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

[Jay Rolette]

On Fri, Mar 10, 2017 at 6:59 AM, Thomas Monjalon <thomas.monjalon at 6wind.com>
wrote:

> 2017-01-18 07:11, Jay Rolette:
> > On Wed, Jan 18, 2017 at 5:05 AM, Sergey Vyazmitinov <
> > s.vyazmitinov at brain4net.com> wrote:
> >
> > > On Thu, Jan 12, 2017 at 12:29 AM, Ferruh Yigit <ferruh.yigit at 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 at 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?
>

I don't believe we should merge the patch.

Jay