Optimizing kernel compilation / alignments for network performance

Rafał Miłecki zajec5 at gmail.com
Tue May 10 04:23:38 PDT 2022

On 6.05.2022 10:45, Arnd Bergmann wrote:
> On Fri, May 6, 2022 at 9:44 AM Rafał Miłecki <zajec5 at gmail.com> wrote:
>> On 5.05.2022 18:04, Andrew Lunn wrote:
>>>> you'll see that most used functions are:
>>>> v7_dma_inv_range
>>>> __irqentry_text_end
>>>> l2c210_inv_range
>>>> v7_dma_clean_range
>>>> bcma_host_soc_read32
>>>> __netif_receive_skb_core
>>>> arch_cpu_idle
>>>> l2c210_clean_range
>>>> fib_table_lookup
>>> There is a lot of cache management functions here.
> Indeed, so optimizing the coherency management (see Felix' reply)
> is likely to help most in making the driver faster, but that does not
> explain why the alignment of the object code has such a big impact
> on performance.
> To investigate the alignment further, what I was actually looking for
> is a comparison of the profile of the slow and fast case. Here I would
> expect that the slow case spends more time in one of the functions
> that don't deal with cache management (maybe fib_table_lookup or
> __netif_receive_skb_core).
> A few other thoughts:
> - bcma_host_soc_read32() is a fundamentally slow operation, maybe
>    some of the calls can turned into a relaxed read, like the readback
>    in bgmac_chip_intrs_off() or the 'poll again' at the end bgmac_poll(),
>    though obviously not the one in bgmac_dma_rx_read().
>    It may be possible to even avoid some of the reads entirely, checking
>    for more data in bgmac_poll() may actually be counterproductive
>    depending on the workload.

I'll experiment with that, hopefully I can optimize it a bit.

> - The higher-end networking SoCs are usually cache-coherent and
>    can avoid the cache management entirely. There is a slim chance
>    that this chip is designed that way and it just needs to be enabled
>    properly. Most low-end chips don't implement the coherent
>    interconnect though, and I suppose you have checked this already.

To my best knowledge Northstar platform doesn't support hw coherency.

I just took an extra look at Broadcom's SDK and them seem to have some
driver for selected chipsets but BCM708 isn't there.

	bool "Global Hardware Cache Coherency"
	default n
	depends on BCM963158 || BCM96846 || BCM96858 || BCM96856 || BCM963178 || BCM947622 || BCM963146  || BCM94912 || BCM96813 || BCM96756 || BCM96855

> - bgmac_dma_rx_update_index() and bgmac_dma_tx_add() appear
>    to have an extraneous dma_wmb(), which should be implied by the
>    non-relaxed writel() in bgmac_write().

I tried dropping wmb() calls.
With wmb(): 421 Mb/s
Without: 418 Mb/s

I also tried dropping bgmac_read() from bgmac_chip_intrs_off() which
seems to be a flushing readback.

With bgmac_read(): 421 Mb/s
Without: 413 Mb/s

> - accesses to the DMA descriptor don't show up in the profile here,
>    but look like they can get misoptimized by the compiler. I would
>    generally use READ_ONCE() and WRITE_ONCE() for these to
>    ensure that you don't end up with extra or out-of-order accesses.
>    This also makes it clearer to the reader that something special
>    happens here.

Should I use something as below?

FWIW it doesn't seem to change NAT performance.
Without WRITE_ONCE: 421 Mb/s
With: 419 Mb/s

diff --git a/drivers/net/ethernet/broadcom/bgmac.c b/drivers/net/ethernet/broadcom/bgmac.c
index 87700072..ce98f2a9 100644
--- a/drivers/net/ethernet/broadcom/bgmac.c
+++ b/drivers/net/ethernet/broadcom/bgmac.c
@@ -119,10 +119,10 @@ bgmac_dma_tx_add_buf(struct bgmac *bgmac, struct bgmac_dma_ring *ring,

  	slot = &ring->slots[i];
  	dma_desc = &ring->cpu_base[i];
-	dma_desc->addr_low = cpu_to_le32(lower_32_bits(slot->dma_addr));
-	dma_desc->addr_high = cpu_to_le32(upper_32_bits(slot->dma_addr));
-	dma_desc->ctl0 = cpu_to_le32(ctl0);
-	dma_desc->ctl1 = cpu_to_le32(ctl1);
+	WRITE_ONCE(dma_desc->addr_low, cpu_to_le32(lower_32_bits(slot->dma_addr)));
+	WRITE_ONCE(dma_desc->addr_high, cpu_to_le32(upper_32_bits(slot->dma_addr)));
+	WRITE_ONCE(dma_desc->ctl0, cpu_to_le32(ctl0));
+	WRITE_ONCE(dma_desc->ctl1, cpu_to_le32(ctl1));

  static netdev_tx_t bgmac_dma_tx_add(struct bgmac *bgmac,
@@ -387,10 +387,10 @@ static void bgmac_dma_rx_setup_desc(struct bgmac *bgmac,

-	dma_desc->addr_low = cpu_to_le32(lower_32_bits(ring->slots[desc_idx].dma_addr));
-	dma_desc->addr_high = cpu_to_le32(upper_32_bits(ring->slots[desc_idx].dma_addr));
-	dma_desc->ctl0 = cpu_to_le32(ctl0);
-	dma_desc->ctl1 = cpu_to_le32(ctl1);
+	WRITE_ONCE(dma_desc->addr_low, cpu_to_le32(lower_32_bits(ring->slots[desc_idx].dma_addr)));
+	WRITE_ONCE(dma_desc->addr_high, cpu_to_le32(upper_32_bits(ring->slots[desc_idx].dma_addr)));
+	WRITE_ONCE(dma_desc->ctl0, cpu_to_le32(ctl0));
+	WRITE_ONCE(dma_desc->ctl1, cpu_to_le32(ctl1));

  	ring->end = desc_idx;

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