From: Tejun Heo <tj@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
mingo@redhat.com, akpm@linux-foundation.org,
peterz@infradead.org
Subject: Re: [PATCHSET] timer: clean up initializers and implement irqsafe timers
Date: Tue, 14 Aug 2012 12:15:49 -0700 [thread overview]
Message-ID: <20120814191549.GX25632@google.com> (raw)
In-Reply-To: <alpine.LFD.2.02.1208131720070.32033@ionos>
Hello, Thomas.
On Tue, Aug 14, 2012 at 08:55:16PM +0200, Thomas Gleixner wrote:
> > * mod_delayed_work() can't be used from IRQ handlers.
>
> This function does not exist. So what?
It makes the workqueue users messy. It's difficult to get completely
correct and subtle errors are difficult to detect / verify.
> > * __cancel_delayed_work() can't use the usual try_to_grab_pending()
> > which handles all three states but instead only deals with the first
> > state using a separate implementation. There's no way to make a
> > delayed_work not pending from IRQ handlers.
>
> And why is that desired and justifies the mess you are trying to
> create in the timer code?
Because API forcing its users to be messy is stupid.
> > * The context / behavior differences among cancel_delayed_work(),
> > __cancel_delayed_work(), cancel_delayed_work_sync() are subtle and
> > confusing (the first two are mostly historical tho).
>
> We have a lot of subtle differences in interfaces for similar
> reasons.
And we should work to make them better.
> > This patchset implements irqsafe timers. For an irqsafe timer, IRQ is
> > not enabled from dispatch till the end of its execution making it safe
> > to drain the timer regardless of context. This will enable cleaning
> > up delayed_work interface.
>
> By burdening crap on the timer code. We had a similar context case
> handling in the original hrtimers code and Linus went berserk on it.
> There is no real good reason to reinvent it in a different flavour.
>
> Your general approach about workqueues seems to be adding hacks into
> other sensitive code paths [ see __schedule() ]. Can we please stop
> that? workqueues are not so special to justify that.
The schedule thing worked out pretty well, didn't it? If it improves
the kernel in general, I don't see why timer shouldn't participate in
it. Timer ain't that special either. However, it does suck to add
one-off feature which isn't used by anyone else but I couldn't find a
better way.
So, if you can think of something better, sure. Let's see.
> Right now delayed work arms a timer, whose callback enqueues the work
> and wakes the worker thread, which then executes the work.
>
> So what about changing delayed_work into:
>
> struct delayed_work {
> struct work_struct work;
> unsigned long expires;
> };
>
> Now when delayed work gets scheduled it gets enqueued into a separate
> list in the workqueue core with the proper worker lock held. Then
> check the expiry time of the new work against the current expiry time
> of a timer in the worker itself.
Work items aren't assigned to worker on queue. It's a shared worker
pool. Workers take work items when they can.
> If the new expiry time is after the
> current expiry time, nothing to do. If the new expiry is before the
> current expiry time or the timer is not armed, then (re)arm the timer.
>
> When the timer expires it wakes the worker and that evaluates the
> delayed list for expired works and executes them and rearms the timer
> if necessary.
How are you gonna decide which worker a delayed work item should be
queued on? What if the work item before it takes a very long time to
finish? Do we migrate those work items to a different worker?
> To cancel delayed work you don't have to worry about the timer
> callback being executed at all, simply because the timer callback is
> just a wakeup of the worker and not fiddling with the work itself. If
> the work is removed before the worker thread runs, life goes on as
> usual.
>
> So all you have to do is to remove the work from the delayed list. If
> the timer is armed, just leave it alone and let it fire. Canceling
> delayed work is probably not a high frequency operation.
>
> In fact that would make cancel_delayed_work and cancel_work basically
> the same operation.
>
> I have no idea how many concurrent delayed works are on the fly, so I
> can't tell whether a simple ordered list is sufficient or if you need
> a tree which is better suited for a large number of sorted items. But
> that's a trivial to solve detail.
Aside from work <-> worker association confusion, you're basically
suggesting for workqueue to implement its own tvec_base in suboptimal
way. Doesn't make much sense to me.
Thanks.
--
tejun
next prev parent reply other threads:[~2012-08-14 19:15 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-08 18:10 Tejun Heo
2012-08-08 18:10 ` [PATCH 1/4] timer: generalize timer->base flags handling Tejun Heo
2012-08-21 16:40 ` [tip:timers/core] timer: Generalize " tip-bot for Tejun Heo
2012-08-08 18:10 ` [PATCH 2/4] timer: relocate declarations of init_timer_on_stack_key() Tejun Heo
2012-08-21 16:41 ` [tip:timers/core] timer: Relocate " tip-bot for Tejun Heo
2012-08-08 18:10 ` [PATCH 3/4] timer: clean up timer initializers Tejun Heo
2012-08-21 16:42 ` [tip:timers/core] timer: Clean " tip-bot for Tejun Heo
2012-08-08 18:10 ` [PATCH 4/4] timer: implement TIMER_IRQSAFE Tejun Heo
2012-08-21 16:43 ` [tip:timers/core] timer: Implement TIMER_IRQSAFE tip-bot for Tejun Heo
2012-08-21 19:26 ` Tejun Heo
2012-08-08 18:13 ` $SUBJ should have been "[PATCHSET] timer: clean up initializers and implement irqsafe timers" Tejun Heo
2012-08-13 23:35 ` [PATCHSET] timer: clean up initializers and implement irqsafe timers Tejun Heo
2012-08-14 8:32 ` Thomas Gleixner
2012-08-14 19:16 ` Thomas Gleixner
2012-08-14 19:22 ` Tejun Heo
2012-08-14 21:03 ` Thomas Gleixner
2012-08-14 21:56 ` Tejun Heo
2012-08-14 22:45 ` Thomas Gleixner
2012-08-14 23:01 ` Tejun Heo
2012-08-14 23:33 ` Thomas Gleixner
2012-08-15 0:18 ` Tejun Heo
2012-08-15 10:58 ` Peter Zijlstra
2012-08-16 19:36 ` Tejun Heo
2012-08-14 18:55 ` Thomas Gleixner
2012-08-14 19:15 ` Tejun Heo [this message]
2012-08-14 20:43 ` Thomas Gleixner
2012-08-14 21:40 ` Tejun Heo
2012-08-14 23:12 ` Thomas Gleixner
2012-08-14 23:27 ` Tejun Heo
2012-08-14 23:46 ` Thomas Gleixner
2012-08-14 23:52 ` Tejun Heo
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