From 57f1f1acfbc6a4de7d97e9f1e957ae4508f713d8 Mon Sep 17 00:00:00 2001 From: Oliver Neukum Date: Mon, 18 Oct 2004 09:03:39 -0700 Subject: [PATCH] [PATCH] additional documentation for power management This is additional documentation for power management. Pavel Machek has given his acknowledgement. Signed-Off-By: Oliver Neukum Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/power/kernel_threads.txt | 41 ++++++++++++++++++++++++++ 1 file changed, 41 insertions(+) create mode 100644 Documentation/power/kernel_threads.txt diff --git a/Documentation/power/kernel_threads.txt b/Documentation/power/kernel_threads.txt new file mode 100644 index 0000000000000..60b548105edf7 --- /dev/null +++ b/Documentation/power/kernel_threads.txt @@ -0,0 +1,41 @@ +KERNEL THREADS + + +Freezer + +Upon entering a suspended state the system will freeze all +tasks. This is done by delivering pseudosignals. This affects +kernel threads, too. To successfully freeze a kernel thread +the thread has to check for the pseudosignal and enter the +refrigerator. Code to do this looks like this: + + do { + hub_events(); + wait_event_interruptible(khubd_wait, !list_empty(&hub_event_list)); + if (current->flags & PF_FREEZE) + refrigerator(PF_FREEZE); + } while (!signal_pending(current)); + +from drivers/usb/core/hub.c::hub_thread() + + +The Unfreezable + +Some kernel threads however, must not be frozen. The kernel must +be able to finish pending IO operations and later on be able to +write the memory image to disk. Kernel threads needed to do IO +must stay awake. Such threads must mark themselves unfreezable +like this: + + /* + * This thread doesn't need any user-level access, + * so get rid of all our resources. + */ + daemonize("usb-storage"); + + current->flags |= PF_NOFREEZE; + +from drivers/usb/storage/usb.c::usb_stor_control_thread() + +Such drivers are themselves responsible for staying quiet during +the actual snapshotting. -- 2.53.0