linux-kernel - Re: [PATCH] tracing: save cmdline only when task does not exist in savecmd for optimization

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Message-ID: <20211014103201.685d3647@gandalf.local.home>
Date:   Thu, 14 Oct 2021 10:32:01 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Yang Jihong <yangjihong1@...wei.com>
Cc:     <mingo@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: save cmdline only when task does not exist in
 savecmd for optimization

On Mon, 11 Oct 2021 19:50:18 +0800
Yang Jihong <yangjihong1@...wei.com> wrote:

> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 7896d30d90f7..a795610a3b37 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2427,8 +2427,11 @@ static int trace_save_cmdline(struct task_struct *tsk)
>  		savedcmd->cmdline_idx = idx;
>  	}
>  
> -	savedcmd->map_cmdline_to_pid[idx] = tsk->pid;
> -	set_cmdline(idx, tsk->comm);
> +	/* save cmdline only when task does not exist in savecmd */
> +	if (savedcmd->map_cmdline_to_pid[idx] != tsk->pid) {
> +		savedcmd->map_cmdline_to_pid[idx] = tsk->pid;
> +		set_cmdline(idx, tsk->comm);
> +	}
>  

I now remember why I never did it this way. This breaks saving the command
line when we do an exec.

That is, just because mapped_pid == tsk->pid does not mean that the comm is
the same.

-- Steve