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

Alex Kogan
2020-01-27T06:06:18Z

> On Jan 26, 2020, at 5:42 PM, Paul E. McKenney <paulmck@kernel.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