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

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Date:   Tue, 21 Jan 2020 22:19:32 +0100
From:   Daniel Bristot de Oliveira <bristot@...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, longman@...hat.com, 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 2:50 PM, Peter Zijlstra wrote:
> On Tue, Jan 21, 2020 at 02:29:49PM +0100, 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.
> Daniel, IIRC you just did a paper on constructing worst case latencies
> from measuring pieces. Do you have data on average lock hold times?
> 

I am still writing the paper, but I do not have the (avg) lock times. It is it
is in the TODO list, though!

-- Daniel