Messages in this thread Patch in this message | | | Date | Tue, 15 Jun 2021 01:21:07 +0200 | From | Matteo Croce <> | Subject | Re: [PATCH net-next] stmmac: align RX buffers |
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On Mon, 14 Jun 2021 12:51:11 -0700 (PDT) David Miller <davem@davemloft.net> wrote:
> > But thois means the ethernet header will be misaliugned and this will > kill performance on some cpus as misaligned accessed are resolved > wioth a trap handler. > > Even on cpus that don't trap, the access will be slower. > > Thanks.
Isn't the IP header which should be aligned to avoid expensive traps? From include/linux/skbuff.h:
* Since an ethernet header is 14 bytes network drivers often end up with * the IP header at an unaligned offset. The IP header can be aligned by * shifting the start of the packet by 2 bytes. Drivers should do this * with: * * skb_reserve(skb, NET_IP_ALIGN);
But the problem here really is not the header alignment, the problem is that the rx buffer is copied into an skb, and the two buffers have different alignments. If I add this print, I get this for every packet:
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c @@ -5460,6 +5460,8 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue) + printk("skb->data alignment: %lu\n", (uintptr_t)skb->data & 7); + printk("xdp.data alignment: %lu\n" , (uintptr_t)xdp.data & 7); skb_copy_to_linear_data(skb, xdp.data, buf1_len);
[ 1060.967768] skb->data alignment: 2 [ 1060.971174] xdp.data alignment: 0 [ 1061.967589] skb->data alignment: 2 [ 1061.970994] xdp.data alignment: 0
And many architectures do an optimized memcpy when the low order bits of the two pointers match, to name a few:
arch/alpha/lib/memcpy.c: /* If both source and dest are word aligned copy words */ if (!((unsigned int)dest_w & 3) && !((unsigned int)src_w & 3)) {
arch/xtensa/lib/memcopy.S: /* * Destination and source are word-aligned, use word copy. */ # copy 16 bytes per iteration for word-aligned dst and word-aligned src
arch/openrisc/lib/memcpy.c: /* If both source and dest are word aligned copy words */ if (!((unsigned int)dest_w & 3) && !((unsigned int)src_w & 3)) { And so on. With my patch I (mis)align the two buffer at an offset 2 (NET_IP_ALIGN) so the data can be copied faster:
[ 16.648485] skb->data alignment: 2 [ 16.651894] xdp.data alignment: 2 [ 16.714260] skb->data alignment: 2 [ 16.717688] xdp.data alignment: 2
Does this make sense?
Regards, -- per aspera ad upstream
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