[PATCH] selftests/ftrace: Fix trace_marker_raw test on 64K page kernels

[PATCH] selftests/ftrace: Fix trace_marker_raw test on 64K page kernels

[Tianchen Ding]

On ARM64 kernels with 64K pages, the trace_marker_raw test fails because
bash's printf builtin uses stdio buffering which splits output into
multiple small write() calls to the tracefs file. Since each individual
write is within TRACE_MARKER_MAX_SIZE (4096), they all succeed, causing
the "too big" write test to incorrectly pass.

Fix by piping make_str output through dd with iflag=fullblock to
guarantee a single atomic write() syscall to trace_marker_raw.

Fixes: 37f46601383a ("selftests/tracing: Add basic test for trace_marker_raw file")
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
---
 .../selftests/ftrace/test.d/00basic/trace_marker_raw.tc     | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
index 8e905d4fe6dd..efd8263e6087 100644
--- a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
+++ b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
@@ -43,8 +43,10 @@ write_buffer() {
 	id=$1
 	size=$2
 
-	# write the string into the raw marker
-	make_str $id $size > trace_marker_raw
+	# Pipe through dd to ensure a single atomic write() syscall.
+	# Shell's printf builtin uses stdio buffering which may split the
+	# output into multiple writes.
+	make_str $id $size | dd of=trace_marker_raw bs=`expr $size + 4` iflag=fullblock
 }
 
 
-- 
2.39.3

Re: [PATCH] selftests/ftrace: Fix trace_marker_raw test on 64K page kernels

[Steven Rostedt]

On Wed, 27 May 2026 17:54:37 +0800
Tianchen Ding <dtcccc@linux.alibaba.com> wrote:

> On ARM64 kernels with 64K pages, the trace_marker_raw test fails because
> bash's printf builtin uses stdio buffering which splits output into
> multiple small write() calls to the tracefs file. Since each individual
> write is within TRACE_MARKER_MAX_SIZE (4096), they all succeed, causing
> the "too big" write test to incorrectly pass.
> 
> Fix by piping make_str output through dd with iflag=fullblock to
> guarantee a single atomic write() syscall to trace_marker_raw.
> 
> Fixes: 37f46601383a ("selftests/tracing: Add basic test for trace_marker_raw file")
> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
> ---
>  .../selftests/ftrace/test.d/00basic/trace_marker_raw.tc     | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
> index 8e905d4fe6dd..efd8263e6087 100644
> --- a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
> +++ b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
> @@ -43,8 +43,10 @@ write_buffer() {
>  	id=$1
>  	size=$2
>  
> -	# write the string into the raw marker
> -	make_str $id $size > trace_marker_raw
> +	# Pipe through dd to ensure a single atomic write() syscall.
> +	# Shell's printf builtin uses stdio buffering which may split the
> +	# output into multiple writes.

Could you comment that this is for architectures with 64K pages too.

Thanks for fixing this,

-- Steve

> +	make_str $id $size | dd of=trace_marker_raw bs=`expr $size + 4` iflag=fullblock
>  }
>  
>  

[PATCH v2] selftests/ftrace: Fix trace_marker_raw test on 64K page kernels

[Tianchen Ding]

On ARM64 kernels with 64K pages, the trace_marker_raw test fails because
bash's printf builtin uses stdio buffering which splits output into
multiple small write() calls to the tracefs file. Since each individual
write is within TRACE_MARKER_MAX_SIZE (4096), they all succeed, causing
the "too big" write test to incorrectly pass.

Fix by piping make_str output through dd with iflag=fullblock to
guarantee a single atomic write() syscall to trace_marker_raw.

Fixes: 37f46601383a ("selftests/tracing: Add basic test for trace_marker_raw file")
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
---
v2:
Update comment about 64K pages.
---
 .../selftests/ftrace/test.d/00basic/trace_marker_raw.tc    | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
index 8e905d4fe6dd..f68f1901f65f 100644
--- a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
+++ b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
@@ -43,8 +43,11 @@ write_buffer() {
 	id=$1
 	size=$2
 
-	# write the string into the raw marker
-	make_str $id $size > trace_marker_raw
+	# Pipe through dd to ensure a single atomic write() syscall
+	# on architectures with 64K pages, where shell's printf builtin
+	# uses stdio buffering which may split the output into multiple
+	# writes.
+	make_str $id $size | dd of=trace_marker_raw bs=`expr $size + 4` iflag=fullblock
 }
 
 
-- 
2.39.3

Re: [PATCH v2] selftests/ftrace: Fix trace_marker_raw test on 64K page kernels

[Steven Rostedt]

On Thu, 28 May 2026 10:24:17 +0800
Tianchen Ding <dtcccc@linux.alibaba.com> wrote:

> 
> diff --git a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
> index 8e905d4fe6dd..f68f1901f65f 100644
> --- a/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
> +++ b/tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc
> @@ -43,8 +43,11 @@ write_buffer() {
>  	id=$1
>  	size=$2
>  
> -	# write the string into the raw marker
> -	make_str $id $size > trace_marker_raw
> +	# Pipe through dd to ensure a single atomic write() syscall
> +	# on architectures with 64K pages, where shell's printf builtin
> +	# uses stdio buffering which may split the output into multiple
> +	# writes.
> +	make_str $id $size | dd of=trace_marker_raw bs=`expr $size + 4` iflag=fullblock

I was looking at this more, and I'm not comfortable with the hard coded
4 above. I rather use the length of the string. Something like:

	str=`make_str $id $size`
	len=${#str}
	echo "$str" | dd of=trace_marker_raw bs=$len iflag=fullblock

-- Steve

>  }
>  
>