Re: [PATCH net-next] netfilter: conntrack: expose gc_scan_interval_max via sysctl

[Fernando Fernandez Mancera]

On 3/11/26 8:40 PM, Prasanna S Panchamukhi wrote:
> The conntrack garbage collection worker uses an adaptive algorithm that
> adjusts the scan interval based on the average timeout of tracked
> entries.  The upper bound of this interval is hardcoded as
> GC_SCAN_INTERVAL_MAX (60 seconds).
>
> Expose the upper bound as a new sysctl,
> net.netfilter.nf_conntrack_gc_scan_interval_max, so it can be tuned at
> runtime without rebuilding the kernel.  The default remains 60 seconds
> to preserve existing behavior.  The sysctl is global and read-only in
> non-init network namespaces, consistent with nf_conntrack_max and
> nf_conntrack_buckets.
>
> In environments where long-lived offloaded flows dominate the table,
> the adaptive average drifts toward the maximum, delaying cleanup
> of short-lived expired entries such as those in TCP CLOSE state
> (10s timeout). Adding sysctl to set the maximum GC scan helps to
> tune according to the evironment.
>
> Signed-off-by: Prasanna S Panchamukhi <panchamukhi@xxxxxxxxxx>
[...]
> ---
>   Documentation/networking/nf_conntrack-sysctl.rst | 11 +++++++++++
>   include/net/netfilter/nf_conntrack.h             |  1 +
>   net/netfilter/nf_conntrack_core.c                |  9 ++++++---
>   net/netfilter/nf_conntrack_standalone.c          | 10 ++++++++++
>   4 files changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/networking/nf_conntrack-sysctl.rst b/Documentation/networking/nf_conntrack-sysctl.rst
> index 35f889259fcd..c848eef9bc4f 100644
> --- a/Documentation/networking/nf_conntrack-sysctl.rst
> +++ b/Documentation/networking/nf_conntrack-sysctl.rst
> @@ -64,6 +64,17 @@ nf_conntrack_frag6_timeout - INTEGER (seconds)
>   
>   	Time to keep an IPv6 fragment in memory.
>   
> +nf_conntrack_gc_scan_interval_max - INTEGER (seconds)
> +	default 60
> +
> +	Maximum interval between garbage collection scans of the connection
> +	tracking table. The GC worker uses an adaptive algorithm that adjusts
> +	the scan interval based on average entry timeouts; this parameter caps
> +	the upper bound. Lower values cause expired entries (e.g. connections
> +	in CLOSE state) to be cleaned up faster, at the cost of slightly more
> +	CPU usage. Minimum value is 1.
> +	This sysctl is only writeable in the initial net namespace.
> +

I think it would be a good idea to add under which situations it is good 
to tweak this setting.

>   nf_conntrack_generic_timeout - INTEGER (seconds)
>   	default 600
>   
> diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
> index bc42dd0e10e6..0449577f322e 100644
> --- a/include/net/netfilter/nf_conntrack.h
> +++ b/include/net/netfilter/nf_conntrack.h
> @@ -331,6 +331,7 @@ extern struct hlist_nulls_head *nf_conntrack_hash;
>   extern unsigned int nf_conntrack_htable_size;
>   extern seqcount_spinlock_t nf_conntrack_generation;
>   extern unsigned int nf_conntrack_max;
> +extern unsigned int nf_conntrack_gc_scan_interval_max;

Could it be just int? so there is no need to cast it to s32 later?

>   /* must be called with rcu read lock held */
>   static inline void
> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index 27ce5fda8993..54949246f329 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -91,7 +91,7 @@ static DEFINE_MUTEX(nf_conntrack_mutex);
>    * allowing non-idle machines to wakeup more often when needed.
>    */
>   #define GC_SCAN_INITIAL_COUNT	100
> -#define GC_SCAN_INTERVAL_INIT	GC_SCAN_INTERVAL_MAX
> +#define GC_SCAN_INTERVAL_INIT	nf_conntrack_gc_scan_interval_max
>   
>   #define GC_SCAN_MAX_DURATION	msecs_to_jiffies(10)
>   #define GC_SCAN_EXPIRED_MAX	(64000u / HZ)
> @@ -204,6 +204,9 @@ EXPORT_SYMBOL_GPL(nf_conntrack_htable_size);
>   
>   unsigned int nf_conntrack_max __read_mostly;
>   EXPORT_SYMBOL_GPL(nf_conntrack_max);
> +
> +unsigned int nf_conntrack_gc_scan_interval_max __read_mostly = GC_SCAN_INTERVAL_MAX;
> +
>   seqcount_spinlock_t nf_conntrack_generation __read_mostly;
>   static siphash_aligned_key_t nf_conntrack_hash_rnd;
>   
> @@ -1568,7 +1571,7 @@ static void gc_worker(struct work_struct *work)
>   				delta_time = nfct_time_stamp - gc_work->start_time;
>   
>   				/* re-sched immediately if total cycle time is exceeded */
> -				next_run = delta_time < (s32)GC_SCAN_INTERVAL_MAX;
> +				next_run = delta_time < (s32)nf_conntrack_gc_scan_interval_max;
>   				goto early_exit;
>   			}
>   

READ_ONCE() is required IMHO as it can be modified from sysctl concurrently.

> @@ -1630,7 +1633,7 @@ static void gc_worker(struct work_struct *work)
>   
>   	gc_work->next_bucket = 0;
>   
> -	next_run = clamp(next_run, GC_SCAN_INTERVAL_MIN, GC_SCAN_INTERVAL_MAX);
> +	next_run = clamp(next_run, GC_SCAN_INTERVAL_MIN, nf_conntrack_gc_scan_interval_max);
>   

Likewise here, READ_ONCE() recommended..

Thanks,
Fernando.