linux-kernel - Re: [PATCH 3/5] lockstat: core infrastructure

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Date:	Fri, 1 Jun 2007 20:30:53 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Baron <jbaron@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/5] lockstat: core infrastructure

* Daniel Walker <dwalker@...sta.com> wrote:

> > So, having two interfaces, one fast and one accurate is the right 
> > answer IMHO.
> 
> In the case of lockstat you have two cases fast and functional, and 
> non-functional .. Right now your patch has no slow and functional 
> state.

let me explain it to you:

1) there is absolutely no problem here to begin with. If a rare 
architecture is lazy enough to not bother implementing a finegrained 
sched_clock() then it certainly does not care about the granularity of 
lockstat fields either. If it does, it can improve scheduling and get 
more finegrained lockstat by implementing a proper sched_clock() 
function - all for the same price! ;-)

2) the 'solution' you suggested for this non-problem is _far worse_ than 
the granularity non-problem, on the _majority_ of server systems today! 
Think about it! Your suggestion would make lockstat _totally unusable_. 
Not "slow and functional" like you claim but "dead-slow and unusable".

in light of all this it is puzzling to me how you can still call Peter's 
code "non-functional" with a straight face. I have just tried lockstat 
with jiffies granular sched_clock() and it was still fully functional. 
So if you want to report some bug then please do it in a proper form.

> As I said before there is no reason why and architectures should be 
> forced to implement sched_clock() .. Is there some specific reason why 
> you think it should be mandatory?

Easy: it's not mandatory, but it's certainly "nice" even today, even 
without lockstat. It will get you:

 - better scheduling
 - better printk timestamps
 - higher-quality blktrace timestamps

With lockstat, append "more finegrained lockstat output" to that list of 
benefits too. That's why every sane server architecture has a 
sched_clock() implementation - go check the kernel source. Now i wouldnt 
mind to clean the API up and call it get_stat_clock() or whatever - but 
that was not your suggestion at all - your suggestion was flawed: to 
implement sched_clock() via the GTOD clocksource.

	Ingo
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