[PATCH 23/25] dynamic_debug: document pending queries, flags-filter, multiple queries

Jim Cromie
2011-07-25T21:44:00Z

Add section on pending-queries. Rework flags description to include
flags-filter, rearrange docs into 3 parts for the 3 components of
the flags-spec. Reflow some paragraphs.

Add example of a command-file which enables dynamic-debugging on
the dynamic debugging facility itself. Example also shows commenting,
describes constraints.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
Documentation/dynamic-debug-howto.txt | 221 ++++++++++++++++++++++-----------
1 files changed, 147 insertions(+), 74 deletions(-)

diff --git a/Documentation/dynamic-debug-howto.txt b/Documentation/dynamic-debug-howto.txt
index f959909..c1a4f42 100644
--- a/Documentation/dynamic-debug-howto.txt
+++ b/Documentation/dynamic-debug-howto.txt
@@ -4,15 +4,15 @@ Introduction

This document describes how to use the dynamic debug (ddebug) feature.

-Dynamic debug is designed to allow you to dynamically enable/disable kernel
-code to obtain additional kernel information. Currently, if
-CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_dbg() calls can be
-dynamically enabled per-callsite.
+Dynamic debug is designed to allow you to dynamically enable/disable
+kernel code to obtain additional kernel information. If kernel is
+built with CONFIG_DYNAMIC_DEBUG=y, then all pr_debug()/dev_dbg() calls
+can be dynamically enabled per-callsite.

Dynamic debug has even more useful features:

- * Simple query language allows turning on and off debugging statements by
- matching any combination of:
+ * Simple query language allows turning on and off debugging
+ statements by matching any combination of:

- source filename
- function name
@@ -20,25 +20,32 @@ Dynamic debug has even more useful features:
- module name
- format string

- * Provides a debugfs control file: <debugfs>/dynamic_debug/control which can be
- read to display the complete list of known debug statements, to help guide you
+ * Provides a debugfs control file: <debugfs>/dynamic_debug/control
+ which can be read to display the complete list of known debug
+ statements, to help guide you.

Controlling dynamic debug Behaviour
===================================

The behaviour of pr_debug()/dev_dbg()s are controlled via writing to a
-control file in the 'debugfs' filesystem. Thus, you must first mount the debugfs
-filesystem, in order to make use of this feature. Subsequently, we refer to the
-control file as: <debugfs>/dynamic_debug/control. For example, if you want to
-enable printing from source file 'svcsock.c', line 1603 you simply do:
+control file in the 'debugfs' filesystem. Thus, you must first mount
+the debugfs filesystem, in order to make use of this feature.
+Subsequently, we refer to the control file as:
+$DEBUGFS/dynamic_debug/control.

-nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
- <debugfs>/dynamic_debug/control
+# a simple helper shell-function, to shorten examples
+function dbg_query() {
+ echo "$*" > $DEBUGFS/dynamic_debug/control
+}
+
+For example, if you want to enable
+printing from source file 'svcsock.c', line 1603 you simply do:
+
+nullarbor:~ # dbg_query file svcsock.c line 1603 +p

If you make a mistake with the syntax, the write will fail thus:

-nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
- <debugfs>/dynamic_debug/control
+nullarbor:~ # dbg_query 'file svcsock.c wtf 1 +p'
-bash: echo: write error: Invalid argument

Viewing Dynamic Debug Behaviour
@@ -47,32 +54,33 @@ Viewing Dynamic Debug Behaviour
You can view the currently configured behaviour of all the debug statements
via:

-nullarbor:~ # cat <debugfs>/dynamic_debug/control
+nullarbor:~ # cat $DEBUGFS/dynamic_debug/control
# filename:lineno [module]function flags format
-/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup - "SVCRDMA Module Removed, deregister RPC RDMA transport\012"
-/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init - "\011max_inline : %d\012"
-/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init - "\011sq_depth : %d\012"
-/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init - "\011max_requests : %d\012"
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup =_ "SVCRDMA Module Removed, deregister RPC RDMA transport\012"
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline : %d\012"
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth : %d\012"
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests : %d\012"
...

You can also apply standard Unix text manipulation filters to this
data, e.g.

-nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control | wc -l
+nullarbor:~ # grep -i rdma $DEBUGFS/dynamic_debug/control | wc -l
62

-nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
+nullarbor:~ # grep -i tcp $DEBUGFS/dynamic_debug/control | wc -l
42

-Note in particular that the third column shows the enabled behaviour
-flags for each debug statement callsite (see below for definitions of the
-flags). The default value, no extra behaviour enabled, is "-". So
-you can view all the debug statement callsites with any non-default flags:
+The third column shows the current flag settings for each debug
+statement callsite (menmonic: what they equal to, see below for
+definitions of the flags). The default value, no extra behaviour
+enabled, is "=_". So you can view all the debug statement callsites
+with any non-default flags:

-nullarbor:~ # awk '$3 != "-"' <debugfs>/dynamic_debug/control
+nullarbor:~ # grep -v "=_" $DEBUGFS/dynamic_debug/control
# filename:lineno [module]function flags format
-/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c:1603 [sunrpc]svc_send p "svc_process: st_sendto returned %d\012"
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c:1603 [sunrpc]svc_send =p "svc_process: st_sendto returned %d\012"

Command Language Reference
@@ -84,16 +92,31 @@ separators and do *not* end a command or allow multiple commands to
be done together. So these are all equivalent:

nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control
nullarbor:~ # echo -c ' file svcsock.c line 1603 +p ' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control
nullarbor:~ # echo -c 'file svcsock.c\nline 1603 +p' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control
nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control
+
+Commands are bounded by a write() system call. Subject to this limit
+(or 1024 for boot-line parameter) you can send multiple commands,
+separated by ';' or '\n'
+
+foo:~ # echo "module nsc_gpio +p ; module pc8736x_gpio +p ; " \
+ > $DEBUGFS/dynamic_debug/control

-Commands are bounded by a write() system call. If you want to do
-multiple commands you need to do a separate "echo" for each, like:
+foo:~ # cat queries
+ module nsc_gpio +p ;
+ module pc8736x_gpio +p
+foo:~ # cat queries > $DEBUGFS/dynamic_debug/control
+
+Multiple commands are processed independently, this allows you to send
+commands which may fail, for example if a module is not present. The
+last failing command returns its error.
+
+Or you can do an "echo" for each, like:

nullarbor:~ # echo 'file svcsock.c line 1603 +p' > /proc/dprintk ;\
> echo 'file svcsock.c line 1563 +p' > /proc/dprintk
@@ -110,11 +133,12 @@ specifications, followed by a flags change specification.

command ::= match-spec* flags-spec

-The match-spec's are used to choose a subset of the known dprintk()
+The match-specs are used to choose a subset of the known dprintk()
callsites to which to apply the flags-spec. Think of them as a query
-with implicit ANDs between each pair. Note that an empty list of
-match-specs is possible, but is not very useful because it will not
-match any debug statement callsites.
+with implicit ANDs between each pair. This means that multiple specs
+of a given type are nonsense; a module cannot match A and B
+simultaneously. Note that an empty list of match-specs matches all
+callsites.

A match specification comprises a keyword, which controls the attribute
of the callsite to be compared, and a value to compare against. Possible
@@ -125,6 +149,7 @@ match-spec ::= 'func' string |
'module' string |
'format' string |
'line' line-range
+Note: no wildcards or regexs are accepted

line-range ::= lineno |
'-'lineno |
@@ -190,36 +215,55 @@ line
line -1605 // the 1605 lines from line 1 to line 1605
line 1600- // all lines from line 1600 to the end of the file

-The flags specification comprises a change operation followed
-by one or more flag characters. The change operation is one
-of the characters:
-
--
- remove the given flags
+The flags specification matches the regexp: ^[flmpta_]*[-+=][flmpta_]*$
+and has 3 parts:

-+
- add the given flags
+flags filter (optional):
+ Constrains matching to thoses callsite with given flags set,
+ allows altering currently enabled callsites.
+ #> dbg_query pm+t # add t to enabled sites which have m
+ #> dbg_query p+t # add t to all enabled sites, both m and !m
+ #> dbg_query tmf-p # disable sites with 'tmf'
+ #> dbg_query tmf+p # re-enable those disabled sites

-=
- set the flags to the given flags
+flags change operation:
+ '-' remove the given flags
+ '+' add the given flags
+ '=' set the flags to the given flags

The flags are:
-
-f
- Include the function name in the printed message
-l
- Include line number in the printed message
-m
- Include module name in the printed message
-p
- Causes a printk() message to be emitted to dmesg
-t
- Include thread ID in messages not generated from interrupt context
-
-Note the regexp ^[-+=][flmpt]+$ matches a flags specification.
-Note also that there is no convenient syntax to remove all
-the flags at once, you need to use "-flmpt".
-
+ 'f' Include the function name in the printed message
+ 'l' Include line number in the printed message
+ 'm' Include module name in the printed message
+ 'p' Causes a printk() message to be emitted to dmesg
+ 't' Include thread ID in messages not generated from interrupt context
+ 'a' Add query to pending queries if not directly applicable
+ '_' default/null flag.
+ Primarily for display, so grep "=_" can avoid " = " in format strings.
+ Also usable (but not required) to clear all flags.
+
+Pending queries
+===============
+
+Queries submitted with 'a' are added to a pending list IFF they don't
+match against current callsites. This allows you to add queries for
+modules before they're loaded, which enables pr_debug()s used in
+initialization. To better support module debugging, pending queries
+remain on the list through modprobe-rmmod cycles. To remove a pending query, resubmit it with zeroed flags:
+
+#> dbg_query module foo +apt # original pending query
+#> dbg_query module foo a= # zero pending query to remove
+#> dbg_query module foo a-a # also zeros, removes
+#> dbg_query module foo a-ap # zeros already 0-flag, removes
+
+There are a few subtleties; removing or altering a pending query
+requires an exact match on the query-spec, but not on the flags
+filter. So 2nd query does not clear 1st.
+
+#> dbg_query module foo line 100-200 +ap
+#> dbg_query module foo ap=
+
+TBR. some issues remain..

Debug messages during boot process
==================================
@@ -232,37 +276,66 @@ QUERY follows the syntax described above, but must not exceed 1023
characters. The enablement of debug messages is done as an arch_initcall.
Thus you can enable debug messages in all code processed after this
arch_initcall via this boot parameter.
-On an x86 system for example ACPI enablement is a subsys_initcall and
-ddebug_query="file ec.c +p"
+
+On an x86 system for example ACPI enablement is a subsys_initcall, so
+ ddebug_query="file ec.c +p"
will show early Embedded Controller transactions during ACPI setup if
your machine (typically a laptop) has an Embedded Controller.
PCI (or other devices) initialization also is a hot candidate for using
this boot parameter for debugging purposes.

-
Examples
========

// enable the message at line 1603 of file svcsock.c
nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control

// enable all the messages in file svcsock.c
nullarbor:~ # echo -n 'file svcsock.c +p' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control

// enable all the messages in the NFS server module
nullarbor:~ # echo -n 'module nfsd +p' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control

// enable all 12 messages in the function svc_process()
nullarbor:~ # echo -n 'func svc_process +p' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control

// disable all 12 messages in the function svc_process()
nullarbor:~ # echo -n 'func svc_process -p' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control

// enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
- <debugfs>/dynamic_debug/control
+ $DEBUGFS/dynamic_debug/control
+
+// enable dynamic-debugging on dynamic-debugging facility
+root@voyage:~# cat debugfs-file
+
+ # blank lines ok
+ module dynamic_debug +p ; # turn on self-debugging
+ # these are quite noisy when grepping
+ # $DEBUGFS/dynamic_debug/{control,pending}
+ # silence them (also, leading spaces allowed in comments)
+ func ddebug_proc_show -p
+ func ddebug_proc_next -p ; # trailing comments need cmd terminator
+ func pending_proc_show -p
+ func pending_proc_next -p
+
+root@voyage:~# cat debugfs-file > /dbg/dynamic_debug/control
+split into words: "module" "dynamic_debug" "+p"
+changed $srcroot/lib/dynamic_debug.c:223 [dynamic_debug]ddebug_change =p
+changed $srcroot/lib/dynamic_debug.c:576 [dynamic_debug]ddebug_save_pending =p
+....
+nfound 23 on func="" file="" module="dynamic_debug" format="" lineno=0-0
+query 1: "func ddebug_proc_show -p "
+split into words: "func" "ddebug_proc_show" "-p"
+parsed func="ddebug_proc_show" file="" module="" format="" lineno=0-0
+filter=0x0 op='-' flags=0x1 *flagsp=0x0 *maskp=0xfffffffe
+changed $srcroot/lib/dynamic_debug.c:881 [dynamic_debug]ddebug_proc_show =_
+nfound 1 on func="ddebug_proc_show" file="" module="" format="" lineno=0-0
+query 2: "func ddebug_proc_next -p"
+...
+
--
1.7.4.1