linux-kernel - Re: [PATCH 1/1] suspend: make sync() on suspend-to-RAM optional

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Date:	Wed, 22 Jul 2015 03:25:41 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Pavel Machek <pavel@....cz>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Brown, Len" <len.brown@...el.com>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Len Brown <lenb@...nel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] suspend: make sync() on suspend-to-RAM optional

On Tuesday, July 21, 2015 05:19:41 PM Pavel Machek wrote:
> On Tue 2015-07-21 16:41:12, Rafael J. Wysocki wrote:
> > Hi Pavel,
> > 
> > On Tue, Jul 21, 2015 at 11:38 AM, Pavel Machek <pavel@....cz> wrote:
> > > On Sat 2015-07-18 01:54:09, Rafael J. Wysocki wrote:
> > >> On Wednesday, July 15, 2015 02:58:22 PM Brown, Len wrote:
> > 
> > [cut]
> > 
> > >> > >> Why do you need CONFIG parameter?
> > >> >
> > >> > So that an OS that doesn't want to change their user-space,
> > >> > can build a kernel that does what they want by default.
> > >> >
> > >> > Originally I had the config parameter remove this code entirely,
> > >> > which would achieve the same goal.
> > >> > But Rafael prefers the sysfs attribute always exist
> > >> > and the config simply set the default.
> > >>
> > >> Indeed.
> > >>
> > >> And so I'm queuing this patch up for 4.3 (with a couple of minor fixups).
> > >
> > > Please don't.
> > >
> > > "OS that doesn't want to change the user-space to speed up suspend by
> > > few milliseconds" is not a valid reason for asking about million users
> > > one more config question.
> > 
> > That's your opinion and I beg to differ.
> 
> Perhaps explaining your opinion would help here? Having to echo value
> to file to improve performance over reliability does not look too
> burdensome on the users.

That's for the people for whom changing the kernel is easier than messing
up with user space.

Also if your user space does the sync before suspending, it's better to
make "no kernel sync" the default, because that saves you some overhead
and energy too (either on the extra redundant sync on every suspend or
on the write to the sysfs attribute on every boot).

> 
> > > Affected users can't run mainline kernel
> > > anyway, and will have to change their userland in non-trivial ways to
> > > get there.
> > 
> > And I'm not sure what you're talking about here.  Who are the
> > "affected users" in particular?
> 
> Who does enter suspend to ram multiple times a second? Only android,
> AFAICT. Can you run android on mainline kernel? No. Can you run
> android on kernel with less that 100k lines of patches? No.
> 
> So who benefits from the new config option? No one.

I am, for one.  None of the systems I use actually needs the sync in the
kernel.  I bet there are more people like me, because I have a stock
distro installed on my systems.

And it is more pain for me to change the user space on each of them to
write to the new sysfs file on every boot than to set a kernel Kconfig
option once.

Thanks,
Rafael


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