linux-kernel - Re: [PATCH v2 2/2] rtmutex: Kill pi_waiters_leftmost from task

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Message-ID: <20160406181529.GU3448@twins.programming.kicks-ass.net>
Date:	Wed, 6 Apr 2016 20:15:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Xunlei Pang <xlpang@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Juri Lelli <juri.lelli@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 2/2] rtmutex: Kill pi_waiters_leftmost from task_struct

On Wed, Apr 06, 2016 at 08:59:16PM +0800, Xunlei Pang wrote:
> Current code use pi_waiters_leftmost to record the leftmost waiter,
> but actually it can be get directly from task_struct::pi_waiters
> using rb_first(). The performance penalty introduced by rb_first()
> should be fine, because normally there aren't that many rtmutexes
> chained together for one task.
> 
> We don't remove rt_mutex:pi_waiters_leftmost, as it is quite possible
> for many tasks sharing one rtmutex.
> 
> Thus, hereby remove pi_waiters_leftmost from task_struct.

Again, I would like Thomas to chime in. This isn't about usual, but very
much about worst case performance -- its RT code after all.