From: Alexander Bluhm <bluhm@openbsd.org>
Subject: Re: limit softnet threads to number of cpu
To: tech@openbsd.org
Date: Tue, 5 Aug 2025 13:34:45 +0200

On Sat, Aug 02, 2025 at 10:18:11PM +0200, Alexander Bluhm wrote:
> Hi,
> 
> Currently always 8 softnet threads are startet, but only up to
> number of CPU are used.  I would like to remove useless threads.
> 
> Problem is that softnet tasks must be initialized before autoconf
> is running.  Drivers need them to attach queues.  But number of CPU
> is known only after autoconf has discovert them.
> 
> So I split the code into softnet_init() and softnet_percpu().  The
> latter removes task queues that are not needed.  Threads have not
> been created yet.
> 
> I have a follow up diff that pins softnet threads to specific CPU.
> But that needs more testing.  I would like to commit this thread
> handling diff first.
> 
> ok?

Anyone?  Or a better idea how to avoid useless threads?

bluhm

> Index: kern/init_main.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/kern/init_main.c,v
> diff -u -p -r1.329 init_main.c
> --- kern/init_main.c	9 Jun 2025 10:57:46 -0000	1.329
> +++ kern/init_main.c	2 Aug 2025 17:13:11 -0000
> @@ -328,6 +328,7 @@ main(void *framep)
>  
>  	/* Initialize the interface/address trees */
>  	ifinit();
> +	softnet_init();
>  
>  	/* Lock the kernel on behalf of proc0. */
>  	KERNEL_LOCK();
> @@ -345,6 +346,9 @@ main(void *framep)
>  
>  	/* Per CPU memory allocation */
>  	percpu_init();
> +
> +	/* Reduce softnet threads to number of CPU */
> +	softnet_percpu();
>  
>  	/* Initialize the file systems. */
>  #if defined(NFSSERVER) || defined(NFSCLIENT)
> Index: net/if.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/net/if.c,v
> diff -u -p -r1.740 if.c
> --- net/if.c	21 Jul 2025 20:36:41 -0000	1.740
> +++ net/if.c	2 Aug 2025 17:14:51 -0000
> @@ -237,7 +237,11 @@ struct softnet {
>  	struct taskq	*sn_taskq;
>  	struct netstack	 sn_netstack;
>  } __aligned(64);
> +#ifdef MULTIPROCESSOR
>  #define NET_TASKQ	8
> +#else
> +#define NET_TASKQ	1
> +#endif
>  struct softnet	softnets[NET_TASKQ];
>  
>  struct task	if_input_task_locked = TASK_INITIALIZER(if_netisr, NULL);
> @@ -255,16 +259,22 @@ struct rwlock netlock = RWLOCK_INITIALIZ
>  void
>  ifinit(void)
>  {
> -	unsigned int	i;
> -
>  	/*
>  	 * most machines boot with 4 or 5 interfaces, so size the initial map
>  	 * to accommodate this
>  	 */
>  	if_idxmap_init(8); /* 8 is a nice power of 2 for malloc */
> +}
>  
> +void
> +softnet_init(void)
> +{
> +	unsigned int i;
> +
> +	/* Number of CPU is unknown, but driver attach needs softnet tasks. */
>  	for (i = 0; i < NET_TASKQ; i++) {
>  		struct softnet *sn = &softnets[i];
> +
>  		snprintf(sn->sn_name, sizeof(sn->sn_name), "softnet%u", i);
>  		sn->sn_taskq = taskq_create(sn->sn_name, 1, IPL_NET,
>  		    TASKQ_MPSAFE);
> @@ -273,6 +283,31 @@ ifinit(void)
>  	}
>  }
>  
> +#ifdef MULTIPROCESSOR
> +
> +void
> +softnet_percpu(void)
> +{
> +	unsigned int i;
> +
> +	/* After attaching all CPUs and interfaces, remove useless threads. */
> +	for (i = softnet_count(); i < NET_TASKQ; i++) {
> +		struct softnet *sn = &softnets[i];
> +
> +		taskq_destroy(sn->sn_taskq);
> +		sn->sn_taskq = NULL;
> +	}
> +}
> +
> +#else /* MULTIPROCESSOR */
> +
> +void
> +softnet_percpu(void)
> +{
> +}
> +
> +#endif /* MULTIPROCESSOR */
> +
>  static struct if_idxmap if_idxmap;
>  
>  /*
> @@ -3642,10 +3677,10 @@ unhandled_af(int af)
>  	panic("unhandled af %d", af);
>  }
>  
> -int
> -net_sn_count(void)
> +unsigned int
> +softnet_count(void)
>  {
> -	static int nsoftnets;
> +	static unsigned int nsoftnets;
>  
>  	if (nsoftnets == 0)
>  		nsoftnets = min(NET_TASKQ, ncpus);
> @@ -3656,7 +3691,7 @@ net_sn_count(void)
>  struct softnet *
>  net_sn(unsigned int ifindex)
>  {
> -	return (&softnets[ifindex % net_sn_count()]);
> +	return (&softnets[ifindex % softnet_count()]);
>  }
>  
>  struct taskq *
> @@ -3672,7 +3707,7 @@ net_tq_barriers(const char *wmesg)
>  	struct refcnt r = REFCNT_INITIALIZER();
>  	int i;
>  
> -	for (i = 0; i < nitems(barriers); i++) {
> +	for (i = 0; i < softnet_count(); i++) {
>  		task_set(&barriers[i], (void (*)(void *))refcnt_rele_wake, &r);
>  		refcnt_take(&r);
>  		task_add(softnets[i].sn_taskq, &barriers[i]);
> Index: net/if.h
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/net/if.h,v
> diff -u -p -r1.220 if.h
> --- net/if.h	19 Jul 2025 16:40:40 -0000	1.220
> +++ net/if.h	2 Aug 2025 17:13:11 -0000
> @@ -561,10 +561,13 @@ void	if_congestion(void);
>  int	if_congested(void);
>  __dead void	unhandled_af(int);
>  int	if_setlladdr(struct ifnet *, const uint8_t *);
> +void	softnet_init(void);
> +void	softnet_percpu(void);
> +unsigned int
> +	softnet_count(void);
>  struct taskq *
>  	net_tq(unsigned int);
>  void	net_tq_barriers(const char *);
> -int	net_sn_count(void);
>  
>  #endif /* _KERNEL */
>  
> Index: net/if_loop.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/net/if_loop.c,v
> diff -u -p -r1.102 if_loop.c
> --- net/if_loop.c	7 Jul 2025 02:28:50 -0000	1.102
> +++ net/if_loop.c	2 Aug 2025 17:13:11 -0000
> @@ -178,8 +178,8 @@ loop_clone_create(struct if_clone *ifc, 
>  		rtable_l2set(0, 0, ifp->if_index);
>  	} else
>  		if_attach(ifp);
> -	if_attach_queues(ifp, net_sn_count());
> -	if_attach_iqueues(ifp, net_sn_count());
> +	if_attach_queues(ifp, softnet_count());
> +	if_attach_iqueues(ifp, softnet_count());
>  	if_alloc_sadl(ifp);
>  #if NBPFILTER > 0
>  	bpfattach(&ifp->if_bpf, ifp, DLT_LOOP, sizeof(u_int32_t));