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

[Johannes Weiner <jweiner@redhat.com>]

On Fri, Aug 12, 2011 at 07:08:23PM +0200, Andrea Arcangeli wrote:
> On Fri, Aug 12, 2011 at 06:57:49PM +0200, Johannes Weiner wrote:
> > 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?
> 
> rcu_time_t would still be an unsigned long long like I suggested?

Do we even need to make this fixed?  It can be unsigned long long for
now, but I could imagine leaving it up to the user depending how much
space she is able/willing to invest to save time:

	void synchronize_rcu_with(unsigned long time, unsigned int bits)
	{
		if (generation_counter & ((1 << bits) - 1) == time)
			synchronize_rcu();
	}

If you have only 3 bits to store the time, you will synchronize
falsely to every 8th phase.  Better than nothing, right?

> About the false positives thing, I failed to see how it's ever
> possible to return only false positives and never false negatives when
> cookie and internal counter are not of the same size (and cookie has
> no enough bits to ever tell if it overflowed or not).

I don't see how.  Even with one bit for the time stamp you get every
second generation right :-)

> I think rcu_generation_t is more appropriate because it's not time but
> a generation/sequence counter.

I intentionally chose a vague name as the unit should be irrelevant to
the outside world.  But I don't feel strongly about this.

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