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[PATCH 05/12] tracing: kdb: Use tracer_tracing_on/off() instead of setting per CPU disabled

Steven Rostedt posted 12 patches 9 months, 1 week ago
There is a newer version of this series
[PATCH 05/12] tracing: kdb: Use tracer_tracing_on/off() instead of setting per CPU disabled
Posted by Steven Rostedt 9 months, 1 week ago 
From: Steven Rostedt <rostedt@goodmis.org>

The per CPU "disabled" value was the original way to disable tracing when
the tracing subsystem was first created. Today, the ring buffer
infrastructure has its own way to disable tracing. In fact, things have
changed so much since 2008 that many things ignore the disable flag.

The kdb_ftdump() function iterates over all the current tracing CPUs and
increments the "disabled" counter before doing the dump, and decrements it
afterward.

As the disabled flag can be ignored, doing this today is not reliable.
Instead, simply call tracer_tracing_off() and then tracer_tracing_on() to
disable and then enabled the entire ring buffer in one go!

Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Daniel Thompson <danielt@kernel.org>
Cc: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/trace_kdb.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index 1e72d20b3c2f..b5cf3fdde8cb 100644
--- a/kernel/trace/trace_kdb.c
+++ b/kernel/trace/trace_kdb.c
@@ -120,9 +120,7 @@ static int kdb_ftdump(int argc, const char **argv)
 	trace_init_global_iter(&iter);
 	iter.buffer_iter = buffer_iter;
 
-	for_each_tracing_cpu(cpu) {
-		atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
-	}
+	tracer_tracing_off(iter.tr);
 
 	/* A negative skip_entries means skip all but the last entries */
 	if (skip_entries < 0) {
@@ -135,9 +133,7 @@ static int kdb_ftdump(int argc, const char **argv)
 
 	ftrace_dump_buf(skip_entries, cpu_file);
 
-	for_each_tracing_cpu(cpu) {
-		atomic_dec(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
-	}
+	tracer_tracing_on(iter.tr);
 
 	kdb_trap_printk--;
 
-- 
2.47.2
Re: [PATCH 05/12] tracing: kdb: Use tracer_tracing_on/off() instead of setting per CPU disabled
Posted by Doug Anderson 9 months, 1 week ago 
Hi,

On Fri, May 2, 2025 at 1:53 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> From: Steven Rostedt <rostedt@goodmis.org>
>
> The per CPU "disabled" value was the original way to disable tracing when
> the tracing subsystem was first created. Today, the ring buffer
> infrastructure has its own way to disable tracing. In fact, things have
> changed so much since 2008 that many things ignore the disable flag.
>
> The kdb_ftdump() function iterates over all the current tracing CPUs and
> increments the "disabled" counter before doing the dump, and decrements it
> afterward.
>
> As the disabled flag can be ignored, doing this today is not reliable.
> Instead, simply call tracer_tracing_off() and then tracer_tracing_on() to
> disable and then enabled the entire ring buffer in one go!
>
> Cc: Jason Wessel <jason.wessel@windriver.com>
> Cc: Daniel Thompson <danielt@kernel.org>
> Cc: Douglas Anderson <dianders@chromium.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  kernel/trace/trace_kdb.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
> index 1e72d20b3c2f..b5cf3fdde8cb 100644
> --- a/kernel/trace/trace_kdb.c
> +++ b/kernel/trace/trace_kdb.c
> @@ -120,9 +120,7 @@ static int kdb_ftdump(int argc, const char **argv)
>         trace_init_global_iter(&iter);
>         iter.buffer_iter = buffer_iter;
>
> -       for_each_tracing_cpu(cpu) {
> -               atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
> -       }
> +       tracer_tracing_off(iter.tr);
>
>         /* A negative skip_entries means skip all but the last entries */
>         if (skip_entries < 0) {
> @@ -135,9 +133,7 @@ static int kdb_ftdump(int argc, const char **argv)
>
>         ftrace_dump_buf(skip_entries, cpu_file);
>
> -       for_each_tracing_cpu(cpu) {
> -               atomic_dec(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
> -       }
> +       tracer_tracing_on(iter.tr);

This new change seems less safe than the old one. Previously you'd
always increment by one at the start of the function and decrement by
one at the end. Now at the start of the function you'll set
"buffer_disabled" to 1 and at the end you'll set it to 0. If
"buffer_disabled" was already 1 at the start of the function your new
sequence will end up having the side effect of changing it to 0.

-Doug
Re: [PATCH 05/12] tracing: kdb: Use tracer_tracing_on/off() instead of setting per CPU disabled
Posted by Steven Rostedt 9 months, 1 week ago 
On Mon, 5 May 2025 08:42:52 -0700
Doug Anderson <dianders@chromium.org> wrote:

> This new change seems less safe than the old one. Previously you'd

Well, it matters what you your definition of "safe" is ;-)

The new change prevents the ring buffer from having anything written to it,
where as the old change didn't disable everything.

> always increment by one at the start of the function and decrement by
> one at the end. Now at the start of the function you'll set
> "buffer_disabled" to 1 and at the end you'll set it to 0. If
> "buffer_disabled" was already 1 at the start of the function your new
> sequence will end up having the side effect of changing it to 0.

Good point. How about I add a tracer_tracing_disable() and
tracer_tracing_enable() that is not an on off switch and uses:
ring_buffer_disable/enable() that decrements/increments disabling of the
ring buffer?

That way it keeps the same semantics.

-- STeve
Re: [PATCH 05/12] tracing: kdb: Use tracer_tracing_on/off() instead of setting per CPU disabled
Posted by kernel test robot 9 months, 1 week ago 
Hi Steven,

kernel test robot noticed the following build warnings:

[auto build test WARNING on trace/for-next]
[also build test WARNING on linus/master v6.15-rc4 next-20250502]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Steven-Rostedt/tracing-mmiotrace-Remove-reference-to-unused-per-CPU-data-pointer/20250503-050317
base:   https://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace for-next
patch link:    https://lore.kernel.org/r/20250502205348.643055437%40goodmis.org
patch subject: [PATCH 05/12] tracing: kdb: Use tracer_tracing_on/off() instead of setting per CPU disabled
config: arc-randconfig-002-20250503 (https://download.01.org/0day-ci/archive/20250505/202505051213.exaXF8qp-lkp@intel.com/config)
compiler: arc-linux-gcc (GCC) 11.5.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250505/202505051213.exaXF8qp-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202505051213.exaXF8qp-lkp@intel.com/

All warnings (new ones prefixed by >>):

   kernel/trace/trace_kdb.c: In function 'kdb_ftdump':
>> kernel/trace/trace_kdb.c:101:13: warning: unused variable 'cpu' [-Wunused-variable]
     101 |         int cpu;
         |             ^~~


vim +/cpu +101 kernel/trace/trace_kdb.c

955b61e5979847 Jason Wessel     2010-08-05   91  
955b61e5979847 Jason Wessel     2010-08-05   92  /*
955b61e5979847 Jason Wessel     2010-08-05   93   * kdb_ftdump - Dump the ftrace log buffer
955b61e5979847 Jason Wessel     2010-08-05   94   */
955b61e5979847 Jason Wessel     2010-08-05   95  static int kdb_ftdump(int argc, const char **argv)
955b61e5979847 Jason Wessel     2010-08-05   96  {
dbfe67334a1767 Douglas Anderson 2019-03-19   97  	int skip_entries = 0;
19063c776fe745 Jason Wessel     2010-08-05   98  	long cpu_file;
0c10cc2435115c Yuran Pereira    2024-10-28   99  	int err;
03197fc02b3566 Douglas Anderson 2019-03-19  100  	int cnt;
03197fc02b3566 Douglas Anderson 2019-03-19 @101  	int cpu;
955b61e5979847 Jason Wessel     2010-08-05  102  
19063c776fe745 Jason Wessel     2010-08-05  103  	if (argc > 2)
955b61e5979847 Jason Wessel     2010-08-05  104  		return KDB_ARGCOUNT;
955b61e5979847 Jason Wessel     2010-08-05  105  
0c10cc2435115c Yuran Pereira    2024-10-28  106  	if (argc && kstrtoint(argv[1], 0, &skip_entries))
0c10cc2435115c Yuran Pereira    2024-10-28  107  		return KDB_BADINT;
955b61e5979847 Jason Wessel     2010-08-05  108  
19063c776fe745 Jason Wessel     2010-08-05  109  	if (argc == 2) {
0c10cc2435115c Yuran Pereira    2024-10-28  110  		err = kstrtol(argv[2], 0, &cpu_file);
0c10cc2435115c Yuran Pereira    2024-10-28  111  		if (err || cpu_file >= NR_CPUS || cpu_file < 0 ||
19063c776fe745 Jason Wessel     2010-08-05  112  		    !cpu_online(cpu_file))
19063c776fe745 Jason Wessel     2010-08-05  113  			return KDB_BADINT;
19063c776fe745 Jason Wessel     2010-08-05  114  	} else {
ae3b5093ad6004 Steven Rostedt   2013-01-23  115  		cpu_file = RING_BUFFER_ALL_CPUS;
19063c776fe745 Jason Wessel     2010-08-05  116  	}
19063c776fe745 Jason Wessel     2010-08-05  117  
955b61e5979847 Jason Wessel     2010-08-05  118  	kdb_trap_printk++;
03197fc02b3566 Douglas Anderson 2019-03-19  119  
03197fc02b3566 Douglas Anderson 2019-03-19  120  	trace_init_global_iter(&iter);
03197fc02b3566 Douglas Anderson 2019-03-19  121  	iter.buffer_iter = buffer_iter;
03197fc02b3566 Douglas Anderson 2019-03-19  122  
6a4611f6bb763d Steven Rostedt   2025-05-02  123  	tracer_tracing_off(iter.tr);
03197fc02b3566 Douglas Anderson 2019-03-19  124  
03197fc02b3566 Douglas Anderson 2019-03-19  125  	/* A negative skip_entries means skip all but the last entries */
03197fc02b3566 Douglas Anderson 2019-03-19  126  	if (skip_entries < 0) {
03197fc02b3566 Douglas Anderson 2019-03-19  127  		if (cpu_file == RING_BUFFER_ALL_CPUS)
03197fc02b3566 Douglas Anderson 2019-03-19  128  			cnt = trace_total_entries(NULL);
03197fc02b3566 Douglas Anderson 2019-03-19  129  		else
03197fc02b3566 Douglas Anderson 2019-03-19  130  			cnt = trace_total_entries_cpu(NULL, cpu_file);
03197fc02b3566 Douglas Anderson 2019-03-19  131  		skip_entries = max(cnt + skip_entries, 0);
03197fc02b3566 Douglas Anderson 2019-03-19  132  	}
03197fc02b3566 Douglas Anderson 2019-03-19  133  
dbfe67334a1767 Douglas Anderson 2019-03-19  134  	ftrace_dump_buf(skip_entries, cpu_file);
03197fc02b3566 Douglas Anderson 2019-03-19  135  
6a4611f6bb763d Steven Rostedt   2025-05-02  136  	tracer_tracing_on(iter.tr);
03197fc02b3566 Douglas Anderson 2019-03-19  137  
955b61e5979847 Jason Wessel     2010-08-05  138  	kdb_trap_printk--;
955b61e5979847 Jason Wessel     2010-08-05  139  
955b61e5979847 Jason Wessel     2010-08-05  140  	return 0;
955b61e5979847 Jason Wessel     2010-08-05  141  }
955b61e5979847 Jason Wessel     2010-08-05  142  

-- 
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