[dpdk-dev] [PATCH v3] net/mlx5: relaxed ordering for multi-packet RQ buffer refcnt

[Alexander Kozyrev]

> Phil Yang <Phil.Yang at arm.com> writes:
> 
> <snip>
> 
> > > > > > > @@ -1790,9 +1792,9 @@ mlx5_rx_burst_mprq(void *dpdk_rxq,
> > > struct
> > > > > > > rte_mbuf **pkts, uint16_t pkts_n)  void *buf_addr;
> > > > > > >
> > > > > > >  /* Increment the refcnt of the whole chunk. */
> > > > > > > -rte_atomic16_add_return(&buf->refcnt, 1);
> > > > rte_atomic16_add_return includes a full barrier along with atomic
> > > operation.
> > > > But is full barrier required here? For ex:
> > > > __atomic_add_fetch(&buf->refcnt, 1,
> > > > __ATOMIC_RELAXED) will offer atomicity, but no barrier. Would that
> > > > be enough?
> > > >
> > > > > > > -MLX5_ASSERT((uint16_t)rte_atomic16_read(&buf-
> > > > > > > >refcnt) <=
> > > > > > > -    strd_n + 1);
> > > > > > > +__atomic_add_fetch(&buf->refcnt, 1,
> > > > > > > __ATOMIC_ACQUIRE);
> > >
> > > The atomic load in MLX5_ASSERT() accesses the same memory space as
> > > the previous __atomic_add_fetch() does.
> > > They will access this memory space in the program order when we
> > > enabled MLX5_PMD_DEBUG. So the ACQUIRE barrier in
> > > __atomic_add_fetch() becomes unnecessary.
> > >
> > > By changing it to RELAXED ordering, this patch got 7.6% performance
> > > improvement on N1 (making it generate A72 alike instructions).
> > >
> > > Could you please also try it on your testbed, Alex?
> >
> > Situation got better with this modification, here are the results:
> >  - no patch:             3.0 Mpps CPU cycles/packet=51.52
> >  - original patch:    2.1 Mpps CPU cycles/packet=71.05
> >  - modified patch: 2.9 Mpps CPU cycles/packet=52.79 Also, I found that
> > the degradation is there only in case I enable bursts stats.
> 
> 
> Great! So this patch will not hurt the normal datapath performance.
> 
> 
> > Could you please turn on the following config options and see if you
> > can reproduce this as well?
> > CONFIG_RTE_TEST_PMD_RECORD_CORE_CYCLES=y
> > CONFIG_RTE_TEST_PMD_RECORD_BURST_STATS=y
> 
> Thanks, Alex. Some updates.
> 
> Slightly (about 1%) throughput degradation was detected after we enabled
> these two config options on N1 SoC.
> 
> If we look insight the perf stats results, with this patch, both mlx5_rx_burst
> and mlx5_tx_burst consume fewer CPU cycles than the original code.
> However, __memcpy_generic takes more cycles. I think that might be the
> reason for CPU cycles per packet increment after applying this patch.
> 
> Original code:
> 98.07%--pkt_burst_io_forward
>         |
>         |--44.53%--__memcpy_generic
>         |
>         |--35.85%--mlx5_rx_burst_mprq
>         |
>         |--15.94%--mlx5_tx_burst_none_empw
>         |          |
>         |          |--7.32%--mlx5_tx_handle_completion.isra.0
>         |          |
>         |           --0.50%--__memcpy_generic
>         |
>          --1.14%--memcpy at plt
> 
> Use C11 with RELAXED ordering:
> 99.36%--pkt_burst_io_forward
>         |
>         |--47.40%--__memcpy_generic
>         |
>         |--34.62%--mlx5_rx_burst_mprq
>         |
>         |--15.55%--mlx5_tx_burst_none_empw
>         |          |
>         |           --7.08%--mlx5_tx_handle_completion.isra.0
>         |
>          --1.17%--memcpy at plt
> 
> BTW, all the atomic operations in this patch are not the hotspot.

Phil, we are seeing much worse degradation on our ARM platform unfortunately.
I don't think that discrepancy in memcpy can explain this behavior.
Your patch is not touching this area of code. Let me collect some perf stat on our side.

> 
> >
> > > >
> > > > Can you replace just the above line with the following lines and test it?
> > > >
> > > > __atomic_add_fetch(&buf->refcnt, 1, __ATOMIC_RELAXED);
> > > > __atomic_thread_fence(__ATOMIC_ACQ_REL);
> > > >
> > > > This should make the generated code same as before this patch. Let
> > > > me know if you would prefer us to re-spin the patch instead (for
> testing).
> > > >
> > > > > > > +MLX5_ASSERT(__atomic_load_n(&buf->refcnt,
> > > > > > > +    __ATOMIC_RELAXED) <= strd_n + 1);
> > > > > > >  buf_addr = RTE_PTR_SUB(addr, RTE_PKTMBUF_HEADROOM);
> > > > > > >  /*
> > > > > > >   * MLX5 device doesn't use iova but it is necessary in a
> > > > > > diff
> > > > > > > --git a/drivers/net/mlx5/mlx5_rxtx.h
> > > > > > > b/drivers/net/mlx5/mlx5_rxtx.h index 26621ff..0fc15f3 100644
> > > > > > > --- a/drivers/net/mlx5/mlx5_rxtx.h
> > > > > > > +++ b/drivers/net/mlx5/mlx5_rxtx.h
> <snip>
> > > >