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

Waiman Long
2020-01-27T17:18:14Z

On 1/27/20 12:12 PM, Waiman Long wrote:
> On 1/27/20 10:09 AM, Paul E. McKenney wrote:
>>> Do you mean within the VM, lscpu showed that the system has one node and
>>> 12 threads per node? If that is the case, it should behave like regular
>>> qspinlock and be fair.
>> I mean that I saw this in dmesg, which I believe to be telling me the
>> same thing as lscpu saying that there is one node, but you tell me!
>>
>> [ 0.007106] No NUMA configuration found
>> [ 0.007107] Faking a node at [mem 0x0000000000000000-0x000000001ffdefff]
>> [ 0.007111] NODE_DATA(0) allocated [mem 0x1ffdb000-0x1ffdefff]
>> [ 0.007126] Zone ranges:
>> [ 0.007127] DMA [mem 0x0000000000001000-0x0000000000ffffff]
>> [ 0.007128] DMA32 [mem 0x0000000001000000-0x000000001ffdefff]
>> [ 0.007128] Normal empty
>> [ 0.007129] Movable zone start for each node
>> [ 0.007129] Early memory node ranges
>> [ 0.007130] node 0: [mem 0x0000000000001000-0x000000000009efff]
>> [ 0.007132] node 0: [mem 0x0000000000100000-0x000000001ffdefff]
>> [ 0.007227] Zeroed struct page in unavailable ranges: 98 pages
>> [ 0.007227] Initmem setup node 0 [mem 0x0000000000001000-0x000000001ffdefff]
>> [ 0.007228] On node 0 totalpages: 130941
>> [ 0.007231] DMA zone: 64 pages used for memmap
>> [ 0.007231] DMA zone: 21 pages reserved
>> [ 0.007232] DMA zone: 3998 pages, LIFO batch:0
>> [ 0.007266] DMA32 zone: 1984 pages used for memmap
>> [ 0.007267] DMA32 zone: 126943 pages, LIFO batch:31
>>
> That does look like just one node.
>>>> 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?
>>> You can certainly play around with CNA in a VM. However, it is generally
>>> not recommended to use CNA in a VM unless the VM cpu topology matches
>>> the host with 1-to-1 vcpu pinning and there is no vcpu overcommit. In
>>> this case, one may see some performance improvement using CNA by using
>>> the "numa_spinlock=on" option to explicitly turn it on.
>> Sorry, but I will not be booting this on bare metal on the systems that
>> I currently have access to. No more than I run rcutorture on bare metal
>> on them, especially not with newly modified variants of RCU. ;-)
>>
>>> Because of the shuffling of queue entries, CNA is inherently less fair
>>> than the regular qspinlock. However, a ratio of 5 seems excessive to me.
>>> vcpu preemption may be a factor in contributing to this large variation.
>>> My testing on bare metal only showed a throughput variation within
>>> 10-20% at most.
>> OK. Any guidance on qemu's -numa, or should I just experiment with it?
>> The latter will take me some time, as I must focus on other things
>> this week.
> To really test it, you should have multiple numa nodes, 2 or 4.
>> Alternatively, would it make sense for you to give it a spin in a VM?
>> After all, it is entirely possible that I still have some configuration
>> or another messed up.
>
> It all depends on what you want to test. If you want just to make sure
> that it won't fail any locking test. Yes, you can certainly do that in
> a VM. If you want to test for performance or fairness of CNA versus
> regular qspinlock, using a VM is not a proper platform. Even with a
> single node, you see 5x difference in locking count. I suspect that
> maybe one or a few vcpus got preempted pretty frequently to perform
> host activities that it screw up the data.
>
BTW, in a VM, the queued_spin_unlock function pointer will still be
pointing to the paravirt version even if CNA is used for the slowpath.
That shouldn't impact correctness, but is not optimal for performance.

Cheers,
Longman