Subject: Re: [PATCH] neighbour.c: Avoid GC directly after state change

From: YOSHIFUJI Hideaki <[email-masked]>
Date: Sun, 15 Mar 2015 17:27:05 +0900

Hello.

Ulf Samuelsson wrote:
> From: Ulf Samuelsson <[email-masked]>
> 
> The neighbour state is changed in the ARP timer handler.
> If the state is changed to NUD_STALE, then the neighbour
> entry becomes a candidate for garbage collection.
> 
> The garbage collection is handled by a "periodic work" routine.
> 
> When :
> 
> 	* noone refers to the entry
> 	* the state is no longer valid (I.E: NUD_STALE).

NUD_STALE is still valid.

> 	* the timeout value has  been reached or state is FAILED
> 
> the "periodic work" routine will notify
> the stack that the entry should be deleted.
> 
> A user application monitoring and controlling the neighbour table
> using NETLINK may fail, if the "period work" routine is run
> directly after the state has been changed to NUD_STALE,
> but before the user application has had a chance to change
> the state to something valid.
> 
> The "period work" routine will detect the NUD_STALE state
> and if the timeout value has been reached, it will notify the stack
> that the entry should be deleted.
> 
> The patch adds a check in the periodic work routine
> which will skip test for garbage collection
> unless a number of ticks has passed since the last time
> the neighbour entry state was changed.
> 
> The feature is controlled through Kconfig
> 
> The featuree is enabled by setting ARP_GC_APPLY_GUARDBAND
> The guardband time (in ticks) is set in ARP_GC_GUARDBAND
> Default time is 100 ms if HZ_### is set.

We have "lower limit" not to start releasing neighbour entries.
Try increasing gc_thresh1.

--yoshfuji

> 
> Signed-off-by: Ulf Samuelsson <[email-masked]>
> ---
>  net/Kconfig          |   32 ++++++++++++++++++++++++++++++++
>  net/core/neighbour.c |   15 ++++++++++++---
>  2 files changed, 44 insertions(+), 3 deletions(-)
> 
> diff --git a/net/Kconfig b/net/Kconfig
> index 44dd578..099a5dd 100644
> --- a/net/Kconfig
> +++ b/net/Kconfig
> @@ -77,6 +77,38 @@ config INET
>  	  Short answer: say Y.
>  
>  if INET
> +
> +#
> +# Core Network configuration
> +#
> +
> +config ARP_GC_APPLY_GUARDBAND
> +	bool "IP: ARP: Avoid garbage collection directly after state change"
> +	default n
> +	---help---
> +	  With this item selected, an entry in the neighbour table
> +	  will not be garbage collected directly after the ARP state
> +	  has changed to STALE of FAILED
> +	  This allows an application program change the state to something valid
> +	  before garbage colllection occurs.
> +
> +	  If unsure, say N.
> +
> +config ARP_GC_GUARDBAND
> +	int "Guardband time on garbage collection"
> +	depends on ARP_GC_APPLY_GUARDBAND
> +	default 10 if HZ_100
> +	default 25 if HZ_250
> +	default 30 if HZ_300
> +	default 100 if HZ_1000
> +	default 100
> +
> +	---help---
> +	  The number of ticks to delay garbage collection
> +	  after the neighbour entry has been updated
> +	  A delay of 100 ms is reasonable.
> +	  With CONFIG_HZ = 250, this value should be 25
> +
>  source "net/ipv4/Kconfig"
>  source "net/ipv6/Kconfig"
>  source "net/netlabel/Kconfig"
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index 70fe9e1..194195d 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -786,13 +786,23 @@ static void neigh_periodic_work(struct work_struct *work)
>  
>  			state = n->nud_state;
>  			if (state & (NUD_PERMANENT | NUD_IN_TIMER)) {
> -				write_unlock(&n->lock);
>  				goto next_elt;
>  			}
>  
>  			if (time_before(n->used, n->confirmed))
>  				n->used = n->confirmed;
>  
> +#if defined(CONFIG_ARP_GC_APPLY_GUARDBAND)
> +			/* Do not garbage collect directly after we
> +			 * updated n->state to allow applications to
> +			 * react to the event
> +			 */
> +			if (time_before(jiffies,
> +					n->updated + CONFIG_ARP_GC_GUARDBAND)) {
> +				goto next_elt;
> +			}
> +#endif
> +
>  			if (atomic_read(&n->refcnt) == 1 &&
>  			    (state == NUD_FAILED ||
>  			     time_after(jiffies, n->used + NEIGH_VAR(n->parms, GC_STALETIME)))) {
> @@ -802,9 +812,8 @@ static void neigh_periodic_work(struct work_struct *work)
>  				neigh_cleanup_and_release(n);
>  				continue;
>  			}
> -			write_unlock(&n->lock);
> -
>  next_elt:
> +			write_unlock(&n->lock);
>  			np = &n->next;
>  		}
>  		/*
>