Re: [RFC] Disable lockref on arm64

[Jayachandran Chandrasekharan Nair]

On Wed, May 22, 2019 at 05:04:17PM +0100, Will Deacon wrote:
> On Sat, May 18, 2019 at 12:00:34PM +0200, Ard Biesheuvel wrote:
> > On Sat, 18 May 2019 at 06:25, Jayachandran Chandrasekharan Nair
> > <jnair@xxxxxxxxxxx> wrote:
> > >
> > > On Mon, May 06, 2019 at 07:10:40PM +0100, Will Deacon wrote:
> > > > On Mon, May 06, 2019 at 06:13:12AM +0000, Jayachandran Chandrasekharan Nair wrote:
> > > > > Perhaps someone from ARM can chime in here how the cas/yield combo
> > > > > is expected to work when there is contention. ThunderX2 does not
> > > > > do much with the yield, but I don't expect any ARM implementation
> > > > > to treat YIELD as a hint not to yield, but to get/keep exclusive
> > > > > access to the last failed CAS location.
> > > >
> > > > Just picking up on this as "someone from ARM".
> > > >
> > > > The yield instruction in our implementation of cpu_relax() is *only* there
> > > > as a scheduling hint to QEMU so that it can treat it as an internal
> > > > scheduling hint and run some other thread; see 1baa82f48030 ("arm64:
> > > > Implement cpu_relax as yield"). We can't use WFE or WFI blindly here, as it
> > > > could be a long time before we see a wake-up event such as an interrupt. Our
> > > > implementation of smp_cond_load_acquire() is much better for that kind of
> > > > thing, but doesn't help at all for a contended CAS loop where the variable
> > > > is actually changing constantly.
> > >
> > > Looking thru the perf output of this case (open/close of a file from
> > > multiple CPUs), I see that refcount is a significant factor in most
> > > kernel configurations - and that too uses cmpxchg (without yield).
> > > x86 has an optimized inline version of refcount that helps
> > > significantly. Do you think this is worth looking at for arm64?
> > >
> > 
> > I looked into this a while ago [0], but at the time, we decided to
> > stick with the generic implementation until we encountered a use case
> > that benefits from it. Worth a try, I suppose ...
> > 
> > [0] https://lore.kernel.org/linux-arm-kernel/20170903101622.12093-1-ard.biesheuvel@xxxxxxxxxx/
> 
> If JC can show that we benefit from this, it would be interesting to see if
> we can implement the refcount-full saturating arithmetic using the
> LDMIN/LDMAX instructions instead of the current cmpxchg() loops.

Now that the lockref change is mainline, I think we need to take another
look at this patch.

Using a fixed up version of Ard's patch above along with Jan's lockref
change upstream, I get significant improvement in scaling for my file
open/read/close testcase[1]. Like I wrote earlier, if I take a
standard Ubuntu arm64 kernel configuration, most of the time for my
test[1] is spent in refcount operations.

With Ard's changes applied[2], I see that the lockref CAS code becomes
the top function and then the retry limit will kick in as expected. In
my testcase, I see that the queued spinlock case is about 2.5 times
faster than the unbound CAS loop when 224 CPUs are enabled (SMT 4,
28core, 2socket).

JC

[1] https://github.com/jchandra-cavm/refcount-test
[2] https://github.com/jchandra-cavm/linux/commits/refcount-fixes