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

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Date:	Sat, 18 Jul 2015 01:54:09 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	"Brown, Len" <len.brown@...el.com>
Cc:	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Pavel Machek <pavel@....cz>, 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 Wednesday, July 15, 2015 02:58:22 PM Brown, Len wrote:
> 
> > -----Original Message-----
> > From: Austin S Hemmelgarn [mailto:ahferroin7@...il.com]
> > Sent: Wednesday, July 15, 2015 10:07 AM
> > To: Pavel Machek; Len Brown
> > Cc: rjw@...ysocki.net; linux-pm@...r.kernel.org; linux-
> > kernel@...r.kernel.org; Brown, Len
> > Subject: Re: [PATCH 1/1] suspend: make sync() on suspend-to-RAM optional
> > 
> > On 2015-07-15 02:43, Pavel Machek wrote:
> > > On Tue 2015-07-14 22:24:51, Len Brown wrote:
> > >> From: Len Brown <len.brown@...el.com>
> > >>
> > >> The Linux kernel suspend path has traditionally invoked sys_sync().
> > >>
> > >> But sys_sync() can be expensive, and some systems do not want
> > >> to pay the cost of sys_sync() on every suspend.
> > >
> > > Have you measured how expensive it can be, and why it is expensive?
> 
> > How expensive it is can vary widely, but it pretty much boils down to
> > how much dirty data still needs written out, and how slow the storage it
> > needs written to is.  There's not really much that can be done in the
> > kernel to change this, and most userspace suspend systems call sync
> > themselves during the suspend sequence.
> 
> Right.
> And now, user-space gets is no longer forced to incur that
> delay on every suspend if they do not want it.
> 
> Yes, have measured this under many conditions.
> The bottom line is that sys_sync() is rarely as fast as 1ms,
> and is sometimes as slow as hundreds of ms.
> 
> >> 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).

Thanks,
Rafael

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