Message-ID: <20090223094441.GM9582@elte.hu>
Date:	Mon, 23 Feb 2009 10:44:41 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Len Brown <lenb@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH 2/2] PM: Rework handling of interrupts during
	suspend-resume


* Eric W. Biederman <ebiederm@...ssion.com> wrote:

> Ingo Molnar <mingo@...e.hu> writes:
> 
> > I think this aspect has been well-understood during the 
> > discussion of this topic and it's just a slightly misleading 
> > changelog.
> 
> As I was a member of that discussion I did not see that.
> 
> It took me several passes through the patches to realize the 
> goal is to allow drivers to be able to sleep while they are in 
> their late pm shutdown routines.
> 
> Why we want this I don't know.  But it seems simple enough to 
> implement, and it makes it harder to get the late pm suspend 
> routines wrong, which is always good.

That's not the only goal. The other goal is to further shrink a 
particular window of suspend fragility: the irqs-disabled stage 
of the suspend/resume sequence.

Since suspend/resume is a mini-reboot sequence, there's a large 
amount of code executed - and the variety of code is large as 
well. We had repeat cases of random drivers re-enabling 
interrupts and thus breaking other drivers - and these are nasty 
to debug.

So this patchset disables device IRQs centrally and serializes 
with pending work - so there's no races with pending IRQs 
anymore.

The fact that we keep the timer irq running is two-fold: firstly 
the timer code is special and not really part of the regular 
suspend/resume sequence.

Drivers want to take timestamps, sometimes they even want to do 
a small usleep(), etc. Ideally the suspend/resume code is pretty 
much _the same_ as a regular bootup (and shutdown) code - so we 
want to provide a similar environment to how drivers initialize 
and deinitialize, and we want to enable them to share code 
between bootup/shutdown and suspend/resume agressively.

So the more generic kernel environment we give these fragile 
handlers, the better we are off in the end. Since we already had 
IRQS_TIMER, that was just the natural thing to do.

> > The new suspend code does not rely on truly disabling IRQs 
> > on the low level. The purpose is to not get IRQs to drivers 
> > - which might crash/hang/race/misbehave.
> 
> Reasonable.  I expect one of the problems with drivers getting 
> it wrong is that the interface is too complex for mortal 
> humans to understand.

The suspend/resume state machine certainly used to be a piece of 
code that makes a seasoned kernel developer weep in fear.

That has changed drastically in the past few months. The 
suspend+hibernation logic got unified (at least as far as driver 
methods go), and all the flow and ordering has been cleaned up 
and has been made more robust.

What makes s2ram fragile is not human failure but the 
combination of a handful of physical property:

1) Psychology: shutting the lid or pushing the suspend button is 
   a deceivingly 'simple' action to the user. But under the 
   hood, a ton of stuff happens: we deinitialize a lot of 
   things, we go through _all hardware state_, and we do so in a 
   serial fashion. If just one piece fails to do the right 
   thing, the box might not resume. Still, the user expects this 
   'simple' thing to just work, all the time. No excuses 
   accepted.

2) Length of code: To get a successful s2ram sequence the kernel
   runs through tens of thousands of lines of code. Code which
   never gets executed on a normal box - only if we s2ram. If 
   just one step fails, we get a hung box.

3) Debuggability: a lot of s2ram code runs with the console off, 
   making any bugs hard to debug. Furthermore we have no 
   meaningful persistent storage either for kernel bug messages. 
   The RTC trick of PM_DEBUG works but is a very narrow channel 
   of information and it takes a lot of time to debug a bug via 
   that method.

The combination of these factors really makes up for a perfect 
storm in terms of kernel technology: we have this 
very-deceivingly-simple-looking but complex-and-rarely-executed 
piece of code, which is very hard to debug.

Even just one of these factors would be enough to make an 
otherwise healthy subsystem fragile - no wonder s2ram has been a 
problem ever since it existed in the upstream kernel.

So now we need just one thing: patience and more of the same 
good stuff that happened lately.

> > Still, it might make sense to not just use the ->disable 
> > sequence but primarily the ->shutdown irqchip method (when 
> > it's available in the irqchip).
> 
> Disable seems fine to me.  This is interesting in the context 
> of all of the irqs that will when masked show up somewhere 
> else (think boot interrupts).
> 
> > While we obviously cannot turn off the PIC that delivers 
> > timer IRQs at this stage - there's no theoretical reason why 
> > the suspend sequence couldnt power down some secondary PICs 
> > as well - in some arch code, or maybe even in the generic 
> > driver suspend sequence if the device tree is structured 
> > carefully enough so that the PIC gets turned off last.
> 
> If the point is simply to prevent deliver of irqs to the 
> drivers I don't see the point of anything more than what the 
> patch does now.

... except for the usecase i described above. Say some PIC sits 
on a piece of silicon which gets turned off. I'm not talking 
about x86 but some custom device. We really dont want that IRQ 
line to send half of an IRQ message (un-ACK-ed) when it gets 
turned off. So physically 'suspending' all IRQ lines does make a 
certain level of long-term sense.

Especially if it's just 3 extra lines of code to the existing 
patch.

There _might_ be one downside: overhead of ->shutdown() methods. 
With a typical IRQ count on the typical netbook i doubt it's 
more than ~50 usecs combined.

	Ingo
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