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From: NeilBrown <neilb@suse.de>
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Date: Mon, 17 May 2021 09:34:38 +1000
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Subject: [PATCH] SUNRPC in case of backlog, hand free slots directly to
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waiting task
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Git-commit: e877a88d1f069edced4160792f42c2a8e2dba942
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Patch-mainline: v5.13-rc4
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References: bsc#1185428
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If sunrpc.tcp_max_slot_table_entries is small and there are tasks
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on the backlog queue, then when a request completes it is freed and the
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first task on the queue is woken. The expectation is that it will wake
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and claim that request. However if it was a sync task and the waiting
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process was killed at just that moment, it will wake and NOT claim the
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request.
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As long as TASK_CONGESTED remains set, requests can only be claimed by
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tasks woken from the backlog, and they are woken only as requests are
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freed, so when a task doesn't claim a request, no other task can ever
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get that request until TASK_CONGESTED is cleared. Each time this
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happens the number of available requests is decreased by one.
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With a sufficiently high workload and sufficiently low setting of
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max_slot (16 in the case where this was seen), TASK_CONGESTED can remain
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set for an extended period, and the above scenario (of a process being
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killed just as its task was woken) can repeat until no requests can be
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allocated. Then traffic stops.
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This patch addresses the problem by introducing a positive handover of a
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request from a completing task to a backlog task - the request is never
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freed when there is a backlog.
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When a task is woken it might already have a request attached in which
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case it is *not* freed (as with current code) but is initialised (if
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needed) and used. If it isn't used it will eventually be freed by
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rpc_exit_task(). xprt_release() is enhanced to be able to correctly
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release an uninitialised request.
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Signed-off-by: NeilBrown <neilb@suse.de>
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Acked-by: NeilBrown <neilb@suse.com>
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---
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net/sunrpc/clnt.c | 7 -----
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net/sunrpc/xprt.c | 68 +++++++++++++++++++++++++++++++++++++-----------------
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2 files changed, 47 insertions(+), 28 deletions(-)
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--- a/net/sunrpc/clnt.c
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+++ b/net/sunrpc/clnt.c
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@@ -1677,13 +1677,6 @@ call_reserveresult(struct rpc_task *task
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return;
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}
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- /*
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- * Even though there was an error, we may have acquired
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- * a request slot somehow. Make sure not to leak it.
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- */
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- if (task->tk_rqstp)
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- xprt_release(task);
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-
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switch (status) {
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case -ENOMEM:
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rpc_delay(task, HZ >> 2);
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--- a/net/sunrpc/xprt.c
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+++ b/net/sunrpc/xprt.c
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@@ -70,6 +70,7 @@
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static void xprt_init(struct rpc_xprt *xprt, struct net *net);
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static __be32 xprt_alloc_xid(struct rpc_xprt *xprt);
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static void xprt_destroy(struct rpc_xprt *xprt);
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+static void xprt_request_init(struct rpc_task *task);
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static DEFINE_SPINLOCK(xprt_list_lock);
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static LIST_HEAD(xprt_list);
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@@ -1541,10 +1542,26 @@ static void xprt_add_backlog(struct rpc_
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rpc_sleep_on(&xprt->backlog, task, NULL);
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}
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-static void xprt_wake_up_backlog(struct rpc_xprt *xprt)
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+static bool __xprt_set_rq(struct rpc_task *task, void *data)
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{
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- if (rpc_wake_up_next(&xprt->backlog) == NULL)
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+ struct rpc_rqst *req = data;
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+
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+ if (task->tk_rqstp == NULL) {
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+ memset(req, 0, sizeof(*req)); /* mark unused */
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+ task->tk_status = -EAGAIN;
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+ task->tk_rqstp = req;
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+ return true;
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+ }
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+ return false;
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+}
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+
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+static bool xprt_wake_up_backlog(struct rpc_xprt *xprt, struct rpc_rqst *req)
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+{
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+ if (rpc_wake_up_first(&xprt->backlog, __xprt_set_rq, req) == NULL) {
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clear_bit(XPRT_CONGESTED, &xprt->state);
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+ return false;
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+ }
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+ return true;
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}
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static bool xprt_throttle_congested(struct rpc_xprt *xprt, struct rpc_task *task)
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@@ -1632,11 +1649,11 @@ EXPORT_SYMBOL_GPL(xprt_alloc_slot);
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void xprt_free_slot(struct rpc_xprt *xprt, struct rpc_rqst *req)
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{
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spin_lock(&xprt->reserve_lock);
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- if (!xprt_dynamic_free_slot(xprt, req)) {
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+ if (!xprt_wake_up_backlog(xprt, req) &&
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+ !xprt_dynamic_free_slot(xprt, req)) {
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memset(req, 0, sizeof(*req)); /* mark unused */
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list_add(&req->rq_list, &xprt->free);
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}
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- xprt_wake_up_backlog(xprt);
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spin_unlock(&xprt->reserve_lock);
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}
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EXPORT_SYMBOL_GPL(xprt_free_slot);
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@@ -1724,6 +1741,10 @@ xprt_request_init(struct rpc_task *task)
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struct rpc_xprt *xprt = task->tk_xprt;
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struct rpc_rqst *req = task->tk_rqstp;
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+ if (req->rq_task)
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+ /* Already initialized */
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+ return;
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+
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req->rq_task = task;
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req->rq_xprt = xprt;
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req->rq_buffer = NULL;
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@@ -1784,8 +1805,10 @@ void xprt_retry_reserve(struct rpc_task
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struct rpc_xprt *xprt = task->tk_xprt;
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task->tk_status = 0;
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- if (task->tk_rqstp != NULL)
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+ if (task->tk_rqstp != NULL) {
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+ xprt_request_init(task);
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return;
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+ }
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task->tk_status = -EAGAIN;
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xprt_do_reserve(xprt, task);
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@@ -1810,23 +1833,26 @@ void xprt_release(struct rpc_task *task)
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}
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xprt = req->rq_xprt;
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- xprt_request_dequeue_xprt(task);
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- spin_lock(&xprt->transport_lock);
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- xprt->ops->release_xprt(xprt, task);
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- if (xprt->ops->release_request)
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- xprt->ops->release_request(task);
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- xprt_schedule_autodisconnect(xprt);
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- spin_unlock(&xprt->transport_lock);
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- if (req->rq_buffer)
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- xprt->ops->buf_free(task);
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- xdr_free_bvec(&req->rq_rcv_buf);
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- xdr_free_bvec(&req->rq_snd_buf);
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- if (req->rq_cred != NULL)
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- put_rpccred(req->rq_cred);
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- task->tk_rqstp = NULL;
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- if (req->rq_release_snd_buf)
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- req->rq_release_snd_buf(req);
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+ if (xprt) {
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+ xprt_request_dequeue_xprt(task);
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+ spin_lock(&xprt->transport_lock);
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+ xprt->ops->release_xprt(xprt, task);
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+ if (xprt->ops->release_request)
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+ xprt->ops->release_request(task);
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+ xprt_schedule_autodisconnect(xprt);
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+ spin_unlock(&xprt->transport_lock);
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+ if (req->rq_buffer)
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+ xprt->ops->buf_free(task);
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+ xdr_free_bvec(&req->rq_rcv_buf);
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+ xdr_free_bvec(&req->rq_snd_buf);
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+ if (req->rq_cred != NULL)
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+ put_rpccred(req->rq_cred);
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+ if (req->rq_release_snd_buf)
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+ req->rq_release_snd_buf(req);
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+ } else
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+ xprt = task->tk_xprt;
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+ task->tk_rqstp = NULL;
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dprintk("RPC: %5u release request %p\n", task->tk_pid, req);
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if (likely(!bc_prealloc(req)))
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xprt->ops->free_slot(xprt, req);
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