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From: Jan Glauber <jglauber@marvell.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	Jayachandran Chandrasekharan Nair <jnair@marvell.com>,
	"will.deacon@arm.com" <will.deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.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 08:27:50 +0000
Message-ID: <20190502082741.GE13955@hc> (raw)
In-Reply-To: <CAHk-=wjPqcPYkiWKFc=R3+18DXqEhV+Nfbo=JWa32Xp8Nze67g@mail.gmail.com>

On Wed, May 01, 2019 at 09:41:08AM -0700, Linus Torvalds wrote:
> On Mon, Apr 29, 2019 at 7:52 AM Jan Glauber <jglauber@marvell.com> wrote:
> >
> > 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
> 
> Ok, excuse me when I rant a bit.
> 
> Since you're at Marvell, maybe you can forward this rant to the proper
> guilty parties?

Sure :)

> Who was the absolute *GENIUS* who went
> 
>  Step 1: "Oh, we have a middling CPU that isn't world-class on its own"
> 
>  Step 2: "BUT! We can put a lot of them on a die, because that's 'easy'"
> 
>  Step 3: "But let's make sure the interconnect isn't all that special,
> because that would negate the the whole 'easy' part, and really strong
> interconnects are even harder than CPU's and use even more power, so
> that wouldn't work"
> 
>  Step 4: "I wonder why this thing scales badly?"
> 
> Seriously. Why are you guys doing this? Has nobody ever looked at the
> fundamental thought process above and gone "Hmm"?
> 
> If you try to compensate for a not-great core by putting twice the
> number of them in a system, you need a cache system and interconnect
> between them that is more than twice as good as the competition.
> 
> And honestly, from everything that I hear, you don't have it. The
> whole chip is designed for "throughput when there is no contention".
> Is it really a huge surprise that it then falls flat on its face when
> there's something fancy going on?

I'll see how x86 runs the same testcase, I thought that playing
cacheline ping-pong is not the optimal use case for any CPU.

My assumption was that x86 probably doesn't suffer that much because
of cpu_relax() -> pause insn could slow down the retry rate.

> So now you want to penalize everybody else in the ARM community
> because you have a badly balanced system?

Not really, as I intentionally did not include a patch and sent this as
RFC.

> Ok, rant over.
> 
> The good news is that we can easily fix _this_ particular case by just
> limiting the CMPXCHG_LOOP to a maximum number of retries, since the
> loop is already designed to fail quickly if the spin lock value isn't
> unlocked, and all the lockref code is already organized to fall back
> to spinlocks.
> 
> So the attached three-liner patch may just work for you. Once _one_
> thread hits the maximum retry case and goes into the spinlocked case,
> everybody else will also fall back to spinlocks because they now see
> that the lockref is contended. So the "retry" value probably isn't all
> that important, but let's make it big enough that it probably never
> happens on a well-balanced system.

Agreed, your patch would solve the issue for ThunderX2. Limiting the
retry attempts was one of the things I tried beside extending the number
of NOPs in cpu_relax().

> But seriously: the whole "let's just do lots of CPU cores because it's
> easy" needs to stop. It's fine if you have a network processor and
> you're doing independent things, but it's not a GP processor approach.
> 
> Your hardware people need to improve on your CPU core (maybe the
> server version of Cortex A76 is starting to approach being good
> enough?) and your interconnect (seriously!) instead of just slapping
> 32 cores on a die and calling it a day.
> 
>                 Linus "not a fan of the flock of chickens" Torvalds

>  lib/lockref.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/lib/lockref.c b/lib/lockref.c
> index 3d468b53d4c9..a6762f8f45c9 100644
> --- a/lib/lockref.c
> +++ b/lib/lockref.c
> @@ -9,6 +9,7 @@
>   * failure case.
>   */
>  #define CMPXCHG_LOOP(CODE, SUCCESS) do {					\
> +	int retry = 15;		/* Guaranteed random number */			\
>  	struct lockref old;							\
>  	BUILD_BUG_ON(sizeof(old) != 8);						\
>  	old.lock_count = READ_ONCE(lockref->lock_count);			\
> @@ -21,6 +22,8 @@
>  		if (likely(old.lock_count == prev.lock_count)) {		\
>  			SUCCESS;						\
>  		}								\
> +		if (!--retry)							\
> +			break;							\
>  		cpu_relax();							\
>  	}									\
>  } while (0)


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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
2019-05-01 16:41 ` Linus Torvalds
2019-05-02  8:27   ` Jan Glauber [this message]
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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