linux-kernel - Re: [PATCH 2/3] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Thu, 31 Jan 2019 12:38:57 -0500
From:   Waiman Long <longman@...hat.com>
To:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        peterz@...radead.org, mingo@...hat.com, will.deacon@....com,
        arnd@...db.de, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com, rahul.x.yadav@...cle.com
Subject: Re: [PATCH 2/3] locking/qspinlock: Introduce CNA into the slow path
 of qspinlock

On 01/30/2019 10:01 PM, Alex Kogan wrote:
> In CNA, spinning threads are organized in two queues, a main queue for
> threads running on the same socket as the current lock holder, and a
> secondary queue for threads running on other sockets. For details,
> see https://arxiv.org/abs/1810.05600.
>
> Note that this variant of CNA may introduce starvation by continuously
> passing the lock to threads running on the same socket. This issue
> will be addressed later in the series.
>
> Signed-off-by: Alex Kogan <alex.kogan@...cle.com>
> Reviewed-by: Steve Sistare <steven.sistare@...cle.com>

Just wondering if you have tried include PARVIRT_SPINLOCKS option to see
if that patch may screw up the PV qspinlock code.

Anyway, I do believe your claim that NUMA-aware qspinlock is good for
large systems with many nodes. However, all these extra code are
overhead for small systems that have a single node/socket, for instance.

I will support doing something similar to what had been done to support
PV qspinlock. IOW, a separate slowpath function that can be patched to
become the default depending on the system being run on or a kernel boot
option setting.

I would like to keep the core slowpath function simple and easy to
understand. So most of the CNA code should be encapsulated into some
helper functions and put into a separated file.

Thanks,
Longman