Messages in this thread | | | Date | Fri, 4 Feb 2011 12:13:01 +0100 | From | Tejun Heo <> | Subject | Re: [PATCH 4/4] workqueue: Remove now superfluous cancel_delayed_work() calls |
| |
Hello, Peter.
On Thu, Feb 03, 2011 at 06:45:41PM +0100, Peter Zijlstra wrote: > On Thu, 2011-02-03 at 17:19 +0100, Tejun Heo wrote: > > On Thu, Feb 03, 2011 at 03:09:44PM +0100, Peter Zijlstra wrote: > > > Since queue_delayed_work() can now deal with existing timers, we don't > > > need to explicitly call cancel_delayed_work() anymore. > > > > This is nice but there's small complication with the way > > queue_delayed_work() behaves. If a delayed work item is already > > pending, another queue_delayed_work() doesn't modify the delay whether > > the new delay is longer or shorter than the current one. The previous > > patch doesn't really change the behavior as the whole thing is gated > > with WORK_STRUCT_PENDING_BIT. > > > > So, cancel_delayed_work() followed by queue_delayed_work() schedules > > the work to be executed at the specified time regardless of the > > current pending state while queue_delayed_work() takes effect iff > > currently the work item is not pending. > > Right, I didn't think it would matter much, the difference is tiny. Only > a small window between the timer triggering and the work getting > scheduled has a different semantics, it used to be the same as before > that window, now its like after that window. > > Since its all timing the code needs to deal with those cases anyway, no?
No, AFAICS the change from add_timer() to mod_timer() doesn't make any difference. The control never reaches there if the work item is already pending. Please consider the following two sequences.
seq1. queue_delayed_work(wq, dwork, 10*HZ); cancel_delayed_work(dwork); queue_delayed_work(wq, dwork, 5*HZ);
seq2. queue_delayed_work(wq, dwork, 10*HZ); queue_delayed_work(wq, dwork, 5*HZ);
With or without the patch, dwork in seq1 will execute in 5 seconds, and, again, with our without the patch dwork in seq2 will execute in 10 seconds, because queueing is gated by WORK_STRUCT_PENDING_BIT and if the bit is already set the timer isn't modified at all.
IOW, those cancel_delayed_work()'s are there not because queue_delayed_work() calls add_timer() instead of mod_timer(). They're there because queue_delayed_work() always uses the first timeout duration and the users want to change it to a new value.
As I wrote before, I'm not a fan of the current behavior but that's how it is currently. This patch series doesn't change the behavior because the timers are guaranteed to be offline when queue_delayed_work_on() calls add_timer(). To actually change the behavior, queue_delayed_work_on() needs to be restructured and all its users audited.
Thank you.
-- tejun
| |