Re: [PATCH net-next] stmmac: align RX buffers

From: Thierry Reding
Date: Wed Aug 11 2021 - 06:26:05 EST


On Tue, Aug 10, 2021 at 08:07:47PM +0100, Marc Zyngier wrote:
> Hi all,
>
> [adding Thierry, Jon and Will to the fun]
>
> On Mon, 14 Jun 2021 03:25:04 +0100,
> Matteo Croce <mcroce@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > From: Matteo Croce <mcroce@xxxxxxxxxxxxx>
> >
> > On RX an SKB is allocated and the received buffer is copied into it.
> > But on some architectures, the memcpy() needs the source and destination
> > buffers to have the same alignment to be efficient.
> >
> > This is not our case, because SKB data pointer is misaligned by two bytes
> > to compensate the ethernet header.
> >
> > Align the RX buffer the same way as the SKB one, so the copy is faster.
> > An iperf3 RX test gives a decent improvement on a RISC-V machine:
> >
> > before:
> > [ ID] Interval Transfer Bitrate Retr
> > [ 5] 0.00-10.00 sec 733 MBytes 615 Mbits/sec 88 sender
> > [ 5] 0.00-10.01 sec 730 MBytes 612 Mbits/sec receiver
> >
> > after:
> > [ ID] Interval Transfer Bitrate Retr
> > [ 5] 0.00-10.00 sec 1.10 GBytes 942 Mbits/sec 0 sender
> > [ 5] 0.00-10.00 sec 1.09 GBytes 940 Mbits/sec receiver
> >
> > And the memcpy() overhead during the RX drops dramatically.
> >
> > before:
> > Overhead Shared O Symbol
> > 43.35% [kernel] [k] memcpy
> > 33.77% [kernel] [k] __asm_copy_to_user
> > 3.64% [kernel] [k] sifive_l2_flush64_range
> >
> > after:
> > Overhead Shared O Symbol
> > 45.40% [kernel] [k] __asm_copy_to_user
> > 28.09% [kernel] [k] memcpy
> > 4.27% [kernel] [k] sifive_l2_flush64_range
> >
> > Signed-off-by: Matteo Croce <mcroce@xxxxxxxxxxxxx>
>
> This patch completely breaks my Jetson TX2 system, composed of 2
> Nvidia Denver and 4 Cortex-A57, in a very "funny" way.
>
> Any significant amount of traffic result in all sort of corruption
> (ssh connections get dropped, Debian packages downloaded have the
> wrong checksums) if any Denver core is involved in any significant way
> (packet processing, interrupt handling). And it is all triggered by
> this very change.
>
> The only way I have to make it work on a Denver core is to route the
> interrupt to that particular core and taskset the workload to it. Any
> other configuration involving a Denver CPU results in some sort of
> corruption. On their own, the A57s are fine.
>
> This smells of memory ordering going really wrong, which this change
> would expose. I haven't had a chance to dig into the driver yet (it
> took me long enough to bisect it), but if someone points me at what is
> supposed to synchronise the DMA when receiving an interrupt, I'll have
> a look.

I recall that Jon was looking into a similar issue recently, though I
think the failure mode was slightly different. I also vaguely recall
that CPU frequency was impacting this to some degree (lower CPU
frequencies would increase the chances of this happening).

Jon's currently out of office, but let me try and dig up the details
on this.

Thierry

>
> Thanks,
>
> M.
>
> > ---
> > drivers/net/ethernet/stmicro/stmmac/stmmac.h | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac.h b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > index b6cd43eda7ac..04bdb3950d63 100644
> > --- a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > @@ -338,9 +338,9 @@ static inline bool stmmac_xdp_is_enabled(struct stmmac_priv *priv)
> > static inline unsigned int stmmac_rx_offset(struct stmmac_priv *priv)
> > {
> > if (stmmac_xdp_is_enabled(priv))
> > - return XDP_PACKET_HEADROOM;
> > + return XDP_PACKET_HEADROOM + NET_IP_ALIGN;
> >
> > - return 0;
> > + return NET_SKB_PAD + NET_IP_ALIGN;
> > }
> >
> > void stmmac_disable_rx_queue(struct stmmac_priv *priv, u32 queue);
> > --
> > 2.31.1
> >
> >
>
> --
> Without deviation from the norm, progress is not possible.

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