> Date: Tue, 5 Aug 2025 13:34:45 +0200
> From: Alexander Bluhm <bluhm@openbsd.org>

I didn't think too deeply about this yet, but creating the threads and
then killing them seems a little bit silly.  So a few ideas:

* By the time we actually start running other kernel threads, the
  number of cCPUs that attached is know.  So I think the softnet
  threads can/should be created later that you do now.

* Another strategy is to create one softnet thread up front and then
  create more (if necessary) later instead of destroying them again.

Cheers,

Mark

> 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));
> 
>