On Fri, Oct 01, 2021 at 12:17:26PM +0800, Kai-Heng Feng wrote:

On Sat, Sep 18, 2021 at 6:09 AM Bjorn Helgaas [off-list ref] wrote:

quoted

On Thu, Sep 16, 2021 at 11:44:14PM +0800, Kai-Heng Feng wrote:

quoted

The purpose of the series is to get comments and reviews so we can merge
and test the series in downstream kernel.

The latest Realtek vendor driver and its Windows driver implements a
feature called "dynamic ASPM" which can improve performance on it's
ethernet NICs.

Heiner Kallweit pointed out the potential root cause can be that the
buffer is too small for its ASPM exit latency.

I looked at the lspci data in your bugzilla
(https://bugzilla.kernel.org/show_bug.cgi?id=214307).

L1.2 is enabled, which requires the Latency Tolerance Reporting
capability, which helps determine when the Link will be put in L1.2.
IIUC, these are analogous to the DevCap "Acceptable Latency" values.
Zero latency values indicate the device will be impacted by any delay
(PCIe r5.0, sec 6.18).

Linux does not currently program those values, so the values there
must have been set by the BIOS.  On the working AMD system, they're
set to 1048576ns, while on the broken Intel system, they're set to
3145728ns.

I don't really understand how these values should be computed, and I
think they depend on some electrical characteristics of the Link, so
I'm not sure it's *necessarily* a problem that they are different.
But a 3X difference does seem pretty large.

So I'm curious whether this is related to the problem.  Here are some
things we could try on the broken Intel system:

Original network speed, tested via iperf3:
TX: ~255 Mbps
RX: ~490 Mbps

quoted

  - What happens if you disable ASPM L1.2 using
    /sys/devices/pci*/.../link/l1_2_aspm?

TX: ~670 Mbps
RX: ~670 Mbps

Do you remember if there were any dropped packets here?  You mentioned
at [1] that you have also seen reports of issues with L0s and L1.1.
If you disable L1.2, L0s and L1.1 *should* still be enabled.

[1] https://lore.kernel.org/r/CAAd53p4v+CmupCu2+3vY5N64WKkxcNvpk1M7+hhNoposx+aYCg@mail.gmail.com (local)