Re: [PATCH] relay-file-read-start-pos-fix.patch

[Masami Hiramatsu]

Hi Tom,

Tom Zanussi wrote:
> Could you send more info on how to reproduce the problem you're seeing?
> And does this patch fix it?

Sure, I'll explain how to reproduce it.

Since current SystemTap is not supporting "overwrite" mode,
you need to apply a patch before trying to reproduce it.
I already posted the patch to bugzilla. You can get it from below.
http://sourceware.org/bugzilla/attachment.cgi?id=1896&action=view

Here is an example script (fillup.stp).
----
global counter=0
probe timer.ms(1) {
        counter++;
        printf("%08d : %020d\n", counter, gettimeofday_ns());
}
----

First of all, run the script with -O (overwrite mode) flag.
(For simplify my explanation, I also use -m flag here.)
$ stap -O fillup.stp -m fillup
Soon after starting, press ^\(Ctrl+\) to detach from it.

The script writes 32 bytes dummy data per 1 milli-second, so
it writes about 32k bytes per 1 second.
And the default size of relay channel of systemtap is 512kB
which contains 4 subbufs (each size of subbufs is 128kB).
Thus, it fills the relay channel at about 16 seconds and
wraparounds because it uses overwrite mode.

So, wait more than 16 seconds (for example, 18 sec),
read the relay channel and count the line number.
And repeat it.
$ while true; do sleep 18; \
 cat /sys/kernel/debug/systemtap/fillup/trace0 | wc -l; done

Ideally, it will show the number from 12288(=3*128k/32) to
16384(=4*128k/32).

However, without my patch, it shows;
4793
5721
9818
9817
9819
13912
0
0
780
1625
5723
5721

And, with my patch;
15742
13273
14901
12430
14056
15682
13215
14840
12370
13996
15624
13154


So, I think my patch (which )fixes the problem.

Thanks,

P.S.
I attached my patch (relay-file-read-overwrite-mode-fix.patch)
which fixed the problem pointed in previous mail.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>

---
 kernel/relay.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

Index: linux-2.6.22-rc4-mm2/kernel/relay.c
===================================================================
--- linux-2.6.22-rc4-mm2.orig/kernel/relay.c	2007-06-13 20:22:02.000000000 +0900
+++ linux-2.6.22-rc4-mm2/kernel/relay.c	2007-06-20 10:53:06.000000000 +0900
@@ -812,7 +812,10 @@
 	}

 	buf->bytes_consumed += bytes_consumed;
-	read_subbuf = read_pos / buf->chan->subbuf_size;
+	if (!read_pos)
+		read_subbuf = buf->subbufs_consumed % n_subbufs;
+	else
+		read_subbuf = read_pos / buf->chan->subbuf_size;
 	if (buf->bytes_consumed + buf->padding[read_subbuf] == subbuf_size) {
 		if ((read_subbuf == buf->subbufs_produced % n_subbufs) &&
 		    (buf->offset == subbuf_size))
@@ -841,8 +844,9 @@
 	}

 	if (unlikely(produced - consumed >= n_subbufs)) {
-		consumed = (produced / n_subbufs) * n_subbufs;
+		consumed = produced - n_subbufs + 1;
 		buf->subbufs_consumed = consumed;
+		buf->bytes_consumed = 0;
 	}
 	
 	produced = (produced % n_subbufs) * subbuf_size + buf->offset;




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