linux-kernel - Re: [External] : Re: [PATCH v14 4/6] locking/qspinlock: Introduce starvation avoidance into CNA

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Message-Id: <7902C919-9624-48C9-89C3-D390A9FF78AB@oracle.com>
Date:   Tue, 13 Apr 2021 22:30:41 -0400
From:   Alex Kogan <alex.kogan@...cle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     linux@...linux.org.uk, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <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: [External] : Re: [PATCH v14 4/6] locking/qspinlock: Introduce
 starvation avoidance into CNA



> On Apr 13, 2021, at 5:22 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> 
>>> ms granularity seems very coarse grained for this. Surely
>>> at some point of spinning you can afford a ktime_get? But ok.
>> We are reading time when we are at the head of the (main) queue, but
>> don’t have the lock yet. Not sure about the latency of ktime_get(), but
>> anything reasonably fast but not necessarily precise should work.
> 
> Actually cpu_clock / sched_clock (see my other email). These should
> be fast without corner cases and also monotonic.
I see, thanks.

> 
>> 
>>> Could you turn that into a moduleparm which can be changed at runtime?
>>> Would be strange to have to reboot just to play with this parameter
>> Yes, good suggestion, thanks.
>> 
>>> This would also make the code a lot shorter I guess.
>> So you don’t think we need the command-line parameter, just the module_param?
> 
> module_params can be changed at the command line too, so yes.
Got it, thanks again.

— Alex