linux-kernel - Re: [RFC] rtmutex: Do not boost fair tasks each other

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Message-ID: <20140504071758.GK17778@laptop.programming.kicks-ass.net>
Date:	Sun, 4 May 2014 09:17:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Kirill Tkhai <tkhai@...dex.ru>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [RFC] rtmutex: Do not boost fair tasks each other

On Sat, May 03, 2014 at 08:54:08PM +0200, Thomas Gleixner wrote:
> On Thu, 1 May 2014, Kirill Tkhai wrote:
> > Higher priority does not provide exclusive privilege
> > of one fair task over the other. In this case priority
> > boosting looks excess.
> > 
> > On RT patch with enabled PREEMPT_RT_FULL I see a lot of
> > rt_mutex_setprio() actions like
> > 
> > 	120 -> 118
> > 	118 -> 120
> > 
> > They harm RT tasks.
> 
> That's not the main problem. The point is that it is useless and
> therefor harming performace and throughput as well.
>  
> > RT patch has lazy preemtion feature, so if idea is we care
> > about excess preemption inside fair class, we should care
> > about excess priority inheritance too.
> > 
> > In case of vanila kernel the problem is the same, but there
> > are no so many rt mutexes. Do I skip anything?
> 
> Almost a decade ago we decided to do the boosting for everything
> including SCHED_OTHER due to the very simple reason that exercising
> that code path more is likely to trigger more bugs.
>  
> But yes in a production environment, it's pointless for SCHED_OTHER
> tasks.
> 
> Though exercising that code path as much as we can is not a bad thing
> either. So I'd like to see that made compile time conditional on one
> of the lock testing CONFIG items.
> 
> And the patch should be made against mainline, where we have the same
> issue (reduced to PI-futexes).

And of course, if we ever get to PEP, we very much want all the classes
to participate :)
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