linux-kernel - Re: [PATCH v9 0/5] Add NUMA-awareness to qspinlock

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Message-Id: <FB96E148-C72B-4D00-95F0-C4B69A3EE454@oracle.com>
Date:   Mon, 27 Jan 2020 01:04:45 -0500
From:   Alex Kogan <alex.kogan@...cle.com>
To:     paulmck@...nel.org
Cc:     Waiman Long <longman@...hat.com>, linux@...linux.org.uk,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linux-arm-kernel <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, dave.dice@...cle.com,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com
Subject: Re: [PATCH v9 0/5] Add NUMA-awareness to qspinlock



> On Jan 26, 2020, at 5:42 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Sun, Jan 26, 2020 at 07:35:35AM -0800, Paul E. McKenney wrote:
>> On Sat, Jan 25, 2020 at 02:41:39PM -0500, Waiman Long wrote:
>>> On 1/24/20 11:58 PM, Paul E. McKenney wrote:
>>>> On Fri, Jan 24, 2020 at 09:17:05PM -0500, Waiman Long wrote:
>>>>> On 1/24/20 8:59 PM, Waiman Long wrote:
>>>>>>> You called it!  I will play with QEMU's -numa argument to see if I can get
>>>>>>> CNA to run for me.  Please accept my apologies for the false alarm.
>>>>>>> 
>>>>>>> 							Thanx, Paul
>>>>>>> 
>>>>>> CNA is not currently supported in a VM guest simply because the numa
>>>>>> information is not reliable. You will have to run it on baremetal to
>>>>>> test it. Sorry for that.
>>>>> Correction. There is a command line option to force CNA lock to be used
>>>>> in a VM. Use the "numa_spinlock=on" boot command line parameter.
>>>> As I understand it, I need to use a series of -numa arguments to qemu
>>>> combined with the numa_spinlock=on (or =1) on the kernel command line.
>>>> If the kernel thinks that there is only one NUMA node, it appears to
>>>> avoid doing CNA.
>>>> 
>>>> Correct?
>>>> 
>>>> 							Thanx, Paul
>>>> 
>>> In auto-detection mode (the default), CNA will only be turned on when
>>> paravirt qspinlock is not enabled first and there are at least 2 numa
>>> nodes. The "numa_spinlock=on" option will force it on even when both of
>>> the above conditions are false.
>> 
>> Hmmm...
>> 
>> Here is my kernel command line taken from the console log:
>> 
>> console=ttyS0 locktorture.onoff_interval=0 numa_spinlock=on locktorture.stat_interval=15 locktorture.shutdown_secs=1800 locktorture.verbose=1
>> 
>> Yet the string "Enabling CNA spinlock" does not appear.
>> 
>> Ah, idiot here needs to enable CONFIG_NUMA_AWARE_SPINLOCKS in his build.
>> Trying again with "--kconfig "CONFIG_NUMA_AWARE_SPINLOCKS=y"...
> 
> And after fixing that, plus adding the other three Kconfig options required
> to enable this, I really do see "Enabling CNA spinlock" in the console log.
> Yay!
Great! Your persistence paid off :)

Yet, CNA does not do much interesting here, as it sees only one numa node.

> 
> At the end of the 30-minute locktorture exclusive-lock run, I see this:
> 
> Writes:  Total: 572176565  Max/Min: 54167704/10878216 ???  Fail: 0
> 
> This is about a five-to-one ratio.  Is this expected behavior, given a
> single NUMA node on a single-socket system with 12 hardware threads?
I’m not sure what is expected here.
I’m guessing that if you boot your guest with the default 
(non-CNA/non-paravirt) qspinlock, you will get a similar result.

> 
> I will try reader-writer lock next.
> 
> Again, should I be using qemu's -numa command-line option to create nodes?
> If so, what would be a sane configuration given 12 CPUs and 512MB of
> memory for the VM?  If not, what is a good way to exercise CNA's NUMA
> capabilities within a guest OS?
That’s a good question. Perhaps Longman knows the answer?

Regards,
— Alex