Subject: Re: [PATCH] workqueues: insert_work: use "list_head *" instead of
	"int tail"
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Andrew Morton <akpm@linux-foundation.org>, Jarek Poplawski <jarkao2@o2.pl>,
       Max Krasnyansky <maxk@qualcomm.com>, linux-kernel@vger.kernel.org
In-Reply-To: <20080613151725.GA9194@tv-sign.ru>
References: <20080612165120.GA12177@tv-sign.ru>
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	 <20080612174433.GA12204@tv-sign.ru> <1213295939.31518.159.camel@twins>
	 <20080613142658.GA9147@tv-sign.ru> <1213368184.16944.20.camel@twins>
	 <20080613151725.GA9194@tv-sign.ru>
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Date: Fri, 13 Jun 2008 17:32:12 +0200
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On Fri, 2008-06-13 at 19:17 +0400, Oleg Nesterov wrote:
> On 06/13, Peter Zijlstra wrote:
> >
> > On Fri, 2008-06-13 at 18:26 +0400, Oleg Nesterov wrote:
> > > 
> > > Yes, we have a free bit... but afaics we can do better.
> > > 
> > > 	struct context_barrier {
> > > 		struct work_struct   work;
> > > 		struct flush_context *fc;
> > > 		...
> > > 	}
> > > 
> > > 	void context_barrier_barrier_func(struct work_struct *work)
> > > 	{
> > > 		struct flush_context *fc = container_of();
> > > 		if (atomic_dec_and_test())
> > > 			...
> > > 	}
> > > 
> > > 	void insert_context_barrier(work, barr)
> > > 	{
> > > 		...insert barr after work, like flush_work() does...
> > > 	}
> > > 
> > > 	queue_work_contex(struct workqueue_struct *wq,
> > > 			  struct work_struct *work,
> > > 			  struct flush_context *fc)
> > > 	{
> > > 		int ret = queue_work(wq, work);
> > > 		if (ret)
> > > 			insert_context_barrier(work, barr);
> > > 		return ret;
> > > 	}
> > > 
> > > this way we shouldn't change run_workqueue() and introduce a "parallel"
> > > larger work_struct which needs its own INIT_()/etc.
> > 
> > Where does do the context_barrier instances come from, are they
> > allocated in insert_context_barrier() ?
> 
> Ah, indeed. We have to kmalloc() or use a larger work_struct... And if we
> use a larger work_struct, we can make
> 
> 	struct work_struct_context
> 	{
> 		struct work_struct work;
> 		work_func_t real_func;
> 		struct flush_context *fc;
> 	};
> 
> 	static void work_context_func(struct work_struct *work)
> 	{
> 		struct work_struct_context *wc = container_of();
> 		struct flush_context *fc = wc->context;
> 
> 		wc->real_func(work);
> 
> 		if (atomic_dec_and_test(fc->count))
> 			...
> 	}
> 
> to avoid changing run_workqueue(), but this is nasty. Perhaps flush_context
> can be just array or list of queued work_structs, then we can do flush_work()
> for each work_struct...

list would grow work_struct with a list_head, and sizeof(list_head) >
sizeof(conetxt *), and an array would require krealloc().

Neither sounds really appealing..

Anyway, I think before we go further down this road, we'd better see if
anybody actually needs this. Not that theorizing about this problem
isn't fun,... but... :-)

> > > > making all this PI savvy for -rt is going to be fun though.. I guess we
> > > > can just queue a normal barrier of the flusher's priority, and cancel it
> > > > once we complete.. hey - that doesn't sound hard at all :-)
> > >
> > > Yes!!! I think this is much better (because _much_ simple) than re-ordering
> > > the pending work_struct's, we can just boost the whole ->worklist. We can
> > > implement flush_work_pi() in the same manner as queue_work_contex() above.
> > > That is why I said previously that flush_() should govern the priority,
> > > not queue.
> > >
> > > But we can also implement queue_work_pi(struct work_struct_pi *work).
> >
> > in -rt all the workqueue stuff is PI already, we replaced the list_head
> > in work_struct with a plist_node and queue_work() enqueues worklets at
> > the ->normal_prio of the calling task.
> 
> Oh well. IIRC, these patches really uglified^Wcompicated the code. Don't
> get me wrong, if I knew how to make this better, I'd sent the patch ;)
> 
> > Hmm, the required barrier might still spoil - or at least complicate the
> > matter.
> >
> > In order to boost the pending work, we'd need to enqueue a barrer before
> > the high prio worklet - otherwise the high prio worklet will complete
> > before the work we're waiting on.
> 
> flush_work_pi() can boost the priority, and then barrier restores it.
> Can't this work?
> 
> > And we can cancel the high prio worklet once all our worklets of
> > interrest are done, but we cannot cancel the barrier.
> 
> Yes, if we cancel the high prio worklet, this doesn't restore the prio
> immediately.

/me reads back that -rt barrier code,... *groan* head explodes.. I'm
sure we can make it drop the prio on cancel, but I'd have to take a
proper look at it.

But getting rid of the barrier will be very tricky. So it will always
have some side-effects..


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