linux-kernel - Re: [RFC PATCH] bit_spinlock: introduce smp_cond_load

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Message-ID: <752f4ea2-7cd7-fff8-ee0b-d9c01204122e@aol.com>
Date:   Sat, 13 Oct 2018 15:44:19 +0800
From:   Gao Xiang <hsiangkao@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Miao Xie <miaoxie@...wei.com>,
        Chao Yu <chao@...nel.org>
Subject: Re: [RFC PATCH] bit_spinlock: introduce smp_cond_load_relaxed

Hi Greg,

On 2018/10/13 15:30, Greg Kroah-Hartman wrote:
> On Sat, Oct 13, 2018 at 03:22:08PM +0800, Gao Xiang wrote:
>> Hi Greg,
>>
>> On 2018/10/13 15:04, Greg Kroah-Hartman wrote:
>>> On Sat, Oct 13, 2018 at 02:47:29PM +0800, Gao Xiang wrote:
>>>> It is better to use smp_cond_load_relaxed instead
>>>> of busy waiting for bit_spinlock.
>>>
>>> Why?  I think we need some kind of "proof" that this is true before
>>> being able to accept a patch like this, don't you agree?
>>
>> There are some materials which discuss smp_cond_load_* earlier.
>> https://patchwork.kernel.org/patch/10335991/
>> https://patchwork.kernel.org/patch/10325057/
>>
>> In ARM64, they implements a function called "cmpwait", which uses
>> hardware instructions to monitor a value change, I think it is more
>> energy efficient than just do a open-code busy loop...
>>
>> And it seem smp_cond_load_* is already used in the current kernel, such as:
>> ./kernel/locking/mcs_spinlock.h
>> ./kernel/locking/qspinlock.c
>> ./kernel/sched/core.c
>> ./kernel/smp.c
>>
>> For other architectures like x86/arm64, I think they could implement
>> smp_cond_load_* later.
> 
> And have you benchmarked this change to show that it provides any
> benifit?
> 
> You need to do that...

OK, it is my responsibility to test this patch in ARM64 indeed.

I will test it later in ARM64 to see if it has any performance difference
after I handled EROFS product landing stuffs (perhaps weeks later,
many urgent stuffs for the current product that I need to solve...)

Or if some warm-hearted folks interest in it, I'm very happy to see other
implementations or comments about that. :)

Thanks,
Gao Xiang

> 
> thanks,
> 
> greg k-h
>