Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend toRAM pathway

[Paul Mackerras]

Alan Stern writes:

> I disagree.  The problem isn't the kernel calling userspace; it's
> userspace trying to do I/O at a time when everything is supposed to be
> quiescing.  Detecting that and blocking it in drivers is hard and
> error-prone; preventing it by freezing userspace is easy and cheap.

And unreliable, and prone to deadlocks, and invasive - requiring
changes to kernel threads that have nothing to do with drivers or
suspend/resume.

> The reasons why the PPC people dislike the whole idea aren't clear to
> me. 

Our experience is that it isn't necessary.  It's extra code that in
practice causes deadlocks and added maintenance burden for no
discernable benefit.

> If it were necessary to have some user task running in order to
> carry out the STR then their objection would make sense -- obviously
> that task couldn't do its job if it were frozen.  But it isn't
> necessary, or at least it should not be.

The freezer doesn't achieve its stated goal of preventing drivers from
getting I/O requests after suspend, since kernel threads can (and do)
initiate I/O.  So then we say that some kernel threads need to be
frozen and others don't, but making that decision is difficult and
error-prone.

Besides, any kernel thread that does I/O is potentially doing that in
order to complete some other I/O request.  So we want to freeze it in
order to prevent new I/O requests from being initiated, but we don't
want to freeze it so that existing I/O requests can be completed.
Thus we have a fundamental conflict in the notion of the freezer.

In fact I believe that making a distinction between user and kernel
threads is wrong and likely to lead to problems, since userspace can
be involved in doing I/O (e.g. FUSE or the user-space driver
framework).  So the argument of the previous paragraph also applies to
some userspace processes.

> Userspace will be effectively "frozen" while the system as a whole is 
> suspended.  So what's wrong with freezing it a little early?  Despite 
> Ben's comments, it seems to me that the freezer doesn't hide problems 
> -- it prevents them.

No, it appears to prevent them, but doesn't in fact.

I remain convinced that the right approach is to fix the drivers to do
one of two things; either do something in the suspend call to block
further requests to the device, or use a late-suspend call to put
their device into a low-power state.  Of course, correctly-written
frameworks can do a lot to help the chipset drivers here.

> Now people may claim that the freezer implementation itself is buggy.  
> I wouldn't dispute it.  But the bugs should be fixable; nobody has 
> pointed out anything fundamentally wrong with the idea AFAICT.

The fundamental problem is the kernel threads and user processes that
we need running to complete existing I/O requests, but which may
initiate new I/O requests in doing so.

The right way to solve the problem is to do the request blocking in
the drivers.

Paul.
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