[PATCH] svcrdma: Cap Read sink allocations at PAGE_ALLOC_COSTLY_ORDER

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

Jonathan Flynn reports that commit 18755b8c2f24 ("svcrdma: Use
contiguous pages for RDMA Read sink buffers") regresses NFS/RDMA
WRITE throughput from 73.9 GiB/s to 30.3 GiB/s on a 128-core
single-NUMA-node server driving dual 400Gb/s links with 640 nfsd
threads. In the regressed configuration, server CPU utilization
rises from 8.5% to 76%, and 73% of all server CPU cycles are spent
in native_queued_spin_lock_slowpath.

The contended lock is zone->lock. The page allocator serves
allocations only up to PAGE_ALLOC_COSTLY_ORDER (3) from its per-CPU
page lists; SVC_RDMA_CONTIG_MAX_ORDER is 4, so every contiguous
sink buffer allocation falls through to rmqueue_buddy() and
acquires the zone lock. The workload above issues roughly half a
million order-4 allocations per second, all serialized on the
single zone lock of the one NUMA node. Replacing the GFP mask with
GFP_NOWAIT did not change the profile because direct reclaim never
ran: the cycles are spent acquiring the lock, not reclaiming
memory.

Cap the allocation order at PAGE_ALLOC_COSTLY_ORDER so contiguous
sink buffer allocations remain eligible for the per-CPU page
lists, where zone lock acquisition is amortized across pcp batch
refills. An order-3 chunk still replaces eight per-page bvecs with
one.

Reported-by: Jonathan Flynn <jonathan.flynn@hammerspace.com>
Fixes: 18755b8c2f24 ("svcrdma: Use contiguous pages for RDMA Read sink buffers")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/xprtrdma/svc_rdma_rw.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/net/sunrpc/xprtrdma/svc_rdma_rw.c b/net/sunrpc/xprtrdma/svc_rdma_rw.c
index efde26cac961..4546e594f2d7 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_rw.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_rw.c
@@ -746,11 +746,12 @@ int svc_rdma_prepare_reply_chunk(struct svcxprt_rdma *rdma,
 }
 
 /*
- * Cap contiguous RDMA Read sink allocations at order-4. Higher orders risk
- * allocation failure under GFP_NOWAIT, which would negate the benefit of
- * the contiguous fast path.
+ * Cap contiguous RDMA Read sink allocations at PAGE_ALLOC_COSTLY_ORDER.
+ * The page allocator serves allocations at or below that order from
+ * its per-CPU page lists; above it, every allocation acquires the
+ * zone lock, which serializes all nfsd threads.
  */
-#define SVC_RDMA_CONTIG_MAX_ORDER	4
+#define SVC_RDMA_CONTIG_MAX_ORDER	PAGE_ALLOC_COSTLY_ORDER
 
 /**
  * svc_rdma_alloc_read_pages - Allocate physically contiguous pages
-- 
2.54.0