cxl: Use call_rcu to reduce latency when releasing the afu fd

The afu fd release path was identified as a significant bottleneck in
the overall performance of cxl. While an optimal AFU design would
minimise the need to close & reopen the AFU fd, it is not always
practical to avoid.

The bottleneck seems to be down to the call to synchronize_rcu(), which
will block until every other thread is guaranteed to be out of an RCU
critical section. Replace it with call_rcu() to free the context
structures later so we can return to the application sooner.

This reduces the time spent in the fd release path from 13356 usec to
13.3 usec - about a 100x speed up.

Reported-by: Fei K Chen <uchen@cn.ibm.com>
Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/misc/cxl/cxl.h b/drivers/misc/cxl/cxl.h
index cfee819bf5d4..b361b48d1b01 100644
--- a/drivers/misc/cxl/cxl.h
+++ b/drivers/misc/cxl/cxl.h
@@ -457,6 +457,8 @@ struct cxl_context {
 	bool pending_irq;
 	bool pending_fault;
 	bool pending_afu_err;
+
+	struct rcu_head rcu;
 };
 
 struct cxl {