Re: [dpdk-dev] [PATCH v2 2/3] kni: fix kni fifo synchronization

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> > > -----Original Message-----
> > > > Date: Wed, 19 Sep 2018 21:42:39 +0800
> > > > From: Phil Yang <phil.yang@arm.com>
> > > > To: dev@dpdk.org
> > > > CC: nd@arm.com, jerin.jacob@caviumnetworks.com,
> > > > kkokkilagadda@caviumnetworks.com, Honnappa.Nagarahalli@arm.com,
> > > > Gavin.Hu@arm.com
> > > > Subject: [PATCH v2 2/3] kni: fix kni fifo synchronization
> > > > X-Mailer: git-send-email 2.7.4
> > > >
> > >
> > > + Ferruh Yigit <ferruh.yigit@intel.com>
> > >
> > > >
> > > > With existing code in kni_fifo_put, rx_q values are not being
> > > > updated before updating fifo_write. While reading rx_q in
> > > > kni_net_rx_normal, This is causing the sync issue on other core.
> > > > The same situation happens in kni_fifo_get as well.
> > > >
> > > > So syncing the values by adding C11 atomic memory barriers to make
> > > > sure the values being synced before updating fifo_write and fifo_read.
> > > >
> > > > Fixes: 3fc5ca2 ("kni: initial import")
> > > > Signed-off-by: Phil Yang <phil.yang@arm.com>
> > > > Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> > > > Reviewed-by: Gavin Hu <Gavin.Hu@arm.com>
> > > > ---
> > > >  .../linuxapp/eal/include/exec-env/rte_kni_common.h |  5 ++++
> > > >  lib/librte_kni/rte_kni_fifo.h                      | 30 +++++++++++++++++++++-
> > > >  2 files changed, 34 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git
> > > > a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> > > > b/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> > > > index cfa9448..1fd713b 100644
> > > > ---
> > > > a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> > > > +++ b/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.
> > > > +++ h
> > > > @@ -54,8 +54,13 @@ struct rte_kni_request {
> > > >   * Writing should never overwrite the read position
> > > >   */
> > > >  struct rte_kni_fifo {
> > > > +#ifndef RTE_USE_C11_MEM_MODEL
> > > >         volatile unsigned write;     /**< Next position to be written*/
> > > >         volatile unsigned read;      /**< Next position to be read */
> > > > +#else
> > > > +       unsigned write;              /**< Next position to be written*/
> > > > +       unsigned read;               /**< Next position to be read */
> > > > +#endif
> > > >         unsigned len;                /**< Circular buffer length */
> > > >         unsigned elem_size;          /**< Pointer size - for 32/64 bit OS */
> > > >         void *volatile buffer[];     /**< The buffer contains mbuf pointers */
> > > > diff --git a/lib/librte_kni/rte_kni_fifo.h
> > > > b/lib/librte_kni/rte_kni_fifo.h index ac26a8c..f4171a1 100644
> > > > --- a/lib/librte_kni/rte_kni_fifo.h
> > > > +++ b/lib/librte_kni/rte_kni_fifo.h
> > > > @@ -28,8 +28,13 @@ kni_fifo_put(struct rte_kni_fifo *fifo, void
> > > > **data, unsigned num)  {
> > > >         unsigned i = 0;
> > > >         unsigned fifo_write = fifo->write;
> > > > -       unsigned fifo_read = fifo->read;
> > > >         unsigned new_write = fifo_write;
> > > > +#ifdef RTE_USE_C11_MEM_MODEL
> > > > +       unsigned fifo_read = __atomic_load_n(&fifo->read,
> > > > +
> > > > +__ATOMIC_ACQUIRE); #else
> > > > +       unsigned fifo_read = fifo->read; #endif
> > >
> > > Correct.
> >
> > My apologies, did not follow your comment here. Do you want us to correct
> anything here? '#endif' is not appearing on the correct line in the email, but it
> shows up fine on the patch work.
> 
> No. What I meant is, code is correct.
> 
> >
> > >
> > >
> > > >
> > > >         for (i = 0; i < num; i++) {
> > > >                 new_write = (new_write + 1) & (fifo->len - 1); @@
> > > > -39,7 +44,12 @@ kni_fifo_put(struct rte_kni_fifo *fifo, void
> > > > **data,
> > > unsigned num)
> > > >                 fifo->buffer[fifo_write] = data[i];
> > > >                 fifo_write = new_write;
> > > >         }
> > > > +#ifdef RTE_USE_C11_MEM_MODEL
> > > > +       __atomic_store_n(&fifo->write, fifo_write,
> > > > +__ATOMIC_RELEASE); #else
> > > > +       rte_smp_wmb();
> > > >         fifo->write = fifo_write;
> > > > +#endif
> > >
> > > Correct.
> > > >         return i;
> > > >  }
> > > >
> > > > @@ -51,7 +61,12 @@ kni_fifo_get(struct rte_kni_fifo *fifo, void
> > > > **data, unsigned num)  {
> > > >         unsigned i = 0;
> > > >         unsigned new_read = fifo->read;
> > > > +#ifdef RTE_USE_C11_MEM_MODEL
> > > > +       unsigned fifo_write = __atomic_load_n(&fifo->write,
> > > > +__ATOMIC_ACQUIRE); #else
> > > >         unsigned fifo_write = fifo->write;
> > > > +#endif
> > >
> > > Correct.
> > >
> > > > +
> > > >         for (i = 0; i < num; i++) {
> > > >                 if (new_read == fifo_write)
> > > >                         break;
> > > > @@ -59,7 +74,12 @@ kni_fifo_get(struct rte_kni_fifo *fifo, void
> > > > **data,
> > > unsigned num)
> > > >                 data[i] = fifo->buffer[new_read];
> > > >                 new_read = (new_read + 1) & (fifo->len - 1);
> > > >         }
> > > > +#ifdef RTE_USE_C11_MEM_MODEL
> > > > +       __atomic_store_n(&fifo->read, new_read, __ATOMIC_RELEASE);
> > > > +#else
> > > > +       rte_smp_wmb();
> > > >         fifo->read = new_read;
> > > > +#endif
> > >
> > > Correct.
> > >
> > > >         return i;
> > > >  }
> > > >
> > > > @@ -69,5 +89,13 @@ kni_fifo_get(struct rte_kni_fifo *fifo, void
> > > > **data, unsigned num)  static inline uint32_t
> > > > kni_fifo_count(struct rte_kni_fifo *fifo)  {
> > > > +#ifdef RTE_USE_C11_MEM_MODEL
> > > > +       unsigned fifo_write = __atomic_load_n(&fifo->write,
> > > > +                                                 __ATOMIC_ACQUIRE);
> > > > +       unsigned fifo_read = __atomic_load_n(&fifo->read,
> > > > +
> > > > +__ATOMIC_ACQUIRE);
> > >
> > > Isn't too  heavy to have two __ATOMIC_ACQUIREs? a simple
> > > rte_smp_rmb() would be enough here. Right?
> > > or
> > > Do we need __ATOMIC_ACQUIRE for fifo_write case?
> > >
> > We also had some amount of debate internally on this:
> > 1) We do not want to use rte_smp_rmb() as we want to keep the memory
> models separated (for ex: while using C11, use C11 everywhere). It is also not
> sufficient, please see 3) below.
> 
> But Nothing technically wrong in using rte_smp_rmb() here in terms
> functionally and code generated by the compiler.

rte_smp_rmb() generates 'DMB ISHLD'. This works fine, but it is not optimal. 'LDAR' is a better option which is generated when C11 atomics are used.

> 
> > 2) This API can get called from writer or reader, so both the loads
> > have to be __ATOMIC_ACQUIRE
> > 3) Other option is to use __ATOMIC_RELAXED. That would allow any
> loads/stores around of this API to get reordered, especially since this is an
> inline function. This would put burden on the application to manage the
> ordering depending on its usage. It will also require the application to
> understand the implementation of this API.
> 
> __ATOMIC_RELAXED may be fine too for _count() case as it may not very
> important to get the exact count for the exact very moment, Application can
> retry.
> 
> I am in favor of performance effective implementation.

The requirement on the correctness of the count depends on the usage of this function. I see the following usage:

In the file kni_net.c, function: kni_net_tx:

       if (kni_fifo_free_count(kni->tx_q) == 0 ||                              
                        kni_fifo_count(kni->alloc_q) == 0) {                    
                /**                                                             
                 * If no free entry in tx_q or no entry in alloc_q,             
                 * drops skb and goes out.                                      
                 */                                                             
                goto drop;                                                      
        }

There is no retry here, the packet is dropped.

> 
> >
> > >
> > > Other than that, I prefer to avoid ifdef clutter by introducing two
> > > separate file just like ring C11 implementation.
> > >
> > > I don't have strong opinion on this this part, I let KNI MAINTAINER
> > > to decide on how to accommodate this change.
> >
> > I prefer to change this as well, I am open for suggestions.
> > Introducing two separate files would be too much for this library. A better
> way would be to have something similar to 'smp_store_release' provided by
> the kernel. i.e. create #defines for loads/stores. Hide the clutter behind the
> #defines.
> 
> No Strong opinion on this, leaving to KNI Maintainer.
Will wait on this before re-spinning the patch

> 
> This patch needs to split by two,
> a) Fixes for non C11 implementation(i.e new addition to rte_smp_wmb())
> b) add support for C11 implementation.
Agree

> 
> >
> > >
> > > > +       return (fifo->len + fifo_write - fifo_read) & (fifo->len -
> > > > +1); #else
> > > >         return (fifo->len + fifo->write - fifo->read) & (fifo->len
> > > > - 1);
> > > > +#endif
> > > >  }
> > > > --
> > > > 2.7.4
> > > >