linux-kernel - Re: [RFC v0] Use swait in completion

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Message-ID: <20160309122423.GD10517@linutronix.de>
Date:	Wed, 9 Mar 2016 13:24:23 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Josh Cartwright <joshc@...com>
Cc:	Daniel Wagner <wagi@...om.org>, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Wagner <daniel.wagner@...-carit.de>
Subject: Re: [RFC v0] Use swait in completion

* Josh Cartwright | 2016-03-08 12:26:56 [-0600]:

>Is it really just about latency?  Does this deferral not lead to an
>inversion in the case where the single woken task isn't the highest
>priority waiter on the completion (and doesn't run due to a
>middle-priority thing spinning)?

This would be case, yes. Not only with deferral. Say you have two
waters: 1st one is MID-prio and the second is HI-prio. Currently after
the wakeup of the MID-prio waiter you get preempted. Waking all of them
at once would put the second waiter first on the CPU.
Samething without the deferral flag.

>In order for this to work, it seems like the chosen waiter would need to
>inherit the highest priority of all waiters (which AFAICT isn't
>happening).

sorting the waiters by priority? This will be fun. This is only done for
the rtmutex waiters.

>  Josh

Sebastian