linux-kernel - Re: ppoll() stuck on POLLIN while TCP peer is sending

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Message-ID: <1357220469.21409.24574.camel@edumazet-glaptop>
Date:	Thu, 03 Jan 2013 05:41:09 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Eric Wong <normalperson@...t.net>
Cc:	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: ppoll() stuck on POLLIN while TCP peer is sending

On Wed, 2013-01-02 at 20:47 +0000, Eric Wong wrote:
> Eric Wong <normalperson@...t.net> wrote:
> > [1] my full setup is very strange.
> > 
> >     Other than the FUSE component I forgot to mention, little depends on
> >     the kernel.  With all this, the standalone toosleepy can get stuck.
> >     I'll try to reproduce it with less...
> 
> I just confirmed my toosleepy processes will get stuck while just
> doing "rsync -a" between local disks.  So this does not depend on
> sendfile or FUSE to reproduce.
> --

How do you tell your 'toosleepy' is stuck ?

If reading its output, you should change its logic, there is no
guarantee the recv() will deliver exactly 16384 bytes each round.

With the following patch, I cant reproduce the 'apparent stuck'

diff --git a/toosleepy.c b/toosleepy.c
index e64b7cd..df3610f 100644
--- a/toosleepy.c
+++ b/toosleepy.c
@@ -15,6 +15,7 @@
 #include <fcntl.h>
 #include <assert.h>
 #include <limits.h>
+#include <time.h>
 
 struct receiver {
 	int rfd;
@@ -53,6 +54,7 @@ static void * recv_loop(void *p)
 	ssize_t r, s;
 	size_t received = 0;
 	size_t sent = 0;
+	time_t t0 = time(NULL), t1;
 
 	for (;;) {
 		r = recv(rcvr->rfd, buf, sizeof(buf), 0);
@@ -80,9 +82,12 @@ static void * recv_loop(void *p)
 				write(-1, buf, sizeof(buf));
 			}
 		}
-		if ((received % (sizeof(buf) * sizeof(buf) * 16) == 0))
+		t1 = time(NULL);
+		if (t1 != t0) {
 			dprintf(2, " %d progress: %zu\n",
 			        rcvr->rfd, received);
+			t0 = t1;
+		}
 	}
 	dprintf(2, "%d got: %zu\n", rcvr->rfd, received);
 	if (rcvr->sfd >= 0) {





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