Re: [dpdk-dev] [PATCH] net/memif: relax barrier for zero copy path

[Phil Yang <Phil.Yang@arm.com>]

Jakub Grajciar -X (jgrajcia - PANTHEON TECH SRO at Cisco) writes:

<snip>

> > Subject: RE: [PATCH] net/memif: relax barrier for zero copy path
> >
> > Hi Jakub,
> > 	I am trying to review this patch. I am having difficulty in
> understanding
> > the implementation for the queue/ring, appreciate if you could help me
> > understand the logic.
> 
> 'ring' refers to a ring buffer holding packet descriptors. These descriptors
> hold metadata about the packet (packet buffer address, length, etc..).
> 'queues' are a representation of rings and buffers  (+ some metadata). In
> more detail, one ring (S2M) and packet buffers allocated for this ring would
> be represented as 'tx queue' for the slave and 'rx queue' for the master.
> 
> >
> > 1) The S2M queues - are used to send packets from slave to master. My
> > understanding is that, the slave thread would call 'eth_memif_tx_zc' and
> the
> > master thread would call 'eth_memif_rx_zc'. Is this correct?
> > 2) The M2S queues - are used to send packets from master to slave. Here
> the
> > slave thread would call 'eth_memif_rx_zc' and the master thread would call
> > 'eth_memif_tx_zc'. Is this correct?
> 
> This is inded correct.
 
<snip>

> > >
> > > Using 'rte_mb' to synchronize the shared ring head/tail between
> > > producer and consumer will stall the pipeline and damage performance
> > > on the weak memory model platforms, such like aarch64.
> > >
> > > Relax the expensive barrier with c11 atomic with explicit memory
> > > ordering can improve 3.6% performance on throughput.
> 
> My question here is: `rte_mb` is supposed to make sure that head/tail
> pointer are not updated before the packets are written into shared memory.
> Does the atomic ensures that the packets are written into shared memory
> before head/tail pointers are updated?

Yes, it does. 
The atomic store-release acts as a one-way barrier here to make sure all the memory accesses before the store-release are observed before it.


Thanks,
Phil