|| ||NeilBrown <firstname.lastname@example.org> |
|| ||Alan Stern <email@example.com>,
John Stultz <firstname.lastname@example.org>,
"Rafael J. Wysocki" <email@example.com>,
mark gross <firstname.lastname@example.org> |
|| ||lsusd - The Linux SUSpend Daemon |
|| ||Fri, 21 Oct 2011 16:23:40 +1100|
|| ||Linux PM list <email@example.com>,
|| ||Article, Thread
I wasn't going to do this... but then I did. I think that sometimes coding is
a bit like chocolate.
you can find a bunch of proof-of-concept sample code that implements a
"Linux SUSpend Daemon" with client support library and test programs.
I haven't actually tested it as root and had it actually suspend and resume
and definitely haven't had it even close to a race condition, but the
various bits seem to work with each other properly when I run them under
strace and watch.
It didn't turn out quite the way I imagined, but then cold harsh reality has
a way of destroying our dreams, doesn't it :-)
Below is the README file. Comment welcome as always.
I'm happy for patches too, but I'm equally happy for someone to re-write it
completely and make something really useful and maintainable.
This directory contains a prototype proof-of-concept system
for managing suspend in Linux.
Thus the Linux SUSpend Daemon.
The main daemon. It is written to run a tight loop and blocks as
required. It obeys the wakeup_count protocol to get race-free
suspend and allows clients to register to find out about
suspend and to block it either briefly or longer term.
It uses files in /var/run/suspend for all communication.
disabled: This file always exists. If any process holds a
shared flock(), suspend will not happen.
immediate: If this file exists, lsusd will try to suspend whenever
request: If this is created, then lsusd will try to suspend
once, and will remove the file when suspend completes or aborts.
watching: This is normally empty. Any process wanting to know
about suspend should take a shared flock and check the file is
still empty, and should watch for modification.
When suspend is imminent, lsusd creates 'watching-next', writes
a byte to 'watching' and waits for an exclusive lock on 'watching'.
Clients should move their lock to 'watching-next' when ready for
When suspend completes, another byte (or 2) is written to
'watching', and 'watching-next' is renamed over it. Clients can
use either of these to know that resume has happened.
watching-next: The file that will be 'watching' in the next awake cycle.
lsusd does not try to be event-loop based because:
- /sys/power/wakeup_count is not pollable. This could probably be
'fixed' but I want code to work with today's kernel. It will probably
only block 100msec at most, but that might be too long???
- I cannot get an event notification when a lock is removed from a
file. :-( And I think locks are an excellent light-weight
mechanism for blocking suspend.
This is an event-loop based daemon that can therefore easily handle
socket connections and client protocols which need prompt
response. It communicates with lsusd and provides extra
services to client.
lsused (which needs a better name) listens on the socket
A client may connect and send a list of file descriptors.
When a suspend is immanent, if any file descriptor is readable,
lsused will send a 'S' message to the client and await an 'R' reply
(S == suspend, R == ready). When all replies are in, lsused will
allow the suspend to complete. When it does (or aborts), it will send
'A' (awake) to those clients to which it sent 'S'.
This allows a client to get a chance to handle any wakeup events,
but not to be woken unnecessarily on every suspend.
This allows clients to register on the socket.
They write a timestamp in seconds since epoch, and will receive
a 'Now' message when that time arrives.
Between the time the connection is made and the time a "seconds"
number is written, suspend will be blocked.
Also between the time that "Now" is sent and when the socket is
closed, suspend is also blocked.
A simple tool to create the 'request' file and then wait for it
to be removed.
libsus.a: A library of client-side interfaces.
suspend_open, suspend_block, suspend_allow, suspend_close:
easy interface to blocking suspend
For use in libevent program to get notifications of
suspend and resume via the 'watching' file.
create a libevent event for an fd which is protected from
suspend. Whenever it is readable, suspend will not be entered.
create a libevent event for a particular time which will
trigger even if system is suspend, and will protect against
suspend while event is happening.
block_test watch_test event_test alarm_test
simple test programs for the above interfaces.
Sample code for detecting suspend/resume from python
Sample code for disabling suspend from shell.
All code is available under GPLv2+. However if you ask for a different
license I am unlikely to refuse (at least with the early prototype).
Patches and comment are welcome, but please also feel free to include
any of this in some more complete framework.