[PATCHSET] netconsole: implement extended console support

[PATCHSET] netconsole: implement extended console support

[Tejun Heo]

This patchset is v2 of netconsole extended console support.  v1 was
part of "printk, netconsole: implement reliable netconsole"
patchset[1].  The printk part is broken off to a separate patchset[2]
"printk: implement extended console support" which this patchset is
dependant upon.

Changes from the last last posting are

* Dynamic ext console de-registration is dropped.  This made most of
  lock restructuring and refactoring in enable/disable path
  unnecessary.  Ext netconsole is now registered on first use and stay
  registered.  While this means that ext console support will stay
  enabled even after a dynamic extended console is disabled, such
  scenarios are likely very rare and the incurred overhead isn't
  drastic enough to justify the complexity.

* Retransmission handling is removed from the patchset.  Handling
  retransmission in kernel doesn't provide enough benefits and is
  moved to userland.

netconsole emits one or more udp messages per each log message and
only transmits the body, which works fine when it's used as a
debugging tool on local network; however, netconsole, due to its
advantages for troubleshooting kernel issues, is also used as a
mechanism to collect kernel messages at larger scale where the packets
may have to travel across congested networks or networks with multiple
paths.

Of the handful large cluster setups that I've seen, two were using
netconsole for fleet-wide kernel logging and having problem with lost
messages.  One was a HPC cluster which had a dedicated slower
management network which was used for all management traffic where
packet losses were fairly common for several different reasons - the
network itself could get fairly overloaded at times and IPMI sharing
the interface didn't seem to help either.  The other is a large web
service cluster where the aggregator is some hops away and packet
losses do happen from time to time.

Because netconsole packets don't carry any metadata, it's impossible
to tell what happened to the messages during transit and even
combining it with messages transmitted via a separate reliable
mechanism is challenging as it boils down to matching message content
textually.

The "printk, netconsole: implement reliable netconsole" patchset[1]
implements extended console support.  If a console driver sets
CON_EXTENDED, printk formats each message in the same way /dev/kmsg
messages are formatted which includes all metadata and, for structured
log messages, KEY=VALUE dictionary.

This patchset implements extended console support for netconsole,
which allows log consumers access to complete log information and to
tell which messages are missing and/or reordered, which can be used to
implement reliable kernel message logging when combined with userland
helpers.

Changes to netconsole are straight-forward.  It optionally registers a
separate extended console driver.  printk passes in extended format
messages which are transmitted the same way.  The only complication is
when the message is longer than the maximum payload size (1k).  As
each message should have proper header and the log receiver should be
able to tell which part the fragment is, netconsole duplicates full
header on each fragment and also adds an extra ncfrag=OFF/LEN header.

 0001-netconsole-make-netconsole_target-enabled-a-bool.patch
 0002-netconsole-make-all-dynamic-netconsoles-share-a-mute.patch
 0003-netconsole-implement-extended-console-support.patch

David, the patchset is small enough that I don't think splitting it
makes much sense.  While the first two patches are mostly independent
cleanups, they can be ignored w/o the third patch.

diffstat follows.  Thanks.

 Documentation/networking/netconsole.txt |   34 ++++++
 drivers/net/netconsole.c                |  175 +++++++++++++++++++++++++++++---
 2 files changed, 192 insertions(+), 17 deletions(-)

--
tejun

[1] http://lkml.kernel.org/g/1429225433-11946-1-git-send-email-tj@kernel.org
[2] http://lkml.kernel.org/g/1430318704-32374-1-git-send-email-tj@kernel.org

[PATCH 1/3] netconsole: make netconsole_target->enabled a bool

[Tejun Heo]

netconsole uses both bool and int for boolean values.  Let's convert
nt->enabled to bool for consistency.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Miller <davem@davemloft.net>
---
 drivers/net/netconsole.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 15731d1..09d4e12 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -104,7 +104,7 @@ struct netconsole_target {
 #ifdef	CONFIG_NETCONSOLE_DYNAMIC
 	struct config_item	item;
 #endif
-	int			enabled;
+	bool			enabled;
 	struct mutex		mutex;
 	struct netpoll		np;
 };
@@ -197,7 +197,7 @@ static struct netconsole_target *alloc_param_target(char *target_config)
 	if (err)
 		goto fail;
 
-	nt->enabled = 1;
+	nt->enabled = true;
 
 	return nt;
 
@@ -322,13 +322,13 @@ static ssize_t store_enabled(struct netconsole_target *nt,
 		return err;
 	if (enabled < 0 || enabled > 1)
 		return -EINVAL;
-	if (enabled == nt->enabled) {
+	if ((bool)enabled == nt->enabled) {
 		pr_info("network logging has already %s\n",
 			nt->enabled ? "started" : "stopped");
 		return -EINVAL;
 	}
 
-	if (enabled) {	/* 1 */
+	if (enabled) {	/* true */
 		/*
 		 * Skip netpoll_parse_options() -- all the attributes are
 		 * already configured via configfs. Just print them out.
@@ -340,13 +340,13 @@ static ssize_t store_enabled(struct netconsole_target *nt,
 			return err;
 
 		pr_info("netconsole: network logging started\n");
-	} else {	/* 0 */
+	} else {	/* false */
 		/* We need to disable the netconsole before cleaning it up
 		 * otherwise we might end up in write_msg() with
-		 * nt->np.dev == NULL and nt->enabled == 1
+		 * nt->np.dev == NULL and nt->enabled == true
 		 */
 		spin_lock_irqsave(&target_list_lock, flags);
-		nt->enabled = 0;
+		nt->enabled = false;
 		spin_unlock_irqrestore(&target_list_lock, flags);
 		netpoll_cleanup(&nt->np);
 	}
@@ -594,7 +594,7 @@ static struct config_item *make_netconsole_target(struct config_group *group,
 
 	/*
 	 * Allocate and initialize with defaults.
-	 * Target is disabled at creation (enabled == 0).
+	 * Target is disabled at creation (!enabled).
 	 */
 	nt = kzalloc(sizeof(*nt), GFP_KERNEL);
 	if (!nt)
@@ -695,7 +695,7 @@ restart:
 				spin_lock_irqsave(&target_list_lock, flags);
 				dev_put(nt->np.dev);
 				nt->np.dev = NULL;
-				nt->enabled = 0;
+				nt->enabled = false;
 				stopped = true;
 				netconsole_target_put(nt);
 				goto restart;
-- 
2.1.0

[PATCH 2/3] netconsole: make all dynamic netconsoles share a mutex

[Tejun Heo]

Currently, each dynamic netconsole_target uses its own separate mutex
to synchronize the configuration operations.  This patch replaces the
per-netconsole_target mutexes with a single mutex -
dynamic_netconsole_mutex.  The reduced granularity doesn't hurt
anything, the code is minutely simpler and this'd allow adding
operations which should be synchronized across all dynamic
netconsoles.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Miller <davem@davemloft.net>
---
 drivers/net/netconsole.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 09d4e12..d9f0f85 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -105,13 +105,13 @@ struct netconsole_target {
 	struct config_item	item;
 #endif
 	bool			enabled;
-	struct mutex		mutex;
 	struct netpoll		np;
 };
 
 #ifdef	CONFIG_NETCONSOLE_DYNAMIC
 
 static struct configfs_subsystem netconsole_subsys;
+static DEFINE_MUTEX(dynamic_netconsole_mutex);
 
 static int __init dynamic_netconsole_init(void)
 {
@@ -185,7 +185,6 @@ static struct netconsole_target *alloc_param_target(char *target_config)
 	strlcpy(nt->np.dev_name, "eth0", IFNAMSIZ);
 	nt->np.local_port = 6665;
 	nt->np.remote_port = 6666;
-	mutex_init(&nt->mutex);
 	eth_broadcast_addr(nt->np.remote_mac);
 
 	/* Parse parameters and setup netpoll */
@@ -562,10 +561,10 @@ static ssize_t netconsole_target_attr_store(struct config_item *item,
 	struct netconsole_target_attr *na =
 		container_of(attr, struct netconsole_target_attr, attr);
 
-	mutex_lock(&nt->mutex);
+	mutex_lock(&dynamic_netconsole_mutex);
 	if (na->store)
 		ret = na->store(nt, buf, count);
-	mutex_unlock(&nt->mutex);
+	mutex_unlock(&dynamic_netconsole_mutex);
 
 	return ret;
 }
@@ -604,7 +603,6 @@ static struct config_item *make_netconsole_target(struct config_group *group,
 	strlcpy(nt->np.dev_name, "eth0", IFNAMSIZ);
 	nt->np.local_port = 6665;
 	nt->np.remote_port = 6666;
-	mutex_init(&nt->mutex);
 	eth_broadcast_addr(nt->np.remote_mac);
 
 	/* Initialize the config_item member */
-- 
2.1.0

[PATCH 3/3] netconsole: implement extended console support

[Tejun Heo]

printk logbuf keeps various metadata and optional key=value dictionary
for structured messages, both of which are stripped when messages are
handed to regular console drivers.

It can be useful to have this metadata and dictionary available to
netconsole consumers.  This obviously makes logging via netconsole
more complete and the sequence number in particular is useful in
environments where messages may be lost or reordered in transit -
e.g. when netconsole is used to collect messages in a large cluster
where packets may have to travel congested hops to reach the
aggregator.  The lost and reordered messages can easily be identified
and handled accordingly using the sequence numbers.

printk recently added extended console support which can be selected
by setting CON_EXTENDED flag.  From console driver side, not much
changes.  The only difference is that the text passed to the write
callback is formatted the same way as /dev/kmsg.

This patch implements extended console support for netconsole which
can be enabled by either prepending "+" to a netconsole boot param
entry or echoing 1 to "extended" file in configfs.  When enabled,
netconsole transmits extended log messages with headers identical to
/dev/kmsg output.

There's one complication due to message fragments.  netconsole limits
the maximum message size to 1k and messages longer than that are split
into multiple fragments.  As all extended console messages should
carry matching headers and be uniquely identifiable, each extended
message fragment carries full copy of the metadata and an extra header
field to identify the specific fragment.  The optional header is of
the form "ncfrag=OFF/LEN" where OFF is the byte offset into the
message body and LEN is the total length.

To avoid unnecessarily making printk format extended messages,
Extended netconsole is registered with printk when the first extended
netconsole is configured.

v2: Dropped dynamic unregistration of extended console driver, which
    added complexity while not being too beneficial given that most
    netconsole configurations are static.  ncfrag updated to use just
    byte offset and message length.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Miller <davem@davemloft.net>
---
 Documentation/networking/netconsole.txt |  34 +++++++-
 drivers/net/netconsole.c                | 149 +++++++++++++++++++++++++++++++-
 2 files changed, 180 insertions(+), 3 deletions(-)

diff --git a/Documentation/networking/netconsole.txt b/Documentation/networking/netconsole.txt
index a5d574a..76c2640 100644
--- a/Documentation/networking/netconsole.txt
+++ b/Documentation/networking/netconsole.txt
@@ -2,6 +2,7 @@
 started by Ingo Molnar <mingo@redhat.com>, 2001.09.17
 2.6 port and netpoll api by Matt Mackall <mpm@selenic.com>, Sep 9 2003
 IPv6 support by Cong Wang <xiyou.wangcong@gmail.com>, Jan 1 2013
+Extended console support by Tejun Heo <tj@kernel.org>, May 1 2015
 
 Please send bug reports to Matt Mackall <mpm@selenic.com>
 Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com>
@@ -24,9 +25,10 @@ Sender and receiver configuration:
 It takes a string configuration parameter "netconsole" in the
 following format:
 
- netconsole=[src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
+ netconsole=[+][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
 
    where
+        +             if present, enable extended console support
         src-port      source for UDP packets (defaults to 6665)
         src-ip        source IP to use (interface address)
         dev           network interface (eth0)
@@ -107,6 +109,7 @@ To remove a target:
 The interface exposes these parameters of a netconsole target to userspace:
 
 	enabled		Is this target currently enabled?	(read-write)
+	extended	Extended mode enabled			(read-write)
 	dev_name	Local network interface name		(read-write)
 	local_port	Source UDP port to use			(read-write)
 	remote_port	Remote agent's UDP port			(read-write)
@@ -132,6 +135,35 @@ You can also update the local interface dynamically. This is especially
 useful if you want to use interfaces that have newly come up (and may not
 have existed when netconsole was loaded / initialized).
 
+Extended console:
+=================
+
+If '+' is prefixed to the configuration line or "extended" config file
+is set to 1, extended console support is enabled. An example boot
+param follows.
+
+ linux netconsole=+4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
+
+Log messages are transmitted with extended metadata header in the
+following format which is the same as /dev/kmsg.
+
+ <level>,<sequnum>,<timestamp>,<contflag>;<message text>
+
+Non printable characters in <message text> are escaped using "\xff"
+notation. If the message contains optional dictionary, verbatim
+newline is used as the delimeter.
+
+If a message doesn't fit in 1000 bytes, the message is split into
+multiple fragments by netconsole. These fragments are transmitted with
+"ncfrag" header field added.
+
+ ncfrag=<byte-offset>/<total-bytes>
+
+For example,
+
+ 6,416,1758426,-,ncfrag=0/33;the first chunk,
+ 6,416,1758426,-,ncfrag=16/33;the second chunk.
+
 Miscellaneous notes:
 ====================
 
diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index d9f0f85..246a76d 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -79,6 +79,12 @@ static LIST_HEAD(target_list);
 /* This needs to be a spinlock because write_msg() cannot sleep */
 static DEFINE_SPINLOCK(target_list_lock);
 
+/*
+ * Console driver for extended netconsoles.  Registered on the first use to
+ * avoid unnecessarily enabling ext message formatting.
+ */
+static struct console netconsole_ext;
+
 /**
  * struct netconsole_target - Represents a configured netconsole target.
  * @list:	Links this target into the target_list.
@@ -105,6 +111,7 @@ struct netconsole_target {
 	struct config_item	item;
 #endif
 	bool			enabled;
+	bool			extended;
 	struct netpoll		np;
 };
 
@@ -187,6 +194,11 @@ static struct netconsole_target *alloc_param_target(char *target_config)
 	nt->np.remote_port = 6666;
 	eth_broadcast_addr(nt->np.remote_mac);
 
+	if (*target_config == '+') {
+		nt->extended = true;
+		target_config++;
+	}
+
 	/* Parse parameters and setup netpoll */
 	err = netpoll_parse_options(&nt->np, target_config);
 	if (err)
@@ -257,6 +269,11 @@ static ssize_t show_enabled(struct netconsole_target *nt, char *buf)
 	return snprintf(buf, PAGE_SIZE, "%d\n", nt->enabled);
 }
 
+static ssize_t show_extended(struct netconsole_target *nt, char *buf)
+{
+	return snprintf(buf, PAGE_SIZE, "%d\n", nt->extended);
+}
+
 static ssize_t show_dev_name(struct netconsole_target *nt, char *buf)
 {
 	return snprintf(buf, PAGE_SIZE, "%s\n", nt->np.dev_name);
@@ -328,6 +345,11 @@ static ssize_t store_enabled(struct netconsole_target *nt,
 	}
 
 	if (enabled) {	/* true */
+		if (nt->extended && !(netconsole_ext.flags & CON_ENABLED)) {
+			netconsole_ext.flags |= CON_ENABLED;
+			register_console(&netconsole_ext);
+		}
+
 		/*
 		 * Skip netpoll_parse_options() -- all the attributes are
 		 * already configured via configfs. Just print them out.
@@ -355,6 +377,30 @@ static ssize_t store_enabled(struct netconsole_target *nt,
 	return strnlen(buf, count);
 }
 
+static ssize_t store_extended(struct netconsole_target *nt,
+			      const char *buf,
+			      size_t count)
+{
+	int extended;
+	int err;
+
+	if (nt->enabled) {
+		pr_err("target (%s) is enabled, disable to update parameters\n",
+		       config_item_name(&nt->item));
+		return -EINVAL;
+	}
+
+	err = kstrtoint(buf, 10, &extended);
+	if (err < 0)
+		return err;
+	if (extended < 0 || extended > 1)
+		return -EINVAL;
+
+	nt->extended = extended;
+
+	return strnlen(buf, count);
+}
+
 static ssize_t store_dev_name(struct netconsole_target *nt,
 			      const char *buf,
 			      size_t count)
@@ -507,6 +553,7 @@ static struct netconsole_target_attr netconsole_target_##_name =	\
 	__CONFIGFS_ATTR(_name, S_IRUGO | S_IWUSR, show_##_name, store_##_name)
 
 NETCONSOLE_TARGET_ATTR_RW(enabled);
+NETCONSOLE_TARGET_ATTR_RW(extended);
 NETCONSOLE_TARGET_ATTR_RW(dev_name);
 NETCONSOLE_TARGET_ATTR_RW(local_port);
 NETCONSOLE_TARGET_ATTR_RW(remote_port);
@@ -517,6 +564,7 @@ NETCONSOLE_TARGET_ATTR_RW(remote_mac);
 
 static struct configfs_attribute *netconsole_target_attrs[] = {
 	&netconsole_target_enabled.attr,
+	&netconsole_target_extended.attr,
 	&netconsole_target_dev_name.attr,
 	&netconsole_target_local_port.attr,
 	&netconsole_target_remote_port.attr,
@@ -727,6 +775,90 @@ static struct notifier_block netconsole_netdev_notifier = {
 	.notifier_call  = netconsole_netdev_event,
 };
 
+/**
+ * send_ext_msg_udp - send extended log message to target
+ * @nt: target to send message to
+ * @msg: extended log message to send
+ * @msg_len: length of message
+ *
+ * Transfer extended log @msg to @nt.  If @msg is longer than
+ * MAX_PRINT_CHUNK, it'll be split and transmitted in multiple chunks with
+ * ncfrag header field added to identify them.
+ */
+static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
+			     int msg_len)
+{
+	static char buf[MAX_PRINT_CHUNK];
+	const int max_extra_len = sizeof(",ncfrag=0000/0000");
+	const char *header, *body;
+	int header_len = msg_len, body_len = 0;
+	int chunk_len, nr_chunks, i;
+
+	if (msg_len <= MAX_PRINT_CHUNK) {
+		netpoll_send_udp(&nt->np, msg, msg_len);
+		return;
+	}
+
+	/* need to insert extra header fields, detect header and body */
+	header = msg;
+	body = memchr(msg, ';', msg_len);
+	if (body) {
+		header_len = body - header;
+		body_len = msg_len - header_len - 1;
+		body++;
+	}
+
+	chunk_len = MAX_PRINT_CHUNK - header_len - max_extra_len;
+	if (WARN_ON_ONCE(chunk_len <= 0))
+		return;
+
+	/*
+	 * Transfer possibly multiple chunks with extra header fields.
+	 *
+	 * If @msg needs to be split to fit MAX_PRINT_CHUNK, add
+	 * "ncfrag=<byte-offset>/<total-bytes>" to identify each chunk.
+	 */
+	memcpy(buf, header, header_len);
+	nr_chunks = DIV_ROUND_UP(body_len, chunk_len);
+
+	for (i = 0; i < nr_chunks; i++) {
+		int offset = i * chunk_len;
+		int this_header = header_len;
+		int this_chunk = min(body_len - offset, chunk_len);
+
+		if (nr_chunks > 1)
+			this_header += scnprintf(buf + this_header,
+						 sizeof(buf) - this_header,
+						 ",ncfrag=%d/%d;",
+						 offset, body_len);
+
+		if (WARN_ON_ONCE(this_header + chunk_len > MAX_PRINT_CHUNK))
+			return;
+
+		memcpy(buf + this_header, body, this_chunk);
+
+		netpoll_send_udp(&nt->np, buf, this_header + this_chunk);
+
+		body += this_chunk;
+	}
+}
+
+static void write_ext_msg(struct console *con, const char *msg,
+			  unsigned int len)
+{
+	struct netconsole_target *nt;
+	unsigned long flags;
+
+	if ((oops_only && !oops_in_progress) || list_empty(&target_list))
+		return;
+
+	spin_lock_irqsave(&target_list_lock, flags);
+	list_for_each_entry(nt, &target_list, list)
+		if (nt->extended && nt->enabled && netif_running(nt->np.dev))
+			send_ext_msg_udp(nt, msg, len);
+	spin_unlock_irqrestore(&target_list_lock, flags);
+}
+
 static void write_msg(struct console *con, const char *msg, unsigned int len)
 {
 	int frag, left;
@@ -743,7 +875,7 @@ static void write_msg(struct console *con, const char *msg, unsigned int len)
 	spin_lock_irqsave(&target_list_lock, flags);
 	list_for_each_entry(nt, &target_list, list) {
 		netconsole_target_get(nt);
-		if (nt->enabled && netif_running(nt->np.dev)) {
+		if (!nt->extended && nt->enabled && netif_running(nt->np.dev)) {
 			/*
 			 * We nest this inside the for-each-target loop above
 			 * so that we're able to get as much logging out to
@@ -763,6 +895,12 @@ static void write_msg(struct console *con, const char *msg, unsigned int len)
 	spin_unlock_irqrestore(&target_list_lock, flags);
 }
 
+static struct console netconsole_ext = {
+	.name	= "netcon_ext",
+	.flags	= CON_EXTENDED,	/* starts disabled, registered on first use */
+	.write	= write_ext_msg,
+};
+
 static struct console netconsole = {
 	.name	= "netcon",
 	.flags	= CON_ENABLED,
@@ -785,7 +923,11 @@ static int __init init_netconsole(void)
 				goto fail;
 			}
 			/* Dump existing printks when we register */
-			netconsole.flags |= CON_PRINTBUFFER;
+			if (nt->extended)
+				netconsole_ext.flags |= CON_PRINTBUFFER |
+							CON_ENABLED;
+			else
+				netconsole.flags |= CON_PRINTBUFFER;
 
 			spin_lock_irqsave(&target_list_lock, flags);
 			list_add(&nt->list, &target_list);
@@ -801,6 +943,8 @@ static int __init init_netconsole(void)
 	if (err)
 		goto undonotifier;
 
+	if (netconsole_ext.flags & CON_ENABLED)
+		register_console(&netconsole_ext);
 	register_console(&netconsole);
 	pr_info("network logging started\n");
 
@@ -829,6 +973,7 @@ static void __exit cleanup_netconsole(void)
 {
 	struct netconsole_target *nt, *tmp;
 
+	unregister_console(&netconsole_ext);
 	unregister_console(&netconsole);
 	dynamic_netconsole_exit();
 	unregister_netdevice_notifier(&netconsole_netdev_notifier);
-- 
2.1.0

Re: [PATCH 3/3] netconsole: implement extended console support

[Tetsuo Handa]

Tejun Heo wrote:
> +If a message doesn't fit in 1000 bytes, the message is split into
> +multiple fragments by netconsole. These fragments are transmitted with
> +"ncfrag" header field added.
> +
> + ncfrag=<byte-offset>/<total-bytes>
> +
> +For example,
> +
> + 6,416,1758426,-,ncfrag=0/33;the first chunk,
> + 6,416,1758426,-,ncfrag=16/33;the second chunk.
> +

Wouldn't total-bytes > 1000 than 33 in this example?

> +/**
> + * send_ext_msg_udp - send extended log message to target
> + * @nt: target to send message to
> + * @msg: extended log message to send
> + * @msg_len: length of message
> + *
> + * Transfer extended log @msg to @nt.  If @msg is longer than
> + * MAX_PRINT_CHUNK, it'll be split and transmitted in multiple chunks with
> + * ncfrag header field added to identify them.
> + */
> +static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
> +			     int msg_len)
> +{
> +	static char buf[MAX_PRINT_CHUNK];
> +	const int max_extra_len = sizeof(",ncfrag=0000/0000");
> +	const char *header, *body;
> +	int header_len = msg_len, body_len = 0;
> +	int chunk_len, nr_chunks, i;
> +
> +	if (msg_len <= MAX_PRINT_CHUNK) {
> +		netpoll_send_udp(&nt->np, msg, msg_len);
> +		return;
> +	}
> +
> +	/* need to insert extra header fields, detect header and body */
> +	header = msg;
> +	body = memchr(msg, ';', msg_len);
> +	if (body) {
> +		header_len = body - header;
> +		body_len = msg_len - header_len - 1;
> +		body++;
> +	}
> +
> +	chunk_len = MAX_PRINT_CHUNK - header_len - max_extra_len;
> +	if (WARN_ON_ONCE(chunk_len <= 0))
> +		return;

This path is executed only when msg_len > MAX_PRINT_CHUNK.
And since header_len == msg_len if body == NULL, chunk_len <= 0 is true.
We will hit this WARN_ON_ONCE() if memchr(msg, ';', msg_len) == NULL
which will fail to send the message. Is this what you want?

> +static void write_ext_msg(struct console *con, const char *msg,
> +			  unsigned int len)
> +{
> +	struct netconsole_target *nt;
> +	unsigned long flags;
> +
> +	if ((oops_only && !oops_in_progress) || list_empty(&target_list))
> +		return;
> +
> +	spin_lock_irqsave(&target_list_lock, flags);
> +	list_for_each_entry(nt, &target_list, list)

Don't you need to call netconsole_target_get() here

> +		if (nt->extended && nt->enabled && netif_running(nt->np.dev))
> +			send_ext_msg_udp(nt, msg, len);

and netconsole_target_put() here as with write_msg()?

> +	spin_unlock_irqrestore(&target_list_lock, flags);
> +}
> +

Re: [PATCH 3/3] netconsole: implement extended console support

[Tejun Heo]

Hello, Tetsuo.

On Sat, May 02, 2015 at 01:54:57PM +0900, Tetsuo Handa wrote:
> Tejun Heo wrote:
> > +For example,
> > +
> > + 6,416,1758426,-,ncfrag=0/33;the first chunk,
> > + 6,416,1758426,-,ncfrag=16/33;the second chunk.
> > +
> 
> Wouldn't total-bytes > 1000 than 33 in this example?

It's trying to give an example on how the ncfrag header looks like but
yeah I'll tweak it a bit and add further explanation.

> > +/**
> > + * send_ext_msg_udp - send extended log message to target
> > + * @nt: target to send message to
> > + * @msg: extended log message to send
> > + * @msg_len: length of message
> > + *
> > + * Transfer extended log @msg to @nt.  If @msg is longer than
> > + * MAX_PRINT_CHUNK, it'll be split and transmitted in multiple chunks with
> > + * ncfrag header field added to identify them.
> > + */
> > +static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
> > +			     int msg_len)
> > +{
> > +	static char buf[MAX_PRINT_CHUNK];
> > +	const int max_extra_len = sizeof(",ncfrag=0000/0000");
> > +	const char *header, *body;
> > +	int header_len = msg_len, body_len = 0;
> > +	int chunk_len, nr_chunks, i;
> > +
> > +	if (msg_len <= MAX_PRINT_CHUNK) {
> > +		netpoll_send_udp(&nt->np, msg, msg_len);
> > +		return;
> > +	}
> > +
> > +	/* need to insert extra header fields, detect header and body */
> > +	header = msg;
> > +	body = memchr(msg, ';', msg_len);
> > +	if (body) {
> > +		header_len = body - header;
> > +		body_len = msg_len - header_len - 1;
> > +		body++;
> > +	}
> > +
> > +	chunk_len = MAX_PRINT_CHUNK - header_len - max_extra_len;
> > +	if (WARN_ON_ONCE(chunk_len <= 0))
> > +		return;
> 
> This path is executed only when msg_len > MAX_PRINT_CHUNK.
> And since header_len == msg_len if body == NULL, chunk_len <= 0 is true.
> We will hit this WARN_ON_ONCE() if memchr(msg, ';', msg_len) == NULL
> which will fail to send the message. Is this what you want?

Yeah, extended consoles should be fed messages w/ proper headers.  If
not, we warn and bail.

> > +static void write_ext_msg(struct console *con, const char *msg,
> > +			  unsigned int len)
> > +{
> > +	struct netconsole_target *nt;
> > +	unsigned long flags;
> > +
> > +	if ((oops_only && !oops_in_progress) || list_empty(&target_list))
> > +		return;
> > +
> > +	spin_lock_irqsave(&target_list_lock, flags);
> > +	list_for_each_entry(nt, &target_list, list)
> 
> Don't you need to call netconsole_target_get() here
> 
> > +		if (nt->extended && nt->enabled && netif_running(nt->np.dev))
> > +			send_ext_msg_udp(nt, msg, len);
> 
> and netconsole_target_put() here as with write_msg()?

Ah, I dropped the patch which removed it from write_msg() but the
following pattern doesn't do anything.  It's just pure confusion.

	lock;
	ref++ on an item on the index;
	do something w/o releasing lock;
	ref-- on the item;
	unlock;

netconsole_netdev_event() has to release lock to process the item so
refcnting is needed there.  It got duplicated in write_msg() for no
reason.  I'll add a patch to remove it.

Thanks.

-- 
tejun

[PATCH 0.5/4] netconsole: remove unnecessary netconsole_target_get/out() from write_msg()

[Tejun Heo]

>From 958d3e14720a35c6103668c69d58751b36053d69 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Mon, 4 May 2015 15:57:54 -0400

write_msg() grabs target_list_lock and walks target_list invoking
netpool_send_udp() on each target.  Curiously, it protects each
iteration with netconsole_target_get/put() even though it never
releases target_list_lock which protects all the members.

While this doesn't harm anything, it doesn't serve any purpose either.
The items on the list can't go away while target_list_lock is held.
Remove the unnecessary get/put pair.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Miller <davem@davemloft.net>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
Hello,

If anyone wants the whole series to be reposted, please let me know.
The updated patchset is available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git review-netconsole-ext-console

Thanks.

 drivers/net/netconsole.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 15731d1..30c0524 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -744,7 +744,6 @@ static void write_msg(struct console *con, const char *msg, unsigned int len)
 
 	spin_lock_irqsave(&target_list_lock, flags);
 	list_for_each_entry(nt, &target_list, list) {
-		netconsole_target_get(nt);
 		if (nt->enabled && netif_running(nt->np.dev)) {
 			/*
 			 * We nest this inside the for-each-target loop above
@@ -760,7 +759,6 @@ static void write_msg(struct console *con, const char *msg, unsigned int len)
 				left -= frag;
 			}
 		}
-		netconsole_target_put(nt);
 	}
 	spin_unlock_irqrestore(&target_list_lock, flags);
 }
-- 
2.1.0

[PATCH v3 3/3] netconsole: implement extended console support

[Tejun Heo]

>From f9530ac244c12a89837736269a1930291a360875 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Mon, 4 May 2015 15:57:54 -0400

printk logbuf keeps various metadata and optional key=value dictionary
for structured messages, both of which are stripped when messages are
handed to regular console drivers.

It can be useful to have this metadata and dictionary available to
netconsole consumers.  This obviously makes logging via netconsole
more complete and the sequence number in particular is useful in
environments where messages may be lost or reordered in transit -
e.g. when netconsole is used to collect messages in a large cluster
where packets may have to travel congested hops to reach the
aggregator.  The lost and reordered messages can easily be identified
and handled accordingly using the sequence numbers.

printk recently added extended console support which can be selected
by setting CON_EXTENDED flag.  From console driver side, not much
changes.  The only difference is that the text passed to the write
callback is formatted the same way as /dev/kmsg.

This patch implements extended console support for netconsole which
can be enabled by either prepending "+" to a netconsole boot param
entry or echoing 1 to "extended" file in configfs.  When enabled,
netconsole transmits extended log messages with headers identical to
/dev/kmsg output.

There's one complication due to message fragments.  netconsole limits
the maximum message size to 1k and messages longer than that are split
into multiple fragments.  As all extended console messages should
carry matching headers and be uniquely identifiable, each extended
message fragment carries full copy of the metadata and an extra header
field to identify the specific fragment.  The optional header is of
the form "ncfrag=OFF/LEN" where OFF is the byte offset into the
message body and LEN is the total length.

To avoid unnecessarily making printk format extended messages,
Extended netconsole is registered with printk when the first extended
netconsole is configured.

v3: Tweaked documentation to make clarify that the example assumes a
    lot smaller chunk size.  Updated to apply on top of spurious
    target get/put removal in write_msg().

v2: Dropped dynamic unregistration of extended console driver, which
    added complexity while not being too beneficial given that most
    netconsole configurations are static.  ncfrag updated to use just
    byte offset and message length.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: David Miller <davem@davemloft.net>
---
Hello,

If anyone wants the whole series to be reposted, please let me know.
The updated patchset is available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git review-netconsole-ext-console

Thanks.

 Documentation/networking/netconsole.txt |  35 +++++++-
 drivers/net/netconsole.c                | 149 +++++++++++++++++++++++++++++++-
 2 files changed, 181 insertions(+), 3 deletions(-)

diff --git a/Documentation/networking/netconsole.txt b/Documentation/networking/netconsole.txt
index a5d574a..30409a3 100644
--- a/Documentation/networking/netconsole.txt
+++ b/Documentation/networking/netconsole.txt
@@ -2,6 +2,7 @@
 started by Ingo Molnar <mingo@redhat.com>, 2001.09.17
 2.6 port and netpoll api by Matt Mackall <mpm@selenic.com>, Sep 9 2003
 IPv6 support by Cong Wang <xiyou.wangcong@gmail.com>, Jan 1 2013
+Extended console support by Tejun Heo <tj@kernel.org>, May 1 2015
 
 Please send bug reports to Matt Mackall <mpm@selenic.com>
 Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com>
@@ -24,9 +25,10 @@ Sender and receiver configuration:
 It takes a string configuration parameter "netconsole" in the
 following format:
 
- netconsole=[src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
+ netconsole=[+][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
 
    where
+        +             if present, enable extended console support
         src-port      source for UDP packets (defaults to 6665)
         src-ip        source IP to use (interface address)
         dev           network interface (eth0)
@@ -107,6 +109,7 @@ To remove a target:
 The interface exposes these parameters of a netconsole target to userspace:
 
 	enabled		Is this target currently enabled?	(read-write)
+	extended	Extended mode enabled			(read-write)
 	dev_name	Local network interface name		(read-write)
 	local_port	Source UDP port to use			(read-write)
 	remote_port	Remote agent's UDP port			(read-write)
@@ -132,6 +135,36 @@ You can also update the local interface dynamically. This is especially
 useful if you want to use interfaces that have newly come up (and may not
 have existed when netconsole was loaded / initialized).
 
+Extended console:
+=================
+
+If '+' is prefixed to the configuration line or "extended" config file
+is set to 1, extended console support is enabled. An example boot
+param follows.
+
+ linux netconsole=+4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
+
+Log messages are transmitted with extended metadata header in the
+following format which is the same as /dev/kmsg.
+
+ <level>,<sequnum>,<timestamp>,<contflag>;<message text>
+
+Non printable characters in <message text> are escaped using "\xff"
+notation. If the message contains optional dictionary, verbatim
+newline is used as the delimeter.
+
+If a message doesn't fit in certain number of bytes (currently 1000),
+the message is split into multiple fragments by netconsole. These
+fragments are transmitted with "ncfrag" header field added.
+
+ ncfrag=<byte-offset>/<total-bytes>
+
+For example, assuming a lot smaller chunk size, a message "the first
+chunk, the 2nd chunk." may be split as follows.
+
+ 6,416,1758426,-,ncfrag=0/31;the first chunk,
+ 6,416,1758426,-,ncfrag=16/31; the 2nd chunk.
+
 Miscellaneous notes:
 ====================
 
diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 9b0c81e..cbc0654 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -79,6 +79,12 @@ static LIST_HEAD(target_list);
 /* This needs to be a spinlock because write_msg() cannot sleep */
 static DEFINE_SPINLOCK(target_list_lock);
 
+/*
+ * Console driver for extended netconsoles.  Registered on the first use to
+ * avoid unnecessarily enabling ext message formatting.
+ */
+static struct console netconsole_ext;
+
 /**
  * struct netconsole_target - Represents a configured netconsole target.
  * @list:	Links this target into the target_list.
@@ -105,6 +111,7 @@ struct netconsole_target {
 	struct config_item	item;
 #endif
 	bool			enabled;
+	bool			extended;
 	struct netpoll		np;
 };
 
@@ -187,6 +194,11 @@ static struct netconsole_target *alloc_param_target(char *target_config)
 	nt->np.remote_port = 6666;
 	eth_broadcast_addr(nt->np.remote_mac);
 
+	if (*target_config == '+') {
+		nt->extended = true;
+		target_config++;
+	}
+
 	/* Parse parameters and setup netpoll */
 	err = netpoll_parse_options(&nt->np, target_config);
 	if (err)
@@ -257,6 +269,11 @@ static ssize_t show_enabled(struct netconsole_target *nt, char *buf)
 	return snprintf(buf, PAGE_SIZE, "%d\n", nt->enabled);
 }
 
+static ssize_t show_extended(struct netconsole_target *nt, char *buf)
+{
+	return snprintf(buf, PAGE_SIZE, "%d\n", nt->extended);
+}
+
 static ssize_t show_dev_name(struct netconsole_target *nt, char *buf)
 {
 	return snprintf(buf, PAGE_SIZE, "%s\n", nt->np.dev_name);
@@ -328,6 +345,11 @@ static ssize_t store_enabled(struct netconsole_target *nt,
 	}
 
 	if (enabled) {	/* true */
+		if (nt->extended && !(netconsole_ext.flags & CON_ENABLED)) {
+			netconsole_ext.flags |= CON_ENABLED;
+			register_console(&netconsole_ext);
+		}
+
 		/*
 		 * Skip netpoll_parse_options() -- all the attributes are
 		 * already configured via configfs. Just print them out.
@@ -355,6 +377,30 @@ static ssize_t store_enabled(struct netconsole_target *nt,
 	return strnlen(buf, count);
 }
 
+static ssize_t store_extended(struct netconsole_target *nt,
+			      const char *buf,
+			      size_t count)
+{
+	int extended;
+	int err;
+
+	if (nt->enabled) {
+		pr_err("target (%s) is enabled, disable to update parameters\n",
+		       config_item_name(&nt->item));
+		return -EINVAL;
+	}
+
+	err = kstrtoint(buf, 10, &extended);
+	if (err < 0)
+		return err;
+	if (extended < 0 || extended > 1)
+		return -EINVAL;
+
+	nt->extended = extended;
+
+	return strnlen(buf, count);
+}
+
 static ssize_t store_dev_name(struct netconsole_target *nt,
 			      const char *buf,
 			      size_t count)
@@ -507,6 +553,7 @@ static struct netconsole_target_attr netconsole_target_##_name =	\
 	__CONFIGFS_ATTR(_name, S_IRUGO | S_IWUSR, show_##_name, store_##_name)
 
 NETCONSOLE_TARGET_ATTR_RW(enabled);
+NETCONSOLE_TARGET_ATTR_RW(extended);
 NETCONSOLE_TARGET_ATTR_RW(dev_name);
 NETCONSOLE_TARGET_ATTR_RW(local_port);
 NETCONSOLE_TARGET_ATTR_RW(remote_port);
@@ -517,6 +564,7 @@ NETCONSOLE_TARGET_ATTR_RW(remote_mac);
 
 static struct configfs_attribute *netconsole_target_attrs[] = {
 	&netconsole_target_enabled.attr,
+	&netconsole_target_extended.attr,
 	&netconsole_target_dev_name.attr,
 	&netconsole_target_local_port.attr,
 	&netconsole_target_remote_port.attr,
@@ -727,6 +775,90 @@ static struct notifier_block netconsole_netdev_notifier = {
 	.notifier_call  = netconsole_netdev_event,
 };
 
+/**
+ * send_ext_msg_udp - send extended log message to target
+ * @nt: target to send message to
+ * @msg: extended log message to send
+ * @msg_len: length of message
+ *
+ * Transfer extended log @msg to @nt.  If @msg is longer than
+ * MAX_PRINT_CHUNK, it'll be split and transmitted in multiple chunks with
+ * ncfrag header field added to identify them.
+ */
+static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
+			     int msg_len)
+{
+	static char buf[MAX_PRINT_CHUNK];
+	const int max_extra_len = sizeof(",ncfrag=0000/0000");
+	const char *header, *body;
+	int header_len = msg_len, body_len = 0;
+	int chunk_len, nr_chunks, i;
+
+	if (msg_len <= MAX_PRINT_CHUNK) {
+		netpoll_send_udp(&nt->np, msg, msg_len);
+		return;
+	}
+
+	/* need to insert extra header fields, detect header and body */
+	header = msg;
+	body = memchr(msg, ';', msg_len);
+	if (body) {
+		header_len = body - header;
+		body_len = msg_len - header_len - 1;
+		body++;
+	}
+
+	chunk_len = MAX_PRINT_CHUNK - header_len - max_extra_len;
+	if (WARN_ON_ONCE(chunk_len <= 0))
+		return;
+
+	/*
+	 * Transfer possibly multiple chunks with extra header fields.
+	 *
+	 * If @msg needs to be split to fit MAX_PRINT_CHUNK, add
+	 * "ncfrag=<byte-offset>/<total-bytes>" to identify each chunk.
+	 */
+	memcpy(buf, header, header_len);
+	nr_chunks = DIV_ROUND_UP(body_len, chunk_len);
+
+	for (i = 0; i < nr_chunks; i++) {
+		int offset = i * chunk_len;
+		int this_header = header_len;
+		int this_chunk = min(body_len - offset, chunk_len);
+
+		if (nr_chunks > 1)
+			this_header += scnprintf(buf + this_header,
+						 sizeof(buf) - this_header,
+						 ",ncfrag=%d/%d;",
+						 offset, body_len);
+
+		if (WARN_ON_ONCE(this_header + chunk_len > MAX_PRINT_CHUNK))
+			return;
+
+		memcpy(buf + this_header, body, this_chunk);
+
+		netpoll_send_udp(&nt->np, buf, this_header + this_chunk);
+
+		body += this_chunk;
+	}
+}
+
+static void write_ext_msg(struct console *con, const char *msg,
+			  unsigned int len)
+{
+	struct netconsole_target *nt;
+	unsigned long flags;
+
+	if ((oops_only && !oops_in_progress) || list_empty(&target_list))
+		return;
+
+	spin_lock_irqsave(&target_list_lock, flags);
+	list_for_each_entry(nt, &target_list, list)
+		if (nt->extended && nt->enabled && netif_running(nt->np.dev))
+			send_ext_msg_udp(nt, msg, len);
+	spin_unlock_irqrestore(&target_list_lock, flags);
+}
+
 static void write_msg(struct console *con, const char *msg, unsigned int len)
 {
 	int frag, left;
@@ -742,7 +874,7 @@ static void write_msg(struct console *con, const char *msg, unsigned int len)
 
 	spin_lock_irqsave(&target_list_lock, flags);
 	list_for_each_entry(nt, &target_list, list) {
-		if (nt->enabled && netif_running(nt->np.dev)) {
+		if (!nt->extended && nt->enabled && netif_running(nt->np.dev)) {
 			/*
 			 * We nest this inside the for-each-target loop above
 			 * so that we're able to get as much logging out to
@@ -761,6 +893,12 @@ static void write_msg(struct console *con, const char *msg, unsigned int len)
 	spin_unlock_irqrestore(&target_list_lock, flags);
 }
 
+static struct console netconsole_ext = {
+	.name	= "netcon_ext",
+	.flags	= CON_EXTENDED,	/* starts disabled, registered on first use */
+	.write	= write_ext_msg,
+};
+
 static struct console netconsole = {
 	.name	= "netcon",
 	.flags	= CON_ENABLED,
@@ -783,7 +921,11 @@ static int __init init_netconsole(void)
 				goto fail;
 			}
 			/* Dump existing printks when we register */
-			netconsole.flags |= CON_PRINTBUFFER;
+			if (nt->extended)
+				netconsole_ext.flags |= CON_PRINTBUFFER |
+							CON_ENABLED;
+			else
+				netconsole.flags |= CON_PRINTBUFFER;
 
 			spin_lock_irqsave(&target_list_lock, flags);
 			list_add(&nt->list, &target_list);
@@ -799,6 +941,8 @@ static int __init init_netconsole(void)
 	if (err)
 		goto undonotifier;
 
+	if (netconsole_ext.flags & CON_ENABLED)
+		register_console(&netconsole_ext);
 	register_console(&netconsole);
 	pr_info("network logging started\n");
 
@@ -827,6 +971,7 @@ static void __exit cleanup_netconsole(void)
 {
 	struct netconsole_target *nt, *tmp;
 
+	unregister_console(&netconsole_ext);
 	unregister_console(&netconsole);
 	dynamic_netconsole_exit();
 	unregister_netdevice_notifier(&netconsole_netdev_notifier);
-- 
2.1.0

Re: [PATCHSET] netconsole: implement extended console support

[Tejun Heo]

Ping?

Thanks.

-- 
tejun

Re: [PATCH v3 3/3] netconsole: implement extended console support

[Sabrina Dubroca]

Hi Tejun,


2015-05-04, 16:04:56 -0400, Tejun Heo wrote:

[...]

> +/**
> + * send_ext_msg_udp - send extended log message to target
> + * @nt: target to send message to
> + * @msg: extended log message to send
> + * @msg_len: length of message
> + *
> + * Transfer extended log @msg to @nt.  If @msg is longer than
> + * MAX_PRINT_CHUNK, it'll be split and transmitted in multiple chunks with
> + * ncfrag header field added to identify them.
> + */
> +static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
> +			     int msg_len)
> +{
> +	static char buf[MAX_PRINT_CHUNK];
> +	const int max_extra_len = sizeof(",ncfrag=0000/0000");

Is msg_len guaranteed < 10000?  Otherwise I think the WARN in the send
loop can trigger.

Also, I think your count is correct because sizeof adds one to the
string's length, but you don't explicitly account for the ';' between
header and body fragment here (and in chunk_len). header_len will stop
before the ;.


> +	const char *header, *body;
> +	int header_len = msg_len, body_len = 0;
> +	int chunk_len, nr_chunks, i;
> +
> +	if (msg_len <= MAX_PRINT_CHUNK) {
> +		netpoll_send_udp(&nt->np, msg, msg_len);
> +		return;
> +	}
> +
> +	/* need to insert extra header fields, detect header and body */
> +	header = msg;
> +	body = memchr(msg, ';', msg_len);
> +	if (body) {
> +		header_len = body - header;
> +		body_len = msg_len - header_len - 1;
> +		body++;
> +	}
> +
> +	chunk_len = MAX_PRINT_CHUNK - header_len - max_extra_len;
> +	if (WARN_ON_ONCE(chunk_len <= 0))
> +		return;
> +
> +	/*
> +	 * Transfer possibly multiple chunks with extra header fields.
> +	 *
> +	 * If @msg needs to be split to fit MAX_PRINT_CHUNK, add
> +	 * "ncfrag=<byte-offset>/<total-bytes>" to identify each chunk.
> +	 */
> +	memcpy(buf, header, header_len);
> +	nr_chunks = DIV_ROUND_UP(body_len, chunk_len);

Wouldn't it be simpler to loop on the remaining size, instead of
doing a division?


> +
> +	for (i = 0; i < nr_chunks; i++) {
> +		int offset = i * chunk_len;
> +		int this_header = header_len;
> +		int this_chunk = min(body_len - offset, chunk_len);
> +
> +		if (nr_chunks > 1)

We already know that there will be more than one chunk, since
you handle msg_len <= MAX_PRINT_CHUNK at the beginning?


> +			this_header += scnprintf(buf + this_header,
> +						 sizeof(buf) - this_header,
> +						 ",ncfrag=%d/%d;",
> +						 offset, body_len);
> +
> +		if (WARN_ON_ONCE(this_header + chunk_len > MAX_PRINT_CHUNK))
> +			return;

This WARN doesn't really seem necessary to me, except for the msg_len
maximum I mentionned earlier.
And if we don't use nr_chunks, we could compute the fragment's length
here in case some computation went wrong.


> +
> +		memcpy(buf + this_header, body, this_chunk);
> +
> +		netpoll_send_udp(&nt->np, buf, this_header + this_chunk);
> +

netpoll_send_udp already does a memcpy (in skb_copy_to_linear_data).
Maybe it would be better to modify netpoll_send_udp, or add a variant
that takes two buffers? or more than two, with something like an iovec?

> +		body += this_chunk;
> +	}
> +}
>
> [...]




Thanks,

-- 
Sabrina

Re: [PATCH v3 3/3] netconsole: implement extended console support

[Tejun Heo]

Hello, Sabrina.

On Sun, May 10, 2015 at 05:11:46PM +0200, Sabrina Dubroca wrote:
> > +static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
> > +			     int msg_len)
> > +{
> > +	static char buf[MAX_PRINT_CHUNK];
> > +	const int max_extra_len = sizeof(",ncfrag=0000/0000");
> 
> Is msg_len guaranteed < 10000?  Otherwise I think the WARN in the send
> loop can trigger.

Yeap, the absolute maximum is 8k.

> Also, I think your count is correct because sizeof adds one to the
> string's length, but you don't explicitly account for the ';' between
> header and body fragment here (and in chunk_len). header_len will stop
> before the ;.

Hmm... ';' would be accounted for in the length of the original
message.  This function just needs to count the extra number of bytes
to be added which means that sizeof() - 1 would be the precise count
here.  Do we care?

> > +	/*
> > +	 * Transfer possibly multiple chunks with extra header fields.
> > +	 *
> > +	 * If @msg needs to be split to fit MAX_PRINT_CHUNK, add
> > +	 * "ncfrag=<byte-offset>/<total-bytes>" to identify each chunk.
> > +	 */
> > +	memcpy(buf, header, header_len);
> > +	nr_chunks = DIV_ROUND_UP(body_len, chunk_len);
> 
> Wouldn't it be simpler to loop on the remaining size, instead of
> doing a division?

Indeed, this is a remnant of earlier code which emitted total number
of chunks.  Will restructure.

> > +
> > +	for (i = 0; i < nr_chunks; i++) {
> > +		int offset = i * chunk_len;
> > +		int this_header = header_len;
> > +		int this_chunk = min(body_len - offset, chunk_len);
> > +
> > +		if (nr_chunks > 1)
> 
> We already know that there will be more than one chunk, since
> you handle msg_len <= MAX_PRINT_CHUNK at the beginning?

Ditto, earlier code had two types of conditions which trigger this
code path.  Will remove.

> > +			this_header += scnprintf(buf + this_header,
> > +						 sizeof(buf) - this_header,
> > +						 ",ncfrag=%d/%d;",
> > +						 offset, body_len);
> > +
> > +		if (WARN_ON_ONCE(this_header + chunk_len > MAX_PRINT_CHUNK))
> > +			return;
> 
> This WARN doesn't really seem necessary to me, except for the msg_len
> maximum I mentionned earlier.
> And if we don't use nr_chunks, we could compute the fragment's length
> here in case some computation went wrong.

I think it'd be better to keep it tho.  It's one unlikely branch in an
already unlikely path.  It doesn't really cost anything.  Stuff
happens, code changes and it'd suck if e.g. netconsole ends up causing
memory corruption following changes / failures in upper layers.

> > +
> > +		memcpy(buf + this_header, body, this_chunk);
> > +
> > +		netpoll_send_udp(&nt->np, buf, this_header + this_chunk);
> > +
> 
> netpoll_send_udp already does a memcpy (in skb_copy_to_linear_data).
> Maybe it would be better to modify netpoll_send_udp, or add a variant
> that takes two buffers? or more than two, with something like an iovec?

The splitting is a very rare event.  I don't think we need to be
worrying about its performance at this level.

Thanks.

-- 
tejun