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

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Message-ID: <20200121135034.GA14946@hirez.programming.kicks-ass.net>
Date:   Tue, 21 Jan 2020 14:50:34 +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,
        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, bristot@...hat.com
Subject: Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance
 into CNA

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?