Re: [PATCH V4] mm: fix kernel crash in khugepaged thread

From: Steven Rostedt <rostedt@goodmis.org>
To: yalin wang <yalin.wang2010@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>, Ingo Molnar <mingo@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ebru Akagunduz <ebru.akagunduz@gmail.com>,
	Rik van Riel <riel@redhat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	jmarchan@redhat.com, mgorman@techsingularity.net,
	willy@linux.intel.com,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org
Subject: Re: [PATCH V4] mm: fix kernel crash in khugepaged thread
Date: Mon, 16 Nov 2015 21:43:04 -0500	[thread overview]
Message-ID: <20151116214304.6fa42a4e@grimm.local.home> (raw)
In-Reply-To: <233209B0-A466-4149-93C6-7173FF0FD4C5@gmail.com>

On Tue, 17 Nov 2015 10:21:47 +0800
yalin wang <yalin.wang2010@gmail.com> wrote:

> i have not tried ,
> just a question,
> if you print a %s , but don’t call trace_define_field() do define this string in
> __entry ,  how does user space perf tool to get this string info and print it ?
> i am curious ..
> i can try this when i have time.  and report to you .

Because the print_fmt has nothing to do with the fields. You can have
as your print_fmt as:

	TP_printk("Message = %s", "hello dolly!")

And both userspace and the kernel with process that correctly (if I got
string processing working in userspace, which I believe I do). The
string is processed, it's not dependent on TP_STRUCT__entry() unless it
references a field there. Which can also be used too:

	TP_printk("Message = %s", __entry->musical ? "Hello dolly!" :
			"Death Trap!")

userspace will see in the entry:

 print_fmt: "Message = %s", REC->musical ? "Hello dolly!" : "Death Trap!"

as long as the field "musical" exists, all is well.

-- Steve