[PATCH] percpu: improve percpu_alloc_percpu_fail event trace

From: George Guo <redacted>

Add do_warn, warn_limit fields to the output of the
percpu_alloc_percpu_fail ftrace event.

This is required to percpu_alloc failed with no warning showing.

Signed-off-by: George Guo <redacted>
---
 include/trace/events/percpu.h | 22 ++++++++++++++--------
 mm/percpu.c                   |  2 +-
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/include/trace/events/percpu.h b/include/trace/events/percpu.h
index 5b8211ca8950..c5f412e84bb8 100644
--- a/include/trace/events/percpu.h
+++ b/include/trace/events/percpu.h

@@ -75,15 +75,18 @@ TRACE_EVENT(percpu_free_percpu,
 
 TRACE_EVENT(percpu_alloc_percpu_fail,
 
-	TP_PROTO(bool reserved, bool is_atomic, size_t size, size_t align),
+	TP_PROTO(bool reserved, bool is_atomic, size_t size, size_t align,
+		 bool do_warn, int warn_limit),
 
-	TP_ARGS(reserved, is_atomic, size, align),
+	TP_ARGS(reserved, is_atomic, size, align, do_warn, warn_limit),
 
 	TP_STRUCT__entry(
-		__field(	bool,	reserved	)
-		__field(	bool,	is_atomic	)
-		__field(	size_t,	size		)
-		__field(	size_t, align		)
+		__field(bool,	reserved)
+		__field(bool,	is_atomic)
+		__field(size_t,	size)
+		__field(size_t, align)
+		__field(bool,	do_warn)
+		__field(int,	warn_limit)
 	),
 
 	TP_fast_assign(

@@ -91,11 +94,14 @@ TRACE_EVENT(percpu_alloc_percpu_fail,
 		__entry->is_atomic	= is_atomic;
 		__entry->size		= size;
 		__entry->align		= align;
+		__entry->do_warn	= do_warn;
+		__entry->warn_limit	= warn_limit;
 	),
 
-	TP_printk("reserved=%d is_atomic=%d size=%zu align=%zu",
+	TP_printk("reserved=%d is_atomic=%d size=%zu align=%zu do_warn=%d, warn_limit=%d",
 		  __entry->reserved, __entry->is_atomic,
-		  __entry->size, __entry->align)
+		  __entry->size, __entry->align,
+		  __entry->do_warn, __entry->warn_limit)
 );
 
 TRACE_EVENT(percpu_create_chunk,

diff --git a/mm/percpu.c b/mm/percpu.c
index 4e11fc1e6def..ac5b48268c99 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c

@@ -1886,7 +1886,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
 fail_unlock:
 	spin_unlock_irqrestore(&pcpu_lock, flags);
 fail:
-	trace_percpu_alloc_percpu_fail(reserved, is_atomic, size, align);
+	trace_percpu_alloc_percpu_fail(reserved, is_atomic, size, align, do_warn, warn_limit);
 
 	if (do_warn && warn_limit) {
 		pr_warn("allocation failed, size=%zu align=%zu atomic=%d, %s\n",

-- 
2.34.1

On Mon, 22 Jan 2024 15:36:29 +0800
George Guo [off-list ref] wrote:

From: George Guo <redacted>

Add do_warn, warn_limit fields to the output of the
percpu_alloc_percpu_fail ftrace event.

This is required to percpu_alloc failed with no warning showing.

You mean to state;

  In order to know why percpu_alloc failed but produces no warnings, the
  do_warn and warn_limit should be traced to let the user know it was
  rate-limited.

Or something like that?

Honestly, I don't think that the trace event is the proper place to do
that. The trace event just shows that it did fail. If you are confused to
why it doesn't print to dmesg, then you can simply add a kprobe to see
those values as well.

-- Steve

Signed-off-by: George Guo <redacted>
---

On Mon, 22 Jan 2024 10:57:00 -0500
Steven Rostedt [off-list ref] wrote:

On Mon, 22 Jan 2024 15:36:29 +0800
George Guo [off-list ref] wrote:

quoted

From: George Guo <redacted>

Add do_warn, warn_limit fields to the output of the
percpu_alloc_percpu_fail ftrace event.

This is required to percpu_alloc failed with no warning showing.  

You mean to state;

  In order to know why percpu_alloc failed but produces no warnings,
the do_warn and warn_limit should be traced to let the user know it
was rate-limited.

Or something like that?

Honestly, I don't think that the trace event is the proper place to do
that. The trace event just shows that it did fail. If you are
confused to why it doesn't print to dmesg, then you can simply add a
kprobe to see those values as well.

-- Steve

quoted

Signed-off-by: George Guo <redacted>
--- 

There are two reasons of percpu_alloc failed without warnings: 

1. do_warn is false
2. do_warn is true and warn_limit is reached the limit.

Showing do_warn and warn_limit makes things simple, maybe dont need
kprobe again.

On Tue, 23 Jan 2024 09:44:43 +0800
George Guo [off-list ref] wrote:

There are two reasons of percpu_alloc failed without warnings: 

1. do_warn is false
2. do_warn is true and warn_limit is reached the limit.

Yes I know the reasons.

Showing do_warn and warn_limit makes things simple, maybe dont need
kprobe again.

It's up to the maintainers of that code to decide if it's worth it or not,
but honestly, my opinion it is not.

The trace event in question is to trace that percpu_alloc failed and why.
It's not there to determine why it did not produce a printk message.

-- Steve

Hello,

On Mon, Jan 22, 2024 at 08:55:39PM -0500, Steven Rostedt wrote:

On Tue, 23 Jan 2024 09:44:43 +0800
George Guo [off-list ref] wrote:

quoted

There are two reasons of percpu_alloc failed without warnings: 

1. do_warn is false
2. do_warn is true and warn_limit is reached the limit.

Yes I know the reasons.

quoted

Showing do_warn and warn_limit makes things simple, maybe dont need
kprobe again.

It's up to the maintainers of that code to decide if it's worth it or not,
but honestly, my opinion it is not.

I agree, I don't think this is a worthwhile change. If we do change
this, I'd like it to be more actionable in some way and as a result
something we can fix or tune accordingly.

George is this a common problem you're seeing?

The trace event in question is to trace that percpu_alloc failed and why.
It's not there to determine why it did not produce a printk message.

-- Steve

Thanks,
Dennis