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From: Jan Glauber <jglauber@marvell.com>
To: Will Deacon <will.deacon@arm.com>
Cc: "peterz@infradead.org" <peterz@infradead.org>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Jayachandran Chandrasekharan Nair <jnair@marvell.com>,
	"torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC] Disable lockref on arm64
Date: Thu, 2 May 2019 08:38:41 +0000
Message-ID: <20190502083834.GG13955@hc> (raw)
In-Reply-To: <20190501160140.GC28109@fuggles.cambridge.arm.com>

On Wed, May 01, 2019 at 05:01:40PM +0100, Will Deacon wrote:
> Hi Jan,
> 
> [+Peter and Linus, since they enjoy this stuff]
> 
> On Mon, Apr 29, 2019 at 02:52:11PM +0000, Jan Glauber wrote:
> > I've been looking into performance issues that were reported for several
> > test-cases, for instance an nginx benchmark.
> 
> Could you share enough specifics here so that we can reproduce the issue
> locally, please? That would help us in our attempts to develop a fix without
> simply disabling the option for everybody else.

I can send my test-case which is a trivial open-read-close loop with one
thread per CPU and increasing read sizes.

> > It turned out the issue we have on ThunderX2 is the file open-close sequence
> > with small read sizes. If the used files are opened read-only the
> > lockref code (enabled by ARCH_USE_CMPXCHG_LOCKREF) is used.
> > 
> > The lockref CMPXCHG_LOOP uses an unbound (as long as the associated
> > spinlock isn't taken) while loop to change the lock count. This behaves
> > badly under heavy contention (~25x retries for one cmpxchg to succeed
> > with 28 threads operating on the same file). In case of a NUMA system
> > it also behaves badly as the access from the other socket is much slower.
> 
> It's surprising that this hasn't been reported on x86. I suspect their
> implementation of cmpxchg is a little more forgiving under contention.
> 
> > The fact that on ThunderX2 cpu_relax() turns only into one NOP
> > instruction doesn't help either. On Intel pause seems to block the thread
> > much longer, avoiding the heavy contention thereby.
> 
> NOPing out the yield instruction seems like a poor choice for an SMT CPU
> such as TX2. That said, the yield was originally added to cpu_relax() as
> a scheduling hint for QEMU.

The issue is not limited to SMT, it also shows without SMT.

> > With the queued spinlocks implementation I can see a major improvement
> > when I disable lockref. A trivial open-close test-case improves by
> > factor 2 while system time is decreasing also 2x. Looking at kernel compile
> > and dbench numbers didn't show any regression with lockref disabled.
> > 
> > Can we simply disable lockref? Is anyone else seeing this issue? Is there
> > an arm64 platform that actually implements yield?
> 
> There are two issues with disabling lockref like this:
> 
>   1. It's a compile-time thing, so systems that would benefit from the code
>      are unfairly penalised.
> 
>   2. You're optimising for the contended case at the cost of the
>      uncontended case, which should actually be the common case as well.

I completely agree with 2). Nevertheless limiting the retry attempts
like Linus suggested looks like a fair change that should not penalize
anyone and would still help the contented case.

--Jan

> Now, nobody expects contended CAS to scale well, so the middle ground
> probably involves backing off to the lock under contention, a bit like
> an optimistic trylock(). Unfortunately, that will need some tuning, hence
> my initial request for a reproducer.
> 
> Cheers,
> 
> Will

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Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-29 14:52 Jan Glauber
2019-05-01 16:01 ` Will Deacon
2019-05-02  8:38   ` Jan Glauber [this message]
2019-05-01 16:41 ` Linus Torvalds
2019-05-02  8:27   ` Jan Glauber
2019-05-02 16:12     ` Linus Torvalds
2019-05-02 23:19       ` Jayachandran Chandrasekharan Nair
2019-05-03 19:40         ` Linus Torvalds
2019-05-06  6:13           ` [EXT] " Jayachandran Chandrasekharan Nair
2019-05-06 17:13             ` Linus Torvalds
2019-05-06 18:10             ` Will Deacon
2019-05-18  4:24               ` Jayachandran Chandrasekharan Nair
2019-05-18 10:00                 ` Ard Biesheuvel
2019-05-22 16:04                   ` Will Deacon
2019-06-12  4:10                     ` Jayachandran Chandrasekharan Nair
2019-06-12  9:31                       ` Will Deacon
2019-06-14  7:09                         ` Jayachandran Chandrasekharan Nair
2019-06-14  9:58                           ` Will Deacon
2019-06-14 10:24                             ` Ard Biesheuvel
2019-06-14 10:38                               ` Will Deacon
2019-06-15  4:21                                 ` Kees Cook
2019-06-15  8:47                                   ` Ard Biesheuvel
2019-06-15 13:59                                     ` Kees Cook
2019-06-15 14:18                                       ` Ard Biesheuvel
2019-06-16 21:31                                         ` Kees Cook
2019-06-17 11:33                                           ` Ard Biesheuvel
2019-06-17 17:26                                             ` Will Deacon
2019-06-17 20:07                                               ` Jayachandran Chandrasekharan Nair
2019-06-18  5:41                                               ` Kees Cook
2019-06-13  9:53                       ` Hanjun Guo
2019-06-05 13:48   ` [PATCH] lockref: Limit number of cmpxchg loop retries Jan Glauber
2019-06-05 20:16     ` Linus Torvalds
2019-06-06  8:03       ` Jan Glauber
2019-06-06  9:41         ` Will Deacon
2019-06-06 10:28           ` Jan Glauber
2019-06-07  7:27             ` Jan Glauber
2019-06-07 20:14               ` Linus Torvalds

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