linux-kernel - Re: [PATCH] tracing: Update snapshot buffer on resize if it is allocated

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Date:   Mon, 11 Dec 2023 21:31:34 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux trace kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] tracing: Update snapshot buffer on resize if it is
 allocated

On Sun, 10 Dec 2023 22:54:47 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> The snapshot buffer is to mimic the main buffer so that when a snapshot is
> needed, the snapshot and main buffer are swapped. When the snapshot buffer
> is allocated, it is set to the minimal size that the ring buffer may be at
> and still functional. When it is allocated it becomes the same size as the
> main ring buffer, and when the main ring buffer changes in size, it should
> do.

nit: There seems two "when the snapshot buffer is allocated" case, maybe latter
"it" means main buffer?

> 
> Currently, the resize only updates the snapshot buffer if it's used by the
> current tracer (ie. the preemptirqsoff tracer). But it needs to be updated
> anytime it is allocated.
> 
> When changing the size of the main buffer, instead of looking to see if
> the current tracer is utilizing the snapshot buffer, just check if it is
> allocated to know if it should be updated or not.
> 
> Also fix typo in comment just above the code change.
> 

Looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

BTW, the historical naming leads this kind of issues.
Maybe we'd better to rename 'max_buffer' to 'snapshot_buffer'?

Thanks,


> Cc: stable@...r.kernel.org
> Fixes: ad909e21bbe69 ("tracing: Add internal tracing_snapshot() functions")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/trace.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index aa8f99f3e5de..6c79548f9574 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6348,7 +6348,7 @@ static int __tracing_resize_ring_buffer(struct trace_array *tr,
>  	if (!tr->array_buffer.buffer)
>  		return 0;
>  
> -	/* Do not allow tracing while resizng ring buffer */
> +	/* Do not allow tracing while resizing ring buffer */
>  	tracing_stop_tr(tr);
>  
>  	ret = ring_buffer_resize(tr->array_buffer.buffer, size, cpu);
> @@ -6356,7 +6356,7 @@ static int __tracing_resize_ring_buffer(struct trace_array *tr,
>  		goto out_start;
>  
>  #ifdef CONFIG_TRACER_MAX_TRACE
> -	if (!tr->current_trace->use_max_tr)
> +	if (!tr->allocated_snapshot)
>  		goto out;
>  
>  	ret = ring_buffer_resize(tr->max_buffer.buffer, size, cpu);
> -- 
> 2.42.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>