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: NFei K Chen <uchen@cn.ibm.com> Signed-off-by: NIan Munsie <imunsie@au1.ibm.com> Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
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