Messages in this thread Patch in this message |  | | | Date | Sun, 23 Oct 2011 19:21:47 +1100 | | From | NeilBrown <> | | Subject | Re: lsusd - The Linux SUSpend Daemon |
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On Fri, 21 Oct 2011 22:00:13 -0400 (EDT) Alan Stern <stern@rowland.harvard.edu> wrote:
> On Sat, 22 Oct 2011, NeilBrown wrote: > > > > > It uses files in /var/run/suspend for all communication. > > > > > > I'm not so keen on using files for communication. At best, they are > > > rather awkward for two-way messaging. If you really want to use them, > > > then at least put them on a non-backed filesystem, like something under > > > /dev. > > > > Isn't /var/run a tmpfs filesystem? It should be. > > Surely /run is, so in the new world order the files should probably go > > there. But that is just a detail. > > On my Fedora-14 systems there is no /run, and /var/run is a regular > directory in a regular filesystem. > > > I like files... I particularly like 'flock' to block suspend. The > > rest.... whatever.. > > With files, you only need a context switch when there is real communication. > > With sockets, every message sent must be read so there will be a context > > switch. > > > > Maybe we could do something with futexes... > > Not easily -- as far as I can tell, futexes enjoy relatively little > support. In any case, they provide the same service as a mutex, which > means you'd have to build a shared lock on top of them. > > > > > 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 > > > > > > Why does this matter? > > > > In my mind an event based program should never block. Every action should be > > non-blocking and only taken when 'poll' says it can. > > Reading /sys/power/wakeup_count can be read non-blocking, but you cannot find > > out when it is sensible to try to read it again. So it doesn't fit. > > There shouldn't be any trouble about making wakeup_count pollable. It > also would need to respect nonblocking reads, which it currently does > not do.
Hmm.. you are correct. I wonder why I thought it did support non-blocking reads... I guess it was the code for handling an interrupted system call.
I feel a bit uncomfortable with the idea of sysfs files that block but I don't think I can convincingly argue against it. A non-blocking flag could be passed in, but it would be a very messy change - lots of function call signatures changing needlessly: we would need a flag to the 'show' method ... or add a 'show_nonblock' method which would also be ugly.
But I think there is a need to block - if there is an in-progress event then it must be possible to wait for it to complete as it may not be visible to userspace until then. We could easily enable 'poll' for wakeup_count and then make it always non-blocking, but I'm not really sure I want to require programs to use poll, only to allow them. And without using poll there is no way to wait.
As wakeup_count really has to be single-access we could possibly fudge something by remembering the last value read (like we remember the last value written).
- if the current count is different from the last value read, then return it even if there are in-progress events. - if the current count is the same as the last value read, then block until there are no in-progress events and return the new value. - enable sysfs_poll on wakeup_count by calling sysfs_notify_dirent at the end of wakeup_source_deactivated .... or calling something in kernel/power/main.c which calls that. However we would need to make sysfs_notify_dirent a lot lighter weight first. Maybe I should do that. Then a process that uses 'poll' could avoid reading wakeup_count except when it has changed, and then it won't block. And a process that doesn't use poll can block by simply reading twice - either explicitly or by going around a read then write and it fails loop a second time.
I'm not sure I'm completely comfortable with that, but it is the best I could come up with.
> > At the worst, you could always have a separate thread to read > wakeup_count.
Maybe. I'm tempted simply not to worry about the short delay after all, but the moment I do that, someone will use a wakeup_source to disable suspend for a longer period of time (just like I suggested) and suddenly it won't be a short delay after all.
> > > > > - 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. > > > > > > Except for this one drawback. Socket connections are superior in that > > > regard. > > > > I'm very happy for someone else write an all-socket based daemon. > > Hmmm... Maybe I'll take you up on that. > > > > > > 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'. > > > > > > But not to the client which failed to send an 'R'? > > > > Every client must send an R before suspend can continue. > > I was referring to the case where you abort before receiving an 'R'. > The current suspend attempt will fail, but then what happens during the > next attempt?
There is no situation in which I abort before receiving an 'R'. I wait until all 'R's are in until I even check if an abort was requested.
NeilBrown
Sample untested patch to allow non-blocking reads and poll on wakeup_count.
diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c index 84f7c7d..54543ba 100644 --- a/drivers/base/power/wakeup.c +++ b/drivers/base/power/wakeup.c @@ -44,6 +44,8 @@ static void split_counters(unsigned int *cnt, unsigned int *inpr) /* A preserved old value of the events counter. */ static unsigned int saved_count; +/* Record last value that was read from events counter. */ +static unsigned int last_read_count; static DEFINE_SPINLOCK(events_lock); @@ -410,6 +412,7 @@ static void wakeup_source_deactivate(struct wakeup_source *ws) { ktime_t duration; ktime_t now; + unsigned int comb; ws->relax_count++; /* @@ -440,7 +443,10 @@ static void wakeup_source_deactivate(struct wakeup_source *ws) * Increment the counter of registered wakeup events and decrement the * couter of wakeup events in progress simultaneously. */ - atomic_add(MAX_IN_PROGRESS, &combined_event_count); + comb = atomic_add_return(MAX_IN_PROGRESS, &combined_event_count); + + if ((comb >> IN_PROGRESS_BITS) == last_read_count + 1) + wakeup_count_changed(); } /** @@ -624,15 +630,24 @@ bool pm_get_wakeup_count(unsigned int *count) for (;;) { split_counters(&cnt, &inpr); - if (inpr == 0 || signal_pending(current)) + if (inpr == 0 || cnt != last_read_count) break; + if (signal_pending(current)) + return false; pm_wakeup_update_hit_counts(); schedule_timeout_interruptible(msecs_to_jiffies(TIMEOUT)); } + last_read_count = cnt; + /* If cnt has just changed, then last_read_count will be a bit + * old, so we won't block on the next read, only the one after. + * However this ensures wakeup_source_deactivate doesn't + * miss out on calling wakeup_count_changed() on a change. + */ + smp_wmb(); split_counters(&cnt, &inpr); *count = cnt; - return !inpr; + return true; } /** diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c index 1ad8c93..41361ba 100644 --- a/fs/sysfs/file.c +++ b/fs/sysfs/file.c @@ -444,18 +444,19 @@ static unsigned int sysfs_poll(struct file *filp, poll_table *wait) void sysfs_notify_dirent(struct sysfs_dirent *sd) { - struct sysfs_open_dirent *od; - unsigned long flags; + if (sd->s_attr.open) { + struct sysfs_open_dirent *od; + unsigned long flags; + spin_lock_irqsave(&sysfs_open_dirent_lock, flags); - spin_lock_irqsave(&sysfs_open_dirent_lock, flags); + od = sd->s_attr.open; + if (od) { + atomic_inc(&od->event); + wake_up_interruptible(&od->poll); + } - od = sd->s_attr.open; - if (od) { - atomic_inc(&od->event); - wake_up_interruptible(&od->poll); + spin_unlock_irqrestore(&sysfs_open_dirent_lock, flags); } - - spin_unlock_irqrestore(&sysfs_open_dirent_lock, flags); } EXPORT_SYMBOL_GPL(sysfs_notify_dirent); diff --git a/include/linux/suspend.h b/include/linux/suspend.h index 6bbcef2..e42b7f9 100644 --- a/include/linux/suspend.h +++ b/include/linux/suspend.h @@ -296,6 +296,7 @@ extern bool events_check_enabled; extern bool pm_wakeup_pending(void); extern bool pm_get_wakeup_count(unsigned int *count); extern bool pm_save_wakeup_count(unsigned int count); +extern void wakeup_count_changed(void); #else /* !CONFIG_PM_SLEEP */ static inline int register_pm_notifier(struct notifier_block *nb) diff --git a/kernel/power/main.c b/kernel/power/main.c index 6c601f8..6b3cd80 100644 --- a/kernel/power/main.c +++ b/kernel/power/main.c @@ -231,6 +231,8 @@ power_attr(state); * are any wakeup events detected after 'wakeup_count' was written to. */ +struct sysfs_dirent *wakeup_count_dirent; + static ssize_t wakeup_count_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) @@ -253,6 +255,12 @@ static ssize_t wakeup_count_store(struct kobject *kobj, return -EINVAL; } +void wakeup_count_change(void) +{ + if (wakeup_count_dirent) + sysfs_notify_dirent(wakeup_count_dirent); +} + power_attr(wakeup_count); #endif /* CONFIG_PM_SLEEP */ @@ -342,7 +350,11 @@ static int __init pm_init(void) power_kobj = kobject_create_and_add("power", NULL); if (!power_kobj) return -ENOMEM; - return sysfs_create_group(power_kobj, &attr_group); + error = sysfs_create_group(power_kobj, &attr_group); +#ifdef CONFIG_PM_SLEEP + wakeup_count_dirent = sysfs_get_dirent(power_kobj->sd, NULL, "wakeup_count"); +#endif + return error; } core_initcall(pm_init);
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