Re: [RFC PATCH 0/3] page count lock for simpler put_page

[Johannes Weiner <jweiner@redhat.com>]

On Fri, Aug 12, 2011 at 09:08:13AM -0700, Paul E. McKenney wrote:
> On Fri, Aug 12, 2011 at 05:36:16PM +0200, Andrea Arcangeli wrote:
> > On Tue, Aug 09, 2011 at 04:04:21AM -0700, Michel Lespinasse wrote:
> > > - Use my proposed page count lock in order to avoid the race. One
> > > would have to convert all get_page_unless_zero() sites to use it. I
> > > expect the cost would be low but still measurable.
> > 
> > I didn't yet focus at your problem after we talked about it at MM
> > summit, but I seem to recall I suggested there to just get to the head
> > page and always take the lock on it. split_huge_page only works at 2M
> > aligned pages, the rest you don't care about. Getting to the head page
> > compound_lock should be always safe. And that will still scale
> > incredibly better than taking the lru_lock for the whole zone (which
> > would also work). And it seems the best way to stop split_huge_page
> > without having to alter the put_page fast path when it works on head
> > pages (the only thing that gets into put_page complex slow path is the
> > release of tail pages after get_user_pages* so it'd be nice if
> > put_page fast path still didn't need to take locks).
> > 
> > > - It'd be sweet if one could somehow record the time a THP page was
> > > created, and wait for at least one RCU grace period *starting from the
> > > recorded THP creation time* before splitting huge pages. In practice,
> > > we would be very unlikely to have to wait since the grace period would
> > > be already expired. However, I don't think RCU currently provides such
> > > a mechanism - Paul, is this something that would seem easy to
> > > implement or not ?
> 
> It should not be hard.  I already have an API for rcutorture testing
> use, but it is not appropriate for your use because it is unsynchronized.
> 
> We need to be careful with what I give you and how you interpret it.
> The most effective approach would be for me to give you an API that
> filled in a cookie given a pointer to one, then another API that took
> pointers to a pair of cookies and returned saying whether or not a
> grace period had elapsed.  You would do something like the following:
> 
> 	rcu_get_gp_cookie(&pagep->rcucookie);
> 	. . .
> 
> 	rcu_get_gp_cookie(&autovarcookie);
> 	if (!rcu_cookie_gp_elapsed(&pagep->rcucookie, &autovarcookie))
> 		synchronize_rcu();
> 
> So, how much space do I get for ->rcucookie?  By default, it is a pair
> of unsigned longs, but I could live with as small as a single byte if
> you didn't mind a high probability of false negatives (me telling you
> to do a grace period despite 16 of them having happened in the meantime
> due to overflow of a 4-bit field in the byte).
> 
> That covers TREE_RCU and TREE_PREEMPT_RCU, on to TINY_RCU and TINY_PREEMPT_RCU.
> 
> TINY_RCU will require more thought, as it doesn't bother counting grace
> periods.  Ah, but in TINY_RCU, synchronize_rcu() is free, so I simply
> make rcu_cookie_gp_elapsed() always return false.
> 
> OK, TINY_PREEMPT_RCU...  It doesn't count grace periods, either.  But it
> is able to reliably detect if there are any RCU readers in flight,
> and there normally won't be, so synchronize_rcu() is again free in the
> common case.  And no, I don't want to count grace periods as this would
> increase the memory footprint.  And the whole point of TINY_PREEMPT_RCU
> is to be tiny, after all.  ;-)

I understand you want to be careful with the promises you make in the
API.  How about not even exposing the check for whether a grace period
elapsed, but instead provide a specialized synchronize_rcu()?

Something like

	void synchronize_rcu_with(rcu_time_t time)

that only promises all readers from the specified time are finished.

[ And synchronize_rcu() would be equivalent to
  synchronize_rcu_with(rcu_current_time()) if I am not mistaken. ]

Then you wouldn't need to worry about how the return value of
rcu_cookie_gp_elapsed() might be interpreted, could freely implement
it equal to synchronize_rcu() on TINY_RCU, the false positives with
small cookies would not be about correctness but merely performance.

And it should still be all that which the THP case requires.

Would that work?

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