[PATCH 5.15 916/917] SUNRPC: Partial revert of commit 6f9f17287e78

Greg Kroah-Hartman
2021-11-16T00:39:36Z

From: Trond Myklebust <trond.myklebust@hammerspace.com>

commit ea7a1019d8baf8503ecd6e3ec8436dec283569e6 upstream.

The premise of commit 6f9f17287e78 ("SUNRPC: Mitigate cond_resched() in
xprt_transmit()") was that cond_resched() is expensive and unnecessary
when there has been just a single send.
The point of cond_resched() is to ensure that tasks that should pre-empt
this one get a chance to do so when it is safe to do so. The code prior
to commit 6f9f17287e78 failed to take into account that it was keeping a
rpc_task pinned for longer than it needed to, and so rather than doing a
full revert, let's just move the cond_resched.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/sunrpc/xprt.c | 28 +++++++++++++++-------------
1 file changed, 15 insertions(+), 13 deletions(-)

--- a/net/sunrpc/xprt.c
+++ b/net/sunrpc/xprt.c
@@ -1603,15 +1603,14 @@ xprt_transmit(struct rpc_task *task)
{
struct rpc_rqst *next, *req = task->tk_rqstp;
struct rpc_xprt *xprt = req->rq_xprt;
- int counter, status;
+ int status;

spin_lock(&xprt->queue_lock);
- counter = 0;
- while (!list_empty(&xprt->xmit_queue)) {
- if (++counter == 20)
+ for (;;) {
+ next = list_first_entry_or_null(&xprt->xmit_queue,
+ struct rpc_rqst, rq_xmit);
+ if (!next)
break;
- next = list_first_entry(&xprt->xmit_queue,
- struct rpc_rqst, rq_xmit);
xprt_pin_rqst(next);
spin_unlock(&xprt->queue_lock);
status = xprt_request_transmit(next, task);
@@ -1619,13 +1618,16 @@ xprt_transmit(struct rpc_task *task)
status = 0;
spin_lock(&xprt->queue_lock);
xprt_unpin_rqst(next);
- if (status == 0) {
- if (!xprt_request_data_received(task) ||
- test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
- continue;
- } else if (test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
- task->tk_status = status;
- break;
+ if (status < 0) {
+ if (test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
+ task->tk_status = status;
+ break;
+ }
+ /* Was @task transmitted, and has it received a reply? */
+ if (xprt_request_data_received(task) &&
+ !test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
+ break;
+ cond_resched_lock(&xprt->queue_lock);
}
spin_unlock(&xprt->queue_lock);
}