linux-kernel - Re: [RFC PATCH] asynchronous page fault.

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Message-ID: <20100104165652.GC6748@linux.vnet.ibm.com>
Date:	Mon, 4 Jan 2010 08:56:52 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	cl@...ux-foundation.org,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH] asynchronous page fault.

On Mon, Jan 04, 2010 at 05:02:54PM +0100, Peter Zijlstra wrote:
> On Mon, 2010-01-04 at 07:55 -0800, Paul E. McKenney wrote:
> > > Well, I was thinking srcu to have this force quiescent state in
> > > call_srcu() much like you did for the preemptible rcu.
> > 
> > Ah, so the idea would be that you register a function with the srcu_struct
> > that is invoked when some readers are stuck for too long in their SRCU
> > read-side critical sections?  Presumably you also supply a time value for
> > "too long" as well.  Hmmm...  What do you do, cancel the corresponding
> > I/O or something? 
> 
> Hmm, I was more thinking along the lines of:
> 
> say IDX is the current counter idx.
> 
> if (pending > thresh) {
>   flush(!IDX)

This flushes pending I/Os?

>   force_flip_counter();

If this is internal to SRCU, what it would do is check for CPUs being
offline or in dyntick-idle state.  Or was your thought that this is
where I invoke callbacks into your code to do whatever can be done to
wake up the sleeping readers?

> }
> 
> Since we explicitly hold a reference on IDX, we can actually wait for !
> IDX to reach 0 and flush those callbacks.

One other thing -- if I merge SRCU into the tree-based infrastructure,
I should be able to eliminate the need for srcu_read_lock() to return
the index (and thus for srcu_read_unlock() to take it as an argument).
So the index would be strictly internal, as it currently is with the
other flavors of RCU.

> We then force-flip the counter, so that even if all callbacks (or the
> majority) were not for !IDX but part of IDX, we'd be able to flush them
> on the next call_srcu() because that will then hold a ref on the new
> counter index.

We can certainly defer callbacks to a later grace period.  What we cannot
do is advance the counter until all readers for the current grace period
have exited their SRCU read-side critical sections.

> Or am I missing something obvious?

Or maybe I am.

							Thanx, Paul
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