selftests: net: tcp_mmap: use madvise(MADV_DONTNEED)

When TCP_ZEROCOPY_RECEIVE operation has been added,
I made the mistake of automatically un-mapping prior
content before mapping new pages.

This has the unfortunate effect of adding potentially long
MMU operations (like TLB flushes) while socket lock is held.

Using madvise(MADV_DONTNEED) right after pages has been used
has two benefits :

1) This releases pages sooner, allowing pages to be recycled
if they were part of a page pool in a NIC driver.

2) No more long unmap operations while preventing immediate
processing of incoming packets.

The cost of the added system call is small enough.

Arjun will submit a kernel patch allowing to opt out from
the unmap attempt in tcp_zerocopy_receive()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Arjun Roy <arjunroy@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/tools/testing/selftests/net/tcp_mmap.c b/tools/testing/selftests/net/tcp_mmap.c
index a61b7b3..59ec0b5 100644 (file)
--- a/tools/testing/selftests/net/tcp_mmap.c
+++ b/tools/testing/selftests/net/tcp_mmap.c
@@ -179,6 +179,10 @@ void *child_thread(void *arg)
                                total_mmap += zc.length;
                                if (xflg)
                                        hash_zone(addr, zc.length);
+                               /* It is more efficient to unmap the pages right now,
+                                * instead of doing this in next TCP_ZEROCOPY_RECEIVE.
+                                */
+                               madvise(addr, zc.length, MADV_DONTNEED);
                                total += zc.length;
                        }
                        if (zc.recv_skip_hint) {