Messages in this thread | | | From | Jayachandran Chandrasekharan Nair <> | Subject | Re: [RFC] Disable lockref on arm64 | Date | Wed, 12 Jun 2019 04:10:20 +0000 |
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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@marvell.com> 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@linaro.org/ > > 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
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