linux-kernel - Re: [PATCH 3/3] locking/qspinlock: Introduce starvation avoidance into CNA

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Date:   Thu, 31 Jan 2019 11:00:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alex Kogan <alex.kogan@...cle.com>
Cc:     linux@...linux.org.uk, mingo@...hat.com, will.deacon@....com,
        arnd@...db.de, longman@...hat.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com, rahul.x.yadav@...cle.com,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/3] locking/qspinlock: Introduce starvation avoidance
 into CNA

On Wed, Jan 30, 2019 at 10:01:35PM -0500, Alex Kogan wrote:
> Choose the next lock holder among spinning threads running on the same
> socket with high probability rather than always. With small probability,
> hand the lock to the first thread in the secondary queue or, if that
> queue is empty, to the immediate successor of the current lock holder
> in the main queue.  Thus, assuming no failures while threads hold the
> lock, every thread would be able to acquire the lock after a bounded
> number of lock transitions, with high probability.
> 
> Note that we could make the inter-socket transition deterministic,
> by sticking a counter of intra-socket transitions in the head node
> of the secondary queue. At the handoff time, we could increment
> the counter and check if it is below a threshold. This adds another
> field to queue nodes and nearly-certain local cache miss to read and
> update this counter during the handoff. While still beating stock,
> this variant adds certain overhead over the probabilistic variant.

(also heavily suffers from the socket == node confusion)

How would you suggest RT 'tunes' this?

RT relies on FIFO fairness of the basic spinlock primitives; you just
completely wrecked that.