Messages in this thread | | | Date | Wed, 16 Jul 2014 09:15:21 +0800 | From | Lai Jiangshan <> | Subject | Re: [PATCH RFC] workqueue: don't grab PENDING bit on some conditions |
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On 07/15/2014 11:58 PM, Tejun Heo wrote: > Hello, Lai. > > On Tue, Jul 15, 2014 at 05:30:10PM +0800, Lai Jiangshan wrote: >> Thread1 expects that, after flush_delayed_work() returns, the known pending >> work is guaranteed finished. But if Thread2 is scheduled a little later than >> Thread1, the known pending work is dequeued and re-queued, it is considered >> as two different works in the workqueue subsystem and the guarantee expected > > They are two separate queueing instances of the same work item.
I think the mod_delayed_work() is expected to modify a queueing instances instead of separate from the name.
> >> by Thread1 is broken. > > The guarantee expected by thread 1 is that the most recent queueing > instance of the work item is finished either through completing > execution or being cancelled. No guarantee is broken.
I don't think the mod_delayed_work() is considered as a cancelling operation to the user. You can add comments to state that it contains a cancelling operation and a requeue operation.
> >> The guarantee expected by Thread1/workqueue-user is reasonable for me, >> the workqueue subsystem should provide this guarantee. In another aspect, > > You're adding a new component to the existing set of guarantees. You > can argue for it but it's a new guarantee regardless.
So, it is an RFC.
> >> the flush_delayed_work() is still working when mod_delayed_work_on() returns, >> it is more acceptable that the flush_delayed_work() beats the >> mod_delayed_work_on(). >> >> It is achieved by introducing a KEEP_FLUSHED flag for try_to_grab_pending(). >> If the work is being flushed and KEEP_FLUSHED flags is set, >> we disallow try_to_grab_pending() to grab the pending of the work. >> >> And there is another condition that the user want to speed up a delayed work. >> >> When the user use "mod_delayed_work_on(..., 0 /* zero delay */);", his >> attention is to accelerate the work and queue the work immediately. >> >> But the work does be slowed down when it is already queued on the worklist >> due to the work is dequeued and re-queued. So we also disallow >> try_to_grab_pending() to grab the pending of the work in this condition >> by introducing KEEP_QUEUED flag. > > Both are extremely marginal. > Do we have any actual cases any of these matters?
No such case.
I only found the WB subsystem (backing-dev.c, fs-writeback.c) uses both mod_delayed_work() and flush_delayed_work(), but it seems that when flush_delayed_work() is called, mod_delayed_work() will can't be called.
> I can't see what we're gaining with the extra complexity.
Will you add some comments or let it as before?
> >> @@ -1212,6 +1220,13 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork, >> */ >> pwq = get_work_pwq(work); >> if (pwq && pwq->pool == pool) { >> + if ((keep_flags | KEEP_QUEUED) || >> + ((keep_flags | KEEP_FLUSHED) && > > This can't be right. > > Thanks. >
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