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From: Hanjun Guo <guohanjun@huawei.com>
To: Jayachandran Chandrasekharan Nair <jnair@marvell.com>,
	Will Deacon <will.deacon@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	Jan Glauber <jglauber@marvell.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Linus Torvalds <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, 13 Jun 2019 17:53:39 +0800
Message-ID: <4210fcce-7ed1-8f93-a0f0-0fc588792fee@huawei.com> (raw)
In-Reply-To: <20190612040933.GA18848@dc5-eodlnx05.marvell.com>

On 2019/6/12 12:10, Jayachandran Chandrasekharan Nair wrote:
> 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

FWIW, with the patch (Ard's patch plus fixes) above, running the
same testcase on ARM64 Kunpeng920 96 CPU core system, I can see about 50%
performance boost.

I also tested Jan's lockref change without Ard's patch, performance
is almost the same.

Thanks
Hanjun


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  parent reply index

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
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 [this message]
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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