_seq=2 ttl=64 time=1.17 ms
> 64 bytes from 1.1.1.2: icmp_seq=3 ttl=64 time=1.17 ms
> 64 bytes from 1.1.1.2: icmp_seq=4 ttl=64 time=1.17 ms
> 64 bytes from 1.1.1.2: icmp_seq=5 ttl=64 time=1.15 ms
> 
> Upon testing, usleep_range(1000, 1100) seems roughly equivalent in
> latency and cpu usage to the variant with schedule_timeout_interruptible(),
> while usleep_range(100, 200) seems to give a decent tradeoff between
> latency and cpu usage, while allowing users to tweak the limits for
> improved precision if they have such use cases.
> 
> Disabling RTE_KNI_PREEMPT_DEFAULT, interestingly seems to lead to a
> softlockup on my kernel.
> 
> Kernel panic - not syncing: softlockup: hung tasks
> CPU: 0 PID: 1226 Comm: kni_single Tainted: G        W  O 3.10 #1
>  <IRQ>  [<ffffffff814f84de>] dump_stack+0x19/0x1b
>  [<ffffffff814f7891>] panic+0xcd/0x1e0
>  [<ffffffff810993b0>] watchdog_timer_fn+0x160/0x160
>  [<ffffffff810644b2>] __run_hrtimer.isra.4+0x42/0xd0
>  [<ffffffff81064b57>] hrtimer_interrupt+0xe7/0x1f0
>  [<ffffffff8102cd57>] smp_apic_timer_interrupt+0x67/0xa0
>  [<ffffffff8150321d>] apic_timer_interrupt+0x6d/0x80
> 
> This patch also attempts to remove this option.
> 
> References:
> [1] https://www.kernel.org/doc/Documentation/timers/timers-howto.txt
> 
> Signed-off-by: Tudor Cornea <tudor.cornea@gmail.com>
> Acked-by: Padraig Connolly <Padraig.J.Connolly@intel.com>
> Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
> ---
> v6:
> * Removed tabs and newline in the description of the
> 
>   min_scheduling_interval and max_scheduling_interval
>   parameters. They seem to be non-standard.

The doc had to be updated a bit as well.

Fixed Kni -> KNI and applied, thanks.