> On Jul 10, 2023, at 2:24 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> On Mon, 2023-07-10 at 12:42 -0400, 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.
>>
>> This patch splits out the use of the sp_lock to protect the set
>> of threads. Svc thread information is now protected by the xarray's
>> lock (when making thread count changes) and the RCU read lock (when
>> only looking up a thread).
>>
>> Since thread count changes are done only via nfsd filesystem API,
>> which runs only in process context, we can safely dispense with the
>> use of a bottom-half-disabled lock.
>>
>> 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, 94 insertions(+), 36 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 9dd3b16cc4c2..86377506a514 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;
>> @@ -196,7 +196,6 @@ 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 */
>>
>> @@ -241,10 +240,10 @@ 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 ad29df00b454..109d7f047385 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);
>> @@ -596,6 +596,8 @@ svc_destroy(struct kref *ref)
>> percpu_counter_destroy(&pool->sp_threads_timedout);
>> percpu_counter_destroy(&pool->sp_threads_starved);
>> percpu_counter_destroy(&pool->sp_threads_no_work);
>> +
>> + xa_destroy(&pool->sp_thread_xa);
>> }
>> kfree(serv->sv_pools);
>> kfree(serv);
>> @@ -676,7 +678,11 @@ EXPORT_SYMBOL_GPL(svc_rqst_alloc);
>> static struct svc_rqst *
>> svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
>> {
>> + struct xa_limit limit = {
>> + .max = U32_MAX,
>> + };
>> struct svc_rqst *rqstp;
>> + int ret;
>>
>> rqstp = svc_rqst_alloc(serv, pool, node);
>> if (!rqstp)
>> @@ -687,11 +693,21 @@ 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);
>> }
>>
>> /**
>> @@ -708,19 +724,17 @@ 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();
>
>
> While it does do its own locking, the resulting object that xa_for_each
> returns needs some protection too. Between xa_for_each returning a rqstp
> and calling test_and_set_bit, could the rqstp be freed? I suspect so,
> and I think you probably need to keep the rcu_read_lock() call above.
Should I keep the rcu_read_lock() even with the bitmap/xa_load
version of svc_pool_wake_idle_thread() in 9/9 ?
>> - 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();
>>
>
> I wonder if this can race with svc_pool_victim below? Can we end up
> waking a thread that's already on its way out of the pool? Maybe this is
> addressed in your next patch though...
>
>> trace_svc_pool_starved(serv, pool);
>> percpu_counter_inc(&pool->sp_threads_starved);
>> @@ -736,32 +750,33 @@ 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 int i;
>>
>> if (pool != NULL) {
>> - spin_lock_bh(&pool->sp_lock);
>> + xa_lock(&pool->sp_thread_xa);
>> + if (!pool->sp_nrthreads)
>> + goto out;
>> } 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 (pool->sp_nrthreads)
>> 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_load(&pool->sp_thread_xa, pool->sp_nrthreads - 1);
>> + if (rqstp) {
>> + __xa_erase(&pool->sp_thread_xa, rqstp->rq_thread_id);
>> task = rqstp->rq_task;
>> }
>> - spin_unlock_bh(&pool->sp_lock);
>> +out:
>> + xa_unlock(&pool->sp_thread_xa);
>> return task;
>> }
>>
>> @@ -843,9 +858,9 @@ 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)
>> @@ -932,11 +947,11 @@ 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 6c2a702aa469..db40f771b60a 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.
>>
>>
>
> Looks like a nice clean conversion otherwise!
> --
> Jeff Layton <jlayton@xxxxxxxxxx>
--
Chuck Lever
