netdev - [PATCH net-next] selftests: net: tcp_mmap: populate pages in send path

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Date:   Fri, 20 Jan 2023 18:11:36 +0000
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com,
        Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next] selftests: net: tcp_mmap: populate pages in send path

In commit 72653ae5303c ("selftests: net: tcp_mmap:
Use huge pages in send path") I made a change to use hugepages
for the buffer used by the client (tx path)

Today, I understood that the cause for poor zerocopy
performance was that after a mmap() for a 512KB memory
zone, kernel uses a single zeropage, mapped 128 times.

This was really the reason for poor tx path performance
in zero copy mode, because this zero page refcount is
under high pressure, especially when TCP ACK packets
are processed on another cpu.

We need either to force a COW on all the memory range,
or use MAP_POPULATE so that a zero page is not abused.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 tools/testing/selftests/net/tcp_mmap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/net/tcp_mmap.c b/tools/testing/selftests/net/tcp_mmap.c
index 00f837c9bc6c4549c19dfc27aa2d08c454ea169e..46a02bbd31d0be4d54b2b372415bce6c0d538b22 100644
--- a/tools/testing/selftests/net/tcp_mmap.c
+++ b/tools/testing/selftests/net/tcp_mmap.c
@@ -137,7 +137,8 @@ static void *mmap_large_buffer(size_t need, size_t *allocated)
 	if (buffer == (void *)-1) {
 		sz = need;
 		buffer = mmap(NULL, sz, PROT_READ | PROT_WRITE,
-			      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+			      MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE,
+			      -1, 0);
 		if (buffer != (void *)-1)
 			fprintf(stderr, "MAP_HUGETLB attempt failed, look at /sys/kernel/mm/hugepages for optimal performance\n");
 	}
-- 
2.39.1.405.gd4c25cc71f-goog