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Date: Wed, 15 Apr 2015 15:40:29 +0200
From: Ulf Samuelsson <ulf.samuelsson@...csson.com>
To: <netdev@...gii.com>
CC: <netdev@...r.kernel.org>
Subject: Re: [PATCH] neighbour.c: Avoid GC directly after state change
No reply on this....
in net/core/neighbour.c: neigh_timer_handler I see:
if (state & NUD_REACHABLE) {
if (time_before_eq(now,
neigh->confirmed + neigh->parms->reachable_time)) {
neigh_dbg(2, "neigh %p is still alive\n", neigh);
next = neigh->confirmed + neigh->parms->reachable_time;
} else if (time_before_eq(now,
neigh->used +
NEIGH_VAR(neigh->parms, DELAY_PROBE_TIME))) {
neigh_dbg(2, "neigh %p is delayed\n", neigh);
neigh->nud_state = NUD_DELAY;
neigh->updated = jiffies;
neigh_suspect(neigh);
next = now + NEIGH_VAR(neigh->parms, DELAY_PROBE_TIME);
} else {
neigh_dbg(2, "neigh %p is suspected\n", neigh);
neigh->nud_state = NUD_STALE;
neigh->updated = jiffies;
neigh_suspect(neigh);
notify = 1;
}
} else ...
Why is the second test there in the first place?
In the normal case, "neigh->used" does not get updated until the entry
is found STALE
in the periodic work.
Why not use "neigh->confirmed" and another sysctl variable?
if (state & NUD_REACHABLE) {
if (time_before_eq(now,
neigh->confirmed + neigh->parms->reachable_time)) {
neigh_dbg(2, "neigh %p is still alive\n", neigh);
next = neigh->confirmed + neigh->parms->reachable_time;
} else if (time_before_eq(now,
- neigh->used +
- NEIGH_VAR(neigh->parms, DELAY_PROBE_TIME))) {
+ neigh->confirmed +
+ NEIGH_VAR(neigh->parms, DELAY_REPROBE_TIME))) {
neigh_dbg(2, "neigh %p is delayed\n", neigh);
neigh->nud_state = NUD_DELAY;
neigh->updated = jiffies;
neigh_suspect(neigh);
next = now + NEIGH_VAR(neigh->parms, DELAY_PROBE_TIME);
} else {
neigh_dbg(2, "neigh %p is suspected\n", neigh);
neigh->nud_state = NUD_STALE;
neigh->updated = jiffies;
neigh_suspect(neigh);
notify = 1;
}
} else ...
if DELAY_REPROBE_TIME is larger than "reachable_time", then
the kernel will send out ARP probes when it is about
to lose communication with a remote machine.
This is what we need.
If it is smaller, then
it will go from REACHABLE to STALE.
The initial value of DELAY_REPROBE_TIME needs to be settable in Kconfig
to allow the selection of functionality.
I am told that setting stuff using sysctl has a performance penalty, when
interfaces are dynamically created and deleted in hundreds.
Best Regards,
Ulf Samuelsson
On 04/10/2015 10:26 AM, Ulf Samuelsson wrote:
>
> On 03/12/2015 07:26 PM, David Miller wrote:
>> I hate changes like this.
>>
>> By making this a Kconfig options it cannot be dynamic, and every
>> distribution is going to have to scratch their head and decide
>> what to set this to.
>>
>> That's seriously suboptimal.
>>
>> If you want to change behavior based upon whether userspace is
>> managing the damn table, make it so the user doing so has to
>> ask for the new behavior at _RUN TIME_ via the netlink interface
>> or similar.
>>
>> Picking the guard time itself at compile time is also undesirable.
>>
>> And you don't even want a damn timer, what you want is for the
>> state of the entry to be frozen and for the user to "release"
>> it by either adjusting the state to something else or marking
>> in some other way to allow it to be unfrozen and released again.
>>
>> Why put it to chance with some timeout? Make things explicit.
>
> The desired functionality is that if communication stops,
> you want to send out ARP probes, before the entry is deleted.
>
> The current (pseudo) code of the neigh timer is:
>
> if (state & NUD_REACHABLE) {
> if (now <= "confirmed + "reachable_time")) {
> ... /* We are OK */
> } else if (now < "used" + DELAY_PROBE_TIME) { /* Never
> happens */
> state = NUD_DELAY;
> } else {
> state = NUD_STALE;
> notify = 1;
> }
>
> We never see the state beeing changed from REACHABLE to DELAY,
> so the probes are not beeing sent out, instead you always go
> from REACHABLE to STALE.
>
> DELAY_PROBE_TIME is set to (5 x HZ) and "used"
> seems to be only set by the periodic_work routine
> when the neigh entry is in STALE state, and then it is too late.
> It is also set by "arp_find" which is used by "broken" devices.
>
> In practice, the second condition: "(now < "used" + DELAY_PROBE_TIME)"
> is never used.
> What is the intention of this test?
>
> By adding a new test + parameter, we would get the desired functionality,
> and no need to listen for notifications or doing ARP state updates
> from applications.
>
> if (now <= "confirmed + "reachable_time")) {
> ... /* We are OK */
> + else if (now <= "confirmed + "reprobe_time")) {
> + state <= NUD_DELAY;
> } else if (now < "used" + DELAY_PROBE_TIME))) { /* Never
> happens */
> state <= NUD_DELAY;
> } else {
> state = NUD_STALE;
> notify = 1;
> }
>
> This way the entry would remain in REACHABLE while normal
> communication occurs,
> then it would enter DELAY state to probe, and if that fails, it goes
> to STALE state.
>
> Alternatively, we just change the second test:
> if (now <= "confirmed + "reachable_time")) {
> ... /* We are OK */
> - } else if (now < "used" + DELAY_PROBE_TIME))) { /* Never
> happens */
> + } else if (now < "confirmed" + DELAY_PROBE_TIME))) {
> state <= NUD_DELAY;
> } else {
> state = NUD_STALE;
> notify = 1;
> }
>
>
> The DELAY_PROBE_TIME, should preferrably be a kernel Kconfig parameter.
>
> Best Regards,
> Ulf Samuelsson
>
>
>> I'm not applying this patch.
>>
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