linux-kernel - Re: [patch] sched: fix single-depth wchan output

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Message-ID: <20081106063012.GA15731@elte.hu>
Date:	Thu, 6 Nov 2008 07:30:12 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Ken Chen <kenchen@...gle.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [patch] sched: fix single-depth wchan output


* Ken Chen <kenchen@...gle.com> wrote:

> On Wed, Nov 5, 2008 at 10:11 PM, Ingo Molnar <mingo@...e.hu> wrote:
>
> > hm, this adds overhead - and the thing is that WCHAN is rather 
> > uninformative to begin with (because it's a single dimension), so 
> > we should phase it out, not expand it.
> >
> > How about adding a /proc/<PID>/stacktrace file that gives us the 
> > stack trace of any task in the system? That would be useful for a 
> > number of other purposes as well, and about 100 times more useful 
> > than wchan. (often it would be more useful than sysrq-t dumps)
> 
> Sure, my main motivation is to remove frame pointer generation. 
> x86_64 unconditionally adds fp for kernel/sched.c right now.  I'm 
> all for phasing out wchan if people don't think there is value in 
> it.

are you interested in adding /proc/<PID>/stacktrace? If yes then we 
could remove fp generation for 64-bit right now and add your 
stacktrace patch when you are done with it.

Generally we want frame pointers for high quality backtraces and 
trouble-shooting. The small cost is almost always worth paying and 
most distros enable framepointers for that reason. On 32-bit a 
no-framepointers kernel image has less register pressure, but on 
64-bit there's little reason to not enable them.

	Ingo
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