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sunrpc: skip svc_xprt_enqueue when transport is busy

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svc_xprt_resource_released() calls svc_xprt_enqueue()
whenever XPT_DATA or XPT_DEFERRED is set. During RPC
processing, svc_reserve_auth() reduces the reservation
counter and triggers this path while the current thread
still holds XPT_BUSY. The enqueue enters svc_xprt_ready(),
executes an smp_rmb(), READ_ONCE(), and tracepoint, then
returns false on seeing XPT_BUSY.

Trace data from a 256KB NFSv3 WRITE workload over TCP
shows this pattern generates roughly 195,000 wasted
enqueue calls -- approximately one per RPC -- each
paying the full svc_xprt_ready() cost for no benefit.

Add a BUSY check alongside the existing DATA|DEFERRED
check in svc_xprt_resource_released(). When the
transport is BUSY, the holder will call
svc_xprt_received() upon completion, which already
checks for pending work flags and re-enqueues.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
This commit is contained in:
Chuck Lever
2026-03-24 09:04:49 -04:00
parent 0a5d7be861
commit e7f558158e
1 changed files with 10 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files

13
net/sunrpc/svc_xprt.c
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@@ -440,16 +440,23 @@ static bool svc_xprt_reserve_slot(struct svc_rqst *rqstp, struct svc_xprt *xprt)
/*
* After a caller releases write-space or a request slot,
* re-enqueue the transport only when there is pending
* work that a thread could act on. The smp_mb() pairs
* work that a thread could act on. The smp_mb() pairs
* with the smp_rmb() in svc_xprt_ready() and orders the
* preceding counter update before the flags read so a
* concurrent set_bit(XPT_DATA) is visible here.
*
* When the transport is BUSY, the thread holding it will
* call svc_xprt_received() upon completion, which checks
* for pending work and re-enqueues as needed.
*/
static void svc_xprt_resource_released(struct svc_xprt *xprt)
{
unsigned long xpt_flags;
smp_mb();
if (READ_ONCE(xprt->xpt_flags) &
(BIT(XPT_DATA) | BIT(XPT_DEFERRED)))
xpt_flags = READ_ONCE(xprt->xpt_flags);
if (xpt_flags & (BIT(XPT_DATA) | BIT(XPT_DEFERRED)) &&
!(xpt_flags & BIT(XPT_BUSY)))
svc_xprt_enqueue(xprt);
}