On Tue, Jul 04, 2023 at 11:11:57AM +1000, NeilBrown wrote:
> On Tue, 04 Jul 2023, Chuck Lever wrote:
> > From: Chuck Lever <chuck.lever@xxxxxxxxxx>
> >
> > We want a thread lookup operation that can be done with RCU only,
> > but also we want to avoid the linked-list walk, which does not scale
> > well in the number of pool threads.
> >
> > BH-disabled locking is no longer necessary because we're no longer
> > sharing the pool's sp_lock to protect either the xarray or the
> > pool's thread count. sp_lock also protects transport activity. There
> > are no callers of svc_set_num_threads() that run outside of process
> > context.
> >
> > Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> > ---
> > fs/nfsd/nfssvc.c | 3 +-
> > include/linux/sunrpc/svc.h | 11 +++----
> > include/trace/events/sunrpc.h | 47 ++++++++++++++++++++++++++++-
> > net/sunrpc/svc.c | 67 ++++++++++++++++++++++++-----------------
> > net/sunrpc/svc_xprt.c | 2 +
> > 5 files changed, 93 insertions(+), 37 deletions(-)
> >
> > diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> > index 2154fa63c5f2..d42b2a40c93c 100644
> > --- a/fs/nfsd/nfssvc.c
> > +++ b/fs/nfsd/nfssvc.c
> > @@ -62,8 +62,7 @@ static __be32 nfsd_init_request(struct svc_rqst *,
> > * If (out side the lock) nn->nfsd_serv is non-NULL, then it must point to a
> > * properly initialised 'struct svc_serv' with ->sv_nrthreads > 0 (unless
> > * nn->keep_active is set). That number of nfsd threads must
> > - * exist and each must be listed in ->sp_all_threads in some entry of
> > - * ->sv_pools[].
> > + * exist and each must be listed in some entry of ->sv_pools[].
> > *
> > * Each active thread holds a counted reference on nn->nfsd_serv, as does
> > * the nn->keep_active flag and various transient calls to svc_get().
> > diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> > index 74ea13270679..6f8bfcd44250 100644
> > --- a/include/linux/sunrpc/svc.h
> > +++ b/include/linux/sunrpc/svc.h
> > @@ -32,10 +32,10 @@
> > */
> > struct svc_pool {
> > unsigned int sp_id; /* pool id; also node id on NUMA */
> > - spinlock_t sp_lock; /* protects all fields */
> > + spinlock_t sp_lock; /* protects sp_sockets */
> > struct list_head sp_sockets; /* pending sockets */
> > unsigned int sp_nrthreads; /* # of threads in pool */
> > - struct list_head sp_all_threads; /* all server threads */
> > + struct xarray sp_thread_xa;
> >
> > /* statistics on pool operation */
> > struct percpu_counter sp_messages_arrived;
> > @@ -195,7 +195,6 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
> > * processed.
> > */
> > struct svc_rqst {
> > - struct list_head rq_all; /* all threads list */
> > struct rcu_head rq_rcu_head; /* for RCU deferred kfree */
> > struct svc_xprt * rq_xprt; /* transport ptr */
> >
> > @@ -240,10 +239,10 @@ struct svc_rqst {
> > #define RQ_SPLICE_OK (4) /* turned off in gss privacy
> > * to prevent encrypting page
> > * cache pages */
> > -#define RQ_VICTIM (5) /* about to be shut down */
> > -#define RQ_BUSY (6) /* request is busy */
> > -#define RQ_DATA (7) /* request has data */
> > +#define RQ_BUSY (5) /* request is busy */
> > +#define RQ_DATA (6) /* request has data */
> > unsigned long rq_flags; /* flags field */
> > + u32 rq_thread_id; /* xarray index */
> > ktime_t rq_qtime; /* enqueue time */
> >
> > void * rq_argp; /* decoded arguments */
> > diff --git a/include/trace/events/sunrpc.h b/include/trace/events/sunrpc.h
> > index 60c8e03268d4..ea43c6059bdb 100644
> > --- a/include/trace/events/sunrpc.h
> > +++ b/include/trace/events/sunrpc.h
> > @@ -1676,7 +1676,6 @@ DEFINE_SVCXDRBUF_EVENT(sendto);
> > svc_rqst_flag(USEDEFERRAL) \
> > svc_rqst_flag(DROPME) \
> > svc_rqst_flag(SPLICE_OK) \
> > - svc_rqst_flag(VICTIM) \
> > svc_rqst_flag(BUSY) \
> > svc_rqst_flag_end(DATA)
> >
> > @@ -2118,6 +2117,52 @@ TRACE_EVENT(svc_pool_starved,
> > )
> > );
> >
> > +DECLARE_EVENT_CLASS(svc_thread_lifetime_class,
> > + TP_PROTO(
> > + const struct svc_serv *serv,
> > + const struct svc_pool *pool,
> > + const struct svc_rqst *rqstp
> > + ),
> > +
> > + TP_ARGS(serv, pool, rqstp),
> > +
> > + TP_STRUCT__entry(
> > + __string(name, serv->sv_name)
> > + __field(int, pool_id)
> > + __field(unsigned int, nrthreads)
> > + __field(unsigned long, pool_flags)
> > + __field(u32, thread_id)
> > + __field(const void *, rqstp)
> > + ),
> > +
> > + TP_fast_assign(
> > + __assign_str(name, serv->sv_name);
> > + __entry->pool_id = pool->sp_id;
> > + __entry->nrthreads = pool->sp_nrthreads;
> > + __entry->pool_flags = pool->sp_flags;
> > + __entry->thread_id = rqstp->rq_thread_id;
> > + __entry->rqstp = rqstp;
> > + ),
> > +
> > + TP_printk("service=%s pool=%d pool_flags=%s nrthreads=%u thread_id=%u",
> > + __get_str(name), __entry->pool_id,
> > + show_svc_pool_flags(__entry->pool_flags),
> > + __entry->nrthreads, __entry->thread_id
> > + )
> > +);
> > +
> > +#define DEFINE_SVC_THREAD_LIFETIME_EVENT(name) \
> > + DEFINE_EVENT(svc_thread_lifetime_class, svc_pool_##name, \
> > + TP_PROTO( \
> > + const struct svc_serv *serv, \
> > + const struct svc_pool *pool, \
> > + const struct svc_rqst *rqstp \
> > + ), \
> > + TP_ARGS(serv, pool, rqstp))
> > +
> > +DEFINE_SVC_THREAD_LIFETIME_EVENT(thread_init);
> > +DEFINE_SVC_THREAD_LIFETIME_EVENT(thread_exit);
> > +
> > DECLARE_EVENT_CLASS(svc_xprt_event,
> > TP_PROTO(
> > const struct svc_xprt *xprt
> > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> > index 9b6701a38e71..ef350f0d8925 100644
> > --- a/net/sunrpc/svc.c
> > +++ b/net/sunrpc/svc.c
> > @@ -507,8 +507,8 @@ __svc_create(struct svc_program *prog, unsigned int bufsize, int npools,
> >
> > pool->sp_id = i;
> > INIT_LIST_HEAD(&pool->sp_sockets);
> > - INIT_LIST_HEAD(&pool->sp_all_threads);
> > spin_lock_init(&pool->sp_lock);
> > + xa_init_flags(&pool->sp_thread_xa, XA_FLAGS_ALLOC);
> >
> > percpu_counter_init(&pool->sp_messages_arrived, 0, GFP_KERNEL);
> > percpu_counter_init(&pool->sp_sockets_queued, 0, GFP_KERNEL);
> > @@ -594,6 +594,8 @@ svc_destroy(struct kref *ref)
> > percpu_counter_destroy(&pool->sp_threads_woken);
> > percpu_counter_destroy(&pool->sp_threads_timedout);
> > percpu_counter_destroy(&pool->sp_threads_starved);
> > +
> > + xa_destroy(&pool->sp_thread_xa);
> > }
> > kfree(serv->sv_pools);
> > kfree(serv);
> > @@ -674,7 +676,11 @@ EXPORT_SYMBOL_GPL(svc_rqst_alloc);
> > static struct svc_rqst *
> > svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
> > {
> > + static const struct xa_limit limit = {
> > + .max = U32_MAX,
> > + };
> > struct svc_rqst *rqstp;
> > + int ret;
> >
> > rqstp = svc_rqst_alloc(serv, pool, node);
> > if (!rqstp)
> > @@ -685,15 +691,25 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
> > serv->sv_nrthreads += 1;
> > spin_unlock_bh(&serv->sv_lock);
> >
> > - spin_lock_bh(&pool->sp_lock);
> > + xa_lock(&pool->sp_thread_xa);
> > + ret = __xa_alloc(&pool->sp_thread_xa, &rqstp->rq_thread_id, rqstp,
> > + limit, GFP_KERNEL);
> > + if (ret) {
> > + xa_unlock(&pool->sp_thread_xa);
> > + goto out_free;
> > + }
> > pool->sp_nrthreads++;
> > - list_add_rcu(&rqstp->rq_all, &pool->sp_all_threads);
> > - spin_unlock_bh(&pool->sp_lock);
> > + xa_unlock(&pool->sp_thread_xa);
> > + trace_svc_pool_thread_init(serv, pool, rqstp);
> > return rqstp;
> > +
> > +out_free:
> > + svc_rqst_free(rqstp);
> > + return ERR_PTR(ret);
> > }
> >
> > /**
> > - * svc_pool_wake_idle_thread - wake an idle thread in @pool
> > + * svc_pool_wake_idle_thread - Find and wake an idle thread in @pool
> > * @serv: RPC service
> > * @pool: service thread pool
> > *
> > @@ -706,19 +722,17 @@ struct svc_rqst *svc_pool_wake_idle_thread(struct svc_serv *serv,
> > struct svc_pool *pool)
> > {
> > struct svc_rqst *rqstp;
> > + unsigned long index;
> >
> > - rcu_read_lock();
> > - list_for_each_entry_rcu(rqstp, &pool->sp_all_threads, rq_all) {
> > + xa_for_each(&pool->sp_thread_xa, index, rqstp) {
> > if (test_and_set_bit(RQ_BUSY, &rqstp->rq_flags))
> > continue;
> >
> > - rcu_read_unlock();
> > WRITE_ONCE(rqstp->rq_qtime, ktime_get());
> > wake_up_process(rqstp->rq_task);
> > percpu_counter_inc(&pool->sp_threads_woken);
> > return rqstp;
> > }
> > - rcu_read_unlock();
> >
> > trace_svc_pool_starved(serv, pool);
> > percpu_counter_inc(&pool->sp_threads_starved);
> > @@ -734,32 +748,31 @@ svc_pool_next(struct svc_serv *serv, struct svc_pool *pool, unsigned int *state)
> > static struct task_struct *
> > svc_pool_victim(struct svc_serv *serv, struct svc_pool *pool, unsigned int *state)
> > {
> > - unsigned int i;
> > struct task_struct *task = NULL;
> > + struct svc_rqst *rqstp;
> > + unsigned long zero = 0;
> > + unsigned int i;
> >
> > if (pool != NULL) {
> > - spin_lock_bh(&pool->sp_lock);
> > + xa_lock(&pool->sp_thread_xa);
> > } else {
> > for (i = 0; i < serv->sv_nrpools; i++) {
> > pool = &serv->sv_pools[--(*state) % serv->sv_nrpools];
> > - spin_lock_bh(&pool->sp_lock);
> > - if (!list_empty(&pool->sp_all_threads))
> > + xa_lock(&pool->sp_thread_xa);
> > + if (!xa_empty(&pool->sp_thread_xa))
> > goto found_pool;
> > - spin_unlock_bh(&pool->sp_lock);
> > + xa_unlock(&pool->sp_thread_xa);
> > }
> > return NULL;
> > }
> >
> > found_pool:
> > - if (!list_empty(&pool->sp_all_threads)) {
> > - struct svc_rqst *rqstp;
> > -
> > - rqstp = list_entry(pool->sp_all_threads.next, struct svc_rqst, rq_all);
> > - set_bit(RQ_VICTIM, &rqstp->rq_flags);
> > - list_del_rcu(&rqstp->rq_all);
> > + rqstp = xa_find(&pool->sp_thread_xa, &zero, U32_MAX, XA_PRESENT);
> > + if (rqstp) {
> > + __xa_erase(&pool->sp_thread_xa, rqstp->rq_thread_id);
>
> This bothers me. We always delete the earliest thread in the xarray.
> So if we create 128 threads, then reduce the number to 64, the remaining
> threads will be numbers 128 to 256.
> This means searching in the bitmap will be (slightly) slower than
> necessary.
>
> xa doesn't have a "find last" interface, but we "know" how many entries
> are in the array - or we would if we decremented the counter when we
> removed an entry.
> Currently we only decrement the counter when the thread exits - at which
> time it is removed the entry - which may already have been removed.
> If we change that code to check if it is still present in the xarray and
> to only erase/decrement if it is, then we can decrement the counter here
> and always reliably be able to find the "last" entry.
>
> ... though I think we wait for a thread to exist before finding the next
> victim, so maybe all we need to do is start the xa_find from
> ->sp_nrthreads-1 rather than from zero ??
>
> Is it worth it? I don't know. But it bothers me.
Well it would be straightforward to change the "pool_wake_idle_thread"
search into a find_next_bit over a range. Store the lowest thread_id
in the svc_pool and use that as the starting point for the loop.
> NeilBrown
>
>
> > task = rqstp->rq_task;
> > }
> > - spin_unlock_bh(&pool->sp_lock);
> > + xa_unlock(&pool->sp_thread_xa);
> > return task;
> > }
> >
> > @@ -841,9 +854,9 @@ svc_set_num_threads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
> > if (pool == NULL) {
> > nrservs -= serv->sv_nrthreads;
> > } else {
> > - spin_lock_bh(&pool->sp_lock);
> > + xa_lock(&pool->sp_thread_xa);
> > nrservs -= pool->sp_nrthreads;
> > - spin_unlock_bh(&pool->sp_lock);
> > + xa_unlock(&pool->sp_thread_xa);
> > }
> >
> > if (nrservs > 0)
> > @@ -930,11 +943,11 @@ svc_exit_thread(struct svc_rqst *rqstp)
> > struct svc_serv *serv = rqstp->rq_server;
> > struct svc_pool *pool = rqstp->rq_pool;
> >
> > - spin_lock_bh(&pool->sp_lock);
> > + xa_lock(&pool->sp_thread_xa);
> > pool->sp_nrthreads--;
> > - if (!test_and_set_bit(RQ_VICTIM, &rqstp->rq_flags))
> > - list_del_rcu(&rqstp->rq_all);
> > - spin_unlock_bh(&pool->sp_lock);
> > + __xa_erase(&pool->sp_thread_xa, rqstp->rq_thread_id);
> > + xa_unlock(&pool->sp_thread_xa);
> > + trace_svc_pool_thread_exit(serv, pool, rqstp);
> >
> > spin_lock_bh(&serv->sv_lock);
> > serv->sv_nrthreads -= 1;
> > diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> > index 8ced7591ce07..7709120b45c1 100644
> > --- a/net/sunrpc/svc_xprt.c
> > +++ b/net/sunrpc/svc_xprt.c
> > @@ -46,7 +46,7 @@ static LIST_HEAD(svc_xprt_class_list);
> >
> > /* SMP locking strategy:
> > *
> > - * svc_pool->sp_lock protects most of the fields of that pool.
> > + * svc_pool->sp_lock protects sp_sockets.
> > * svc_serv->sv_lock protects sv_tempsocks, sv_permsocks, sv_tmpcnt.
> > * when both need to be taken (rare), svc_serv->sv_lock is first.
> > * The "service mutex" protects svc_serv->sv_nrthread.
> >
> >
> >
>
