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Date: Wed, 4 Jan 2017 13:49:29 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [RFC PATCH 0/7] locking/rtqspinlock: Realtime queued spinlocks
Message-ID: <20170104124929.GI25813@worktop.programming.kicks-ass.net>
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On Tue, Jan 03, 2017 at 01:00:23PM -0500, Waiman Long wrote:
> This patchset introduces a new variant of queued spinlocks - the
> realtime queued spinlocks. The purpose of this new variant is to
> support real spinlock in a realtime environment where high priority
> RT tasks should be allowed to complete its work ASAP. This means as
> little waiting time for spinlocks as possible.
> 
> Non-RT tasks will wait for spinlocks in the MCS waiting queue as
> usual. RT tasks and interrupts will spin directly on the spinlocks
> and use the priority value in the pending byte to arbitrate who get
> the lock first.
> 
> Patch 1 removes the unused spin_lock_bh_nested() API.
> 
> Patch 2 introduces the basic realtime queued spinlocks where the
> pending byte is used for storing the priority of the highest priority
> RT task that is waiting on the spinlock. All the RT tasks will spin
> directly on the spinlock instead of waiting in the queue.
> 


OK, so a single numerical field isn't sufficient to describe priority
anymore, since we added DEADLINE support things have gotten a lot more
complex.

Also, the whole approach worries me, it has the very real possibility of
re-introducing a bunch of starvation cases avoided by the fair lock.


Is there a real problem with -RT that inspired these patches?