[lkml]   [2004]   [Jan]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: Is this too ugly to merge?

    > I'm wanting to the opinion, if I may, of more experienced people
    > regarding changes I have implemented in my version of Software Suspend,
    > which I want to merge with Patrick and Pavel. Since I'm don't expect
    > that you're all familiar with how my version works, I'll give a fair bit
    > of background before I come to the question.
    > One of the problems I ran into in developing the code was the issue of
    > getting activity stopped so that (1) locks which are required in writing
    > the image aren't still held, (2) we can ensure that dirty buffers are
    > synced to disk and (3) freezing doesn't fail because of races between
    > processes entering the freezer. Point three is especially important.
    > With the implementation in the kernel at the moment, deadlocks can
    > easily happen under load. (I could provide examples but I'm sure you can
    > imagine).
    > To get around these problems, I tried a number of different approaches.
    > In the end, the one that has worked best has been to add hooks to the
    > entrance and exit for critical paths in the kernel, and maintain a count
    > of the number processes in those sections. There are also hooks to
    > temporarily decrement the counter at points where a thread can block in
    > kernel code, in cases where it can safely sit there until resume time.
    > These hooks also provide a means whereby processes that want to begin
    > work on a critical path can be held until post-resume.
    > Finally, processes can be marked as needed for syncing ('syncthreads'),
    > and allowed to continue through these hooks where 'normal' threads would
    > be held.
    > When we want to freeze activity, then, it is simply a matter of toggling
    > a flag and waiting for the number of active processes to reduce to zero.
    > During this time, user space threads that want to start new activity are
    > frozen (via the hook at the start of the critical path they try to
    > enter) until post suspend. Threads already in critical sections run
    > until they exit the critical path or pause at one of the 'safe points',
    > and syncthreads such as kjournald run normally.

    Okay, I can now remember (and agree to) that we need to suspend
    userspace first, and only then suspend kernelspace. Bug I don't see
    why we can't suspend userspace using old, SIGSTOP-like, method.

    When do you have a heart between your knees?
    [Johanka's followup: and *two* hearts?]
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to
    More majordomo info at
    Please read the FAQ at

     \ /
      Last update: 2005-03-22 14:00    [W:0.045 / U:13.464 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site