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SubjectRe: [PATCH net-next] net: dsa: mv88e6xxx: avoid some redundant vtu load/purge operations
Hi Rasmus,

On Mon, 22 Jul 2019 23:37:26 +0000, Rasmus Villemoes <rasmus.villemoes@prevas.dk> wrote:
> We have an ERPS (Ethernet Ring Protection Switching) setup involving
> mv88e6250 switches which we're in the process of switching to a BSP
> based on the mainline driver. Breaking any link in the ring works as
> expected, with the ring reconfiguring itself quickly and traffic
> continuing with almost no noticable drops. However, when plugging back
> the cable, we see 5+ second stalls.
>
> This has been tracked down to the userspace application in charge of
> the protocol missing a few CCM messages on the good link (the one that
> was not unplugged), causing it to broadcast a "signal fail". That
> message eventually reaches its link partner, which responds by
> blocking the port. Meanwhile, the first node has continued to block
> the port with the just plugged-in cable, breaking the network. And the
> reason for those missing CCM messages has in turn been tracked down to
> the VTU apparently being too busy servicing load/purge operations that
> the normal lookups are delayed.
>
> Initial state, the link between C and D is blocked in software.
>
> _____________________
> / \
> | |
> A ----- B ----- C *---- D
>
> Unplug the cable between C and D.
>
> _____________________
> / \
> | |
> A ----- B ----- C * * D
>
> Reestablish the link between C and D.
> _____________________
> / \
> | |
> A ----- B ----- C *---- D
>
> Somehow, enough VTU/ATU operations happen inside C that prevents
> the application from receving the CCM messages from B in a timely
> manner, so a Signal Fail message is sent by C. When B receives
> that, it responds by blocking its port.
>
> _____________________
> / \
> | |
> A ----- B *---* C *---- D
>
> Very shortly after this, the signal fail condition clears on the
> BC link (some CCM messages finally make it through), so C
> unblocks the port. However, a guard timer inside B prevents it
> from removing the blocking before 5 seconds have elapsed.
>
> It is not unlikely that our userspace ERPS implementation could be
> smarter and/or is simply buggy. However, this patch fixes the symptoms
> we see, and is a small optimization that should not break anything
> (knock wood). The idea is simply to avoid doing an VTU load of an
> entry identical to the one already present. To do that, we need to
> know whether mv88e6xxx_vtu_get() actually found an existing entry, or
> has just prepared a struct mv88e6xxx_vtu_entry for us to load. To that
> end, let vlan->valid be an output parameter. The other two callers of
> mv88e6xxx_vtu_get() are not affected by this patch since they pass
> new=false.
>
> Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
> ---
> drivers/net/dsa/mv88e6xxx/chip.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
> index 6b17cd961d06..2e500428670c 100644
> --- a/drivers/net/dsa/mv88e6xxx/chip.c
> +++ b/drivers/net/dsa/mv88e6xxx/chip.c
> @@ -1423,7 +1423,6 @@ static int mv88e6xxx_vtu_get(struct mv88e6xxx_chip *chip, u16 vid,
>
> /* Initialize a fresh VLAN entry */
> memset(entry, 0, sizeof(*entry));
> - entry->valid = true;
> entry->vid = vid;
>
> /* Exclude all ports */
> @@ -1618,6 +1617,9 @@ static int _mv88e6xxx_port_vlan_add(struct mv88e6xxx_chip *chip, int port,
> if (err)
> return err;
>
> + if (vlan.valid && vlan.member[port] == member)
> + return 0;
> + vlan.valid = true;
> vlan.member[port] = member;
>
> err = mv88e6xxx_vtu_loadpurge(chip, &vlan);

I'd prefer not to use the mv88e6xxx_vtu_entry structure for output
parameters, this can be confusing. As you correctly mentioned, this
initialization is only done for _mv88e6xxx_port_vlan_add, so I'll
prepare a patch which gets rid of the boolean parameter and move that
logic up.


Thanks,
Vivien

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