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

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Date:   Sat, 13 Oct 2018 09:30:04 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Gao Xiang <hsiangkao@....com>
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

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...

thanks,

greg k-h