Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention

From: George Spelvin
Date: Wed Aug 26 2015 - 21:28:35 EST

> I can include your patch in the series and then use boolean for the new
> checking_timer field. However, it looks like this applies on an old
> kernel. For example, the spin_lock field has already been removed from
> the structure.

Apologies; that was 4.1.6. A 4.2-rc8 patch is appended (it's a pretty
trivial merge once you look at it).

> The spinlock call has already been removed from a previous patch. The
> issue now is with contention with the sighand lock.

I'll look some more and try to wrap my head around it.

>> Or is it basically okay if this is massively racey, since process-wide
>> CPU timers are inherently sloppy. A race will just cause an expiration
>> check to be missed, but it will be retried soon anyway.

> Yes, the worst case scenario is that we wait until the next thread to
> come along and handle the next expired timer. However, this "delay"
> already occurs now (for example: a timer can expire right after a thread
> exits check_process_timers()).

Ad is this polled, or is there some non-polled system that will trigger
another call to check_process_timers().

E.g. suppose a process fails to notice that it blew past a CPU time
timeout before blocking. Does anything guarantee that it will get
the timeout signal in finite real time?