Re: [dpdk-dev] [PATCH v1] net/i40e: remove the SMP barrier in HW scanning func

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

<snip>

> >
> > > >
> > > > Add the logic to determine how many DD bits have been set for
> > > > contiguous packets, for removing the SMP barrier while reading descs.
> > >
> > > I didn't understand this.
> > > The current logic already guarantee the read out DD bits are from
> > > continue packets, as it read Rx descriptor in a reversed order from the
> ring.
> > Qi, the comments in the code mention that there is a race condition if
> > the descriptors are not read in the reverse order. But, they do not
> > mention what the race condition is and how it can occur. Appreciate if
> > you could explain that.
> 
> The Race condition happens between the NIC and CPU, if write and read DD
> bit in the same order, there might be a hole (e.g. 1011)  with the reverse read
> order, we make sure no more "1" after the first "0"
> as the read address are declared as volatile, compiler will not re-ordered
> them.
My understanding is that

1) the NIC will write an entire cache line of descriptors to memory "atomically" (i.e. the entire cache line is visible to the CPU at once) if there are enough descriptors ready to fill one cache line.
2) But, if there are not enough descriptors ready (because for ex: there is not enough traffic), then it might write partial cache lines.

Please correct me if I am wrong.

For #1, I do not think it matters if we read the descriptors in reverse order or not as the cache line is written atomically.
For #1, if we read in reverse order, does it make sense to not check the DD bits of descriptors that are earlier in the order once we encounter a descriptor that has its DD bit set? This is because NIC updates the descriptors in order.

> 
> >
> > On x86, the reads are not re-ordered (though the compiler can
> > re-order). On ARM, the reads can get re-ordered and hence the barriers
> > are required. In order to avoid the barriers, we are trying to process
> > only those descriptors whose DD bits are set such that they are
> > contiguous. i.e. if the DD bits are 1011, we process only the first descriptor.
> 
> Ok, I see. thanks for the explanation.
> At this moment, I may prefer not change the behavior of x86, so compile
> option for arm can be added, in future when we observe no performance
> impact for x86 as well, we can consider to remove it, what do you think?
I am ok with this approach.

> 
> >
> > > So I didn't see the a new logic be added, would you describe more
> > > clear about the purpose of this patch?
> > >
> > > >
> > > > Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> > > > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > > ---
> > > >  drivers/net/i40e/i40e_rxtx.c | 13 ++++++++-----
> > > >  1 file changed, 8 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/drivers/net/i40e/i40e_rxtx.c
> > > > b/drivers/net/i40e/i40e_rxtx.c index
> > > > 6c58decec..410a81f30 100644
> > > > --- a/drivers/net/i40e/i40e_rxtx.c
> > > > +++ b/drivers/net/i40e/i40e_rxtx.c
> > > > @@ -452,7 +452,7 @@ i40e_rx_scan_hw_ring(struct i40e_rx_queue
> *rxq)
> > > >  	uint16_t pkt_len;
> > > >  	uint64_t qword1;
> > > >  	uint32_t rx_status;
> > > > -	int32_t s[I40E_LOOK_AHEAD], nb_dd;
> > > > +	int32_t s[I40E_LOOK_AHEAD], var, nb_dd;
> > > >  	int32_t i, j, nb_rx = 0;
> > > >  	uint64_t pkt_flags;
> > > >  	uint32_t *ptype_tbl = rxq->vsi->adapter->ptype_tbl; @@ -482,11
> > > > +482,14 @@ i40e_rx_scan_hw_ring(struct i40e_rx_queue *rxq)
> > > >  					I40E_RXD_QW1_STATUS_SHIFT;
> > > >  		}
> > > >
> > > > -		rte_smp_rmb();
> > >
> > > Any performance gain by removing this? and it is not necessary to be
> > > combined with below change, right?
> > >
> > > > -
> > > >  		/* Compute how many status bits were set */
> > > > -		for (j = 0, nb_dd = 0; j < I40E_LOOK_AHEAD; j++)
> > > > -			nb_dd += s[j] & (1 <<
> > > I40E_RX_DESC_STATUS_DD_SHIFT);
> > > > +		for (j = 0, nb_dd = 0; j < I40E_LOOK_AHEAD; j++) {
> > > > +			var = s[j] & (1 << I40E_RX_DESC_STATUS_DD_SHIFT);
> > > > +			if (var)
> > > > +				nb_dd += 1;
> > > > +			else
> > > > +				break;
> > > > +		}
> > > >
> > > >  		nb_rx += nb_dd;
> > > >
> > > > --
> > > > 2.17.1