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. _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
