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Date:   Mon, 14 Feb 2022 13:42:38 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Sven Schnelle <svens@linux.ibm.com>
Cc:     Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ftrace: ensure trace buffer is at least 4096 bytes
 large
Message-ID: <20220214134238.20255872@gandalf.local.home>
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On Mon, 14 Feb 2022 14:44:56 +0100
Sven Schnelle <svens@linux.ibm.com> wrote:

> Booting the kernel with 'trace_buf_size=1' give a warning at
> boot during the ftrace selftests:
> 
> [    0.892809] Running postponed tracer tests:
> [    0.892893] Testing tracer function:
> [    0.901899] Callback from call_rcu_tasks_trace() invoked.
> [    0.983829] Callback from call_rcu_tasks_rude() invoked.
> [    1.072003] .. bad ring buffer .. corrupted trace buffer ..
> [    1.091944] Callback from call_rcu_tasks() invoked.
> [    1.097695] PASSED
> [    1.097701] Testing dynamic ftrace: .. filter failed count=0 ..FAILED!
> [    1.353474] ------------[ cut here ]------------
> [    1.353478] WARNING: CPU: 0 PID: 1 at kernel/trace/trace.c:1951 run_tracer_selftest+0x13c/0x1b0
> 
> Therefore enforce a minimum of 4096 bytes to make the selftest pass.
> 
> Signed-off-by: Sven Schnelle <svens@linux.ibm.com>

Thanks,

I'll queue this up for the next merge window.

-- Steve

> ---
>  kernel/trace/trace.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 7c2578efde26..3050892d1812 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -1474,10 +1474,12 @@ static int __init set_buf_size(char *str)
>  	if (!str)
>  		return 0;
>  	buf_size = memparse(str, &str);
> -	/* nr_entries can not be zero */
> -	if (buf_size == 0)
> -		return 0;
> -	trace_buf_size = buf_size;
> +	/*
> +	 * nr_entries can not be zero and the startup
> +	 * tests require some buffer space. Therefore
> +	 * ensure we have at least 4096 bytes of buffer.
> +	 */
> +	trace_buf_size = max(4096UL, buf_size);
>  	return 1;
>  }
>  __setup("trace_buf_size=", set_buf_size);