Re: [dpdk-dev] [PATCH] kni: fast data availability check in thread_single loop

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2017-01-18 07:11, Jay Rolette:
> On Wed, Jan 18, 2017 at 5:05 AM, Sergey Vyazmitinov <
> s.vyazmitinov@brain4net.com> wrote:
> 
> > On Thu, Jan 12, 2017 at 12:29 AM, Ferruh Yigit <ferruh.yigit@intel.com>
> >  wrote:
> >
> > > On 12/29/2016 11:23 PM, Sergey Vyazmitinov wrote:
> > > > This allow to significant reduces packets processing latency.
> > > >
> > > > Signed-off-by: Sergey Vyazmitinov <s.vyazmitinov@brain4net.com>
[...]
> > > > --- a/lib/librte_eal/linuxapp/kni/kni_misc.c
> > > > +++ b/lib/librte_eal/linuxapp/kni/kni_misc.c
> > > > @@ -45,6 +45,7 @@ MODULE_AUTHOR("Intel Corporation");
> > > >  MODULE_DESCRIPTION("Kernel Module for managing kni devices");
> > > >
> > > >  #define KNI_RX_LOOP_NUM 1000
> > > > +#define KNI_RX_DATA_LOOP_NUM 2500
> > > >
> > > >  #define KNI_MAX_DEVICES 32
> > > >
> > > > @@ -129,25 +130,39 @@ static struct pernet_operations kni_net_ops = {
> > > >  #endif
> > > >  };
> > > >
> > > > -static int
> > > > -kni_thread_single(void *data)
> > > > +static inline void
> > > > +kni_thread_single_rx_data_loop(struct kni_net *knet)
> > > >  {
> > > > -     struct kni_net *knet = data;
> > > > -     int j;
> > > >       struct kni_dev *dev;
> > > > +     int i;
> > > >
> > > > -     while (!kthread_should_stop()) {
> > > > -             down_read(&knet->kni_list_lock);
> > > > -             for (j = 0; j < KNI_RX_LOOP_NUM; j++) {
> > > > -                     list_for_each_entry(dev, &knet->kni_list_head,
> > > list) {
> > > > +     for (i = 0; i < KNI_RX_DATA_LOOP_NUM; ++i) {
> > >
> > > When there are multiple KNI interfaces, and lets assume there is traffic
> > > too, this will behave like:
> > >
> > > KNI1x2500 data_packets + KNI2x2500 data_packets .... KNI10x2500
> > >
> > > After data packets, KNI1 resp_packet + KNI2 resp_packets ...
> > >
> > > Won't this scenario also may cause latency? And perhaps jitter according
> > > KNI interface traffic loads?
> > >
> > > This may be good for some use cases, but not sure if this is good for
> > all.
> > >
> > We can decrease KNI_RX_DATA_LOOP_NUM to some reasonable value.
> > I can make test to find lower bound.
> > Also, the point is in fast check for a new data in interface rx queue.
> > May be will be better add some kind of break after several kni_net_rx
> > calls.
> > Without them loop ends very quickly.
> > Anyway, this patch decrease average latency in my case from 4.5ms to
> > 0.011ms in ping test with 100000 packets.
> >
> 
> If you were seeing latency of 4.5ms, then it is more likely a different
> issue.
> 
> At the end of the loop where KNI is reading packets from the queue, it
> calls *schedule_timeout_interruptible()* with (by default) a 5us timeout.
> However, that call just guarantees that the thread will sleep for AT LEAST
> 5us.
> 
> For most x86 Linux distros, HZ = 250 in the kernel, which works out to 4ms.
> I'm reasonably certain the latency you are seeing is because the KNI thread
> is sleeping and not getting woken up like you might expect.
> 
> When you increased the number of loops happening before the sleep, you
> increased how long KNI spends before it sleeps and it happened to be long
> enough in your particular test to change your average latency. If you ran
> your test for a few minutes and built a histogram of ping times, I bet
> you'll see ~4ms of latency pop up regularly.
> 
> More details from when I dug into this behavior previously:
> http://dpdk.org/ml/archives/dev/2015-June/018858.html

No answer in this discussion.
Should we close it in patchwork?