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

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Date:   Sun, 26 Jan 2020 14:42:45 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Alex Kogan <alex.kogan@...cle.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 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?

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?

							Thanx, Paul