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

Paul E. McKenney
2020-01-27T15:09:11Z

On Mon, Jan 27, 2020 at 09:11:43AM -0500, Waiman Long wrote:
> On 1/26/20 5:42 PM, Paul E. McKenney 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!
> >
> > 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?
> 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

> > 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.

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.

Thanx, Paul