Re: [PATCH] Increase nptl test case buffer size so we really block on current Linux kernels.

[David Miller <davem at davemloft dot net>]

From: Andreas Jaeger <aj@suse.com>
Date: Tue, 23 Jul 2013 06:54:22 +0200

> Since this hit us, could you document the reason for the buffer size?

Sure, how does this look?

--------------------
Increase nptl test case buffer size so we really block on current Linux kernels.

	* tst-cancel4.c (WRITE_BUFFER_SIZE): Increase to 16384.

diff --git a/nptl/ChangeLog b/nptl/ChangeLog
index fc2ef41..0e4747e 100644
--- a/nptl/ChangeLog
+++ b/nptl/ChangeLog
@@ -1,3 +1,7 @@
+2013-07-22  David S. Miller  <davem@davemloft.net>
+
+	* tst-cancel4.c (WRITE_BUFFER_SIZE): Increase to 16384.
+
 2013-07-19  Dominik Vogt  <vogt@de.ibm.com>
 
 	* pthread_mutex_lock.c: Fix whitespace.
diff --git a/nptl/tst-cancel4.c b/nptl/tst-cancel4.c
index 9ffd5d1..10b7c6e 100644
--- a/nptl/tst-cancel4.c
+++ b/nptl/tst-cancel4.c
@@ -83,7 +83,30 @@ static pthread_barrier_t b2;
 # define IPC_ADDVAL 0
 #endif
 
-#define WRITE_BUFFER_SIZE 4096
+/* The WRITE_BUFFER_SIZE value needs to be choosen such that if we set
+   the socket send buffer size to '1', a write of this size on that
+   socket will block.
+
+   The Linux kernel imposes a minimum send socket buffer size
+   which has changed over the years.  Currently the value is:
+
+     2 * (2048 + SKB_DATA_ALIGN(sizeof(struct sk_buff)))
+
+   which is attempting to make sure that with standard MTUs,
+   TCP can always queue up at least 2 full sized packets.
+
+   Furthermore, there is logic in the socket send paths that
+   will allow one more packet (of any size) to be queued up as
+   long as some socket buffer space remains.   Blocking only
+   occurs when we try to queue up a new packet and the send
+   buffer space has already been fully consumed.
+
+   Therefore we must set this value to the largest possible value of
+   the formula above (and since it depends upon the size of "struct
+   sk_buff", it is dependent upon machine word size etc.) plus some
+   slack space.  */
+
+#define WRITE_BUFFER_SIZE 16384
 
 /* Cleanup handling test.  */
 static int cl_called;