From: Alexander Bluhm Subject: Re: limit softnet threads to number of cpu To: Mark Kettenis Cc: tech@openbsd.org Date: Sat, 9 Aug 2025 17:07:29 +0200 On Sat, Aug 09, 2025 at 02:50:32PM +0200, Mark Kettenis wrote: > > Date: Tue, 5 Aug 2025 13:34:45 +0200 > > From: Alexander Bluhm > > 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: The threads are not really killed. The task is created, threads are defered. Before the threads are created, needless tasks are destroyed. At least i understand the code this way. > * 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. The problem are the tasks. The interface driver attach code calls ifq_init() for each queue. And here each softnet task is linked to the queue. Task must exist when driveres are in autoconf. > * Another strategy is to create one softnet thread up front and then > create more (if necessary) later instead of destroying them again. And how to get that into the attch code of every multiqueue driver? bluhm > > 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)); > > > >