linux-kernel - Re: [PATCH v2 0/2] enable hires timer to timeout datagram socket

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Message-ID: <1504891402.32080.5.camel@infradead.org>
Date:   Fri, 08 Sep 2017 18:23:22 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     David Miller <davem@...emloft.net>, eduval@...zon.com
Cc:     vallish@...zon.com, shuah@...nel.org, richardcochran@...il.com,
        xiyou.wangcong@...il.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, anchalag@...zon.com, dwmw@...zon.com
Subject: Re: [PATCH v2 0/2] enable hires timer to timeout datagram socket

On Fri, 2017-09-08 at 10:16 -0700, David Miller wrote:
> From: Eduardo Valentin <eduval@...zon.com>
> Date: Fri, 8 Sep 2017 10:04:09 -0700
> 
> > 
> > However, this is a clear, the system call, from the net subsystem,
> > has changed in behavior across kernel versions. From application /
> > userspace perspective, changing the system call without clear
> > documentation or deprecation path, to me, looks like breaking
> > userspace, isn't it?
>
> Where is the chapter and verse of the system call documentation that
> guaranteed this level of timer granularity for you?
> 
> Or were you simply relying upon implementation dependent behavior?
> I can't see anything which ever guarateed the granularity of timers
> to the extent upon which you were relying.
> 
> And most importantly, letting the kernel have flexibility in this area
> is absolutely essential for various forms of optimizations and power
> savings.

The rule we normally use, typically enforced very shoutily by Linus, is
that *however* stupid userspace was to rely on something, if they *do*
rely on it then we shouldn't change it.

I don't know that anyone's ever tried saying "show me the chapter and
verse of the documentation" to Linus when he's in full rant mode, as he
tends to get in such discussions. You could try it, I suppose.

I don't think 'HZ==100' was documented per se either, was it? Perhaps
we *could* change that, after all? :)

(Not that I've actually looked at the patch or the userspace in
question yet, mind you. Just commenting on the absurdity of the
response.)
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