linux-kernel - Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance into CNA

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Message-ID: <cfdf635d-be2e-9d4b-c4ca-6bcbddc6868f@redhat.com>
Date:   Tue, 21 Jan 2020 10:45:48 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Alex Kogan <alex.kogan@...cle.com>
Cc:     linux@...linux.org.uk, 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,
        tglx@...utronix.de, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        guohanjun@...wei.com, jglauber@...vell.com,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com
Subject: Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance
 into CNA

On 1/21/20 8:29 AM, Peter Zijlstra wrote:
> On Mon, Dec 30, 2019 at 02:40:41PM -0500, Alex Kogan wrote:
>
>> +/*
>> + * Controls the threshold for the number of intra-node lock hand-offs before
>> + * the NUMA-aware variant of spinlock is forced to be passed to a thread on
>> + * another NUMA node. By default, the chosen value provides reasonable
>> + * long-term fairness without sacrificing performance compared to a lock
>> + * that does not have any fairness guarantees. The default setting can
>> + * be changed with the "numa_spinlock_threshold" boot option.
>> + */
>> +int intra_node_handoff_threshold __ro_after_init = 1 << 16;
> There is a distinct lack of quantitative data to back up that
> 'reasonable' claim there.
>
> Where is the table of inter-node latencies observed for the various
> values tested, and on what criteria is this number deemed reasonable?
>
> To me, 64k lock hold times seems like a giant number, entirely outside
> of reasonable.

I actually had similar question before, but having the capability of
changing the default with boot time parameter alleviate some of my
concern. I will certainly like to see actual data on how different
values will affect the performance of the code.

Cheers,
Longman