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

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Message-ID: <4a9dbfa7-db68-a2dc-9018-a5b74f0f421c@redhat.com>
Date:   Wed, 14 Apr 2021 12:41:58 -0400
From:   Waiman Long <llong@...hat.com>
To:     Alex Kogan <alex.kogan@...cle.com>, Andi Kleen <ak@...ux.intel.com>
Cc:     linux@...linux.org.uk, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <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: [External] : Re: [PATCH v14 4/6] locking/qspinlock: Introduce
 starvation avoidance into CNA

On 4/13/21 5:01 PM, Alex Kogan wrote:
> Hi, Andi.
>
> Thanks for your comments!
>
>> On Apr 13, 2021, at 2:03 AM, Andi Kleen <ak@...ux.intel.com> wrote:
>>
>> Alex Kogan <alex.kogan@...cle.com> writes:
>>> +	numa_spinlock_threshold=	[NUMA, PV_OPS]
>>> +			Set the time threshold in milliseconds for the
>>> +			number of intra-node lock hand-offs before the
>>> +			NUMA-aware spinlock is forced to be passed to
>>> +			a thread on another NUMA node.	Valid values
>>> +			are in the [1..100] range. Smaller values result
>>> +			in a more fair, but less performant spinlock,
>>> +			and vice versa. The default value is 10.
>> 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.
>
>> 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?

The CNA code, if enabled, will be in vmlinux, not in a kernel module. As 
a result, I think a module parameter will be no different from a kernel 
command line parameter in this regard.

Cheers,
Longman