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Subject: Re: [PATCH] tracing: save cmdline only when task does not exist in
 savecmd for optimization
To:     Steven Rostedt <rostedt@goodmis.org>
CC:     <mingo@redhat.com>, <linux-kernel@vger.kernel.org>
References: <20211011115018.88948-1-yangjihong1@huawei.com>
 <20211014103201.685d3647@gandalf.local.home>
From:   Yang Jihong <yangjihong1@huawei.com>
Message-ID: <3d07f3c9-3636-9563-fab5-603bfad4a3af@huawei.com>
Date:   Fri, 15 Oct 2021 09:27:35 +0800
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Hi Steven,

On 2021/10/14 22:32, Steven Rostedt wrote:
> On Mon, 11 Oct 2021 19:50:18 +0800
> Yang Jihong <yangjihong1@huawei.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.
> 
If do an exec, the original process is replaced with a new binary. 
Therefore, the command changes but the tsk->pid does not change. 
Therefore, we need to savecmd here again?

Okay, I see. Thank you for the detailed explanation. :)

Thanks,
Jihong
> -- Steve
> .
>