Re: [PATCH net-next V2 00/11] net/mlx5e: Add support for devmem and io_uring TCP zero-copy

Stanislav Fomichev
2025-05-28T23:04:20Z

On 05/28, Mina Almasry wrote:
> On Wed, May 28, 2025 at 8:45 AM Stanislav Fomichev <stfomichev@gmail.com> wrote:
> >
> > On 05/28, Dragos Tatulea wrote:
> > > On Tue, May 27, 2025 at 09:05:49AM -0700, Stanislav Fomichev wrote:
> > > > On 05/23, Tariq Toukan wrote:
> > > > > This series from the team adds support for zerocopy rx TCP with devmem
> > > > > and io_uring for ConnectX7 NICs and above. For performance reasons and
> > > > > simplicity HW-GRO will also be turned on when header-data split mode is
> > > > > on.
> > > > >
> > > > > Find more details below.
> > > > >
> > > > > Regards,
> > > > > Tariq
> > > > >
> > > > > Performance
> > > > > ===========
> > > > >
> > > > > Test setup:
> > > > >
> > > > > * CPU: Intel(R) Xeon(R) Platinum 8380 CPU @ 2.30GHz (single NUMA)
> > > > > * NIC: ConnectX7
> > > > > * Benchmarking tool: kperf [1]
> > > > > * Single TCP flow
> > > > > * Test duration: 60s
> > > > >
> > > > > With application thread and interrupts pinned to the *same* core:
> > > > >
> > > > > |------+-----------+----------|
> > > > > | MTU | epoll | io_uring |
> > > > > |------+-----------+----------|
> > > > > | 1500 | 61.6 Gbps | 114 Gbps |
> > > > > | 4096 | 69.3 Gbps | 151 Gbps |
> > > > > | 9000 | 67.8 Gbps | 187 Gbps |
> > > > > |------+-----------+----------|
> > > > >
> > > > > The CPU usage for io_uring is 95%.
> > > > >
> > > > > Reproduction steps for io_uring:
> > > > >
> > > > > server --no-daemon -a 2001:db8::1 --no-memcmp --iou --iou_sendzc \
> > > > > --iou_zcrx --iou_dev_name eth2 --iou_zcrx_queue_id 2
> > > > >
> > > > > server --no-daemon -a 2001:db8::2 --no-memcmp --iou --iou_sendzc
> > > > >
> > > > > client --src 2001:db8::2 --dst 2001:db8::1 \
> > > > > --msg-zerocopy -t 60 --cpu-min=2 --cpu-max=2
> > > > >
> > > > > Patch overview:
> > > > > ================
> > > > >
> > > > > First, a netmem API for skb_can_coalesce is added to the core to be able
> > > > > to do skb fragment coalescing on netmems.
> > > > >
> > > > > The next patches introduce some cleanups in the internal SHAMPO code and
> > > > > improvements to hw gro capability checks in FW.
> > > > >
> > > > > A separate page_pool is introduced for headers. Ethtool stats are added
> > > > > as well.
> > > > >
> > > > > Then the driver is converted to use the netmem API and to allow support
> > > > > for unreadable netmem page pool.
> > > > >
> > > > > The queue management ops are implemented.
> > > > >
> > > > > Finally, the tcp-data-split ring parameter is exposed.
> > > > >
> > > > > Changelog
> > > > > =========
> > > > >
> > > > > Changes from v1 [0]:
> > > > > - Added support for skb_can_coalesce_netmem().
> > > > > - Avoid netmem_to_page() casts in the driver.
> > > > > - Fixed code to abide 80 char limit with some exceptions to avoid
> > > > > code churn.
> > > >
> > > > Since there is gonna be 2-3 weeks of closed net-next, can you
> > > > also add a patch for the tx side? It should be trivial (skip dma unmap
> > > > for niovs in tx completions plus netdev->netmem_tx=1).
> > > >
> > > Seems indeed trivial. We will add it.
> > >
> > > > And, btw, what about the issue that Cosmin raised in [0]? Is it addressed
> > > > in this series?
> > > >
> > > > 0: https://lore.kernel.org/netdev/9322c3c4826ed1072ddc9a2103cc641060665864.camel@nvidia.com/
> > > We wanted to fix this afterwards as it needs to change a more subtle
> > > part in the code that replenishes pages. This needs more thinking and
> > > testing.
> >
> > Thanks! For my understanding: does the issue occur only during initial
> > queue refill? Or the same problem will happen any time there is a burst
> > of traffic that might exhaust all rx descriptors?
> >
>
> Minor: a burst in traffic likely won't reproduce this case, I'm sure
> mlx5 can drive the hardware to line rate consistently. It's more if
> the machine is under extreme memory pressure, I think,
> page_pool_alloc_pages and friends may return ENOMEM, which reproduces
> the same edge case as the dma-buf being extremely small which also
> makes page_pool_alloc_netmems return -ENOMEM.

What I want to understand is whether the kernel/driver will oops when dmabuf
runs out of buffers after initial setup. Either traffic burst and/or userspace
being slow on refill - doesn't matter.