Re: [PATCH] TOMOYO: Add garbage collector support. (v3)

[Paul E. McKenney]

On Sat, Jun 20, 2009 at 04:04:43PM +0900, Tetsuo Handa wrote:
> Hello.
> 
> Paul E. McKenney wrote:
> > On Fri, Jun 19, 2009 at 01:57:46PM +0900, Tetsuo Handa wrote:
> > > Hello.
> > > 
> > > The GC thread is a loop of
> > > 
> > > (1) Take gc_mutex
> > > (2) Remove an element from the list using RCU
> > > (3) Wait for readers without releasing gc_mutex
> > > (4) Free up that element
> > > (5) Release gc_mutex
> > > 
> > > A new round will not see element which was removed by previous round.
> > 
> > Understood.
> > 
> > > Paul E. McKenney wrote:
> > > > > > Consider the following sequence of events:
> > > > > > 
> > > > > > o	CPU 0 picks up users_counter_idx int local variable idx.
> > > > > > 	Let's assume that the value is zero.
> > > > > > 
> > > > > > o	CPU 0 is now preempted, interrupted, or otherwise delayed.
> > > > > > 
> > >       o	CPU 1 takes gc_mutex.
> > 
> > Your (1).
> > 
> > > 
> > > > > > o	CPU 1 starts garbage collection, finding some elements to
> > > > > > 	delete, thus setting "element_deleted" to true.
> > 
> > Your (2).
> > 
> > > > > > o	CPU 1 continues garbage collection, inverting the value of
> > > > > > 	users_counter_idx, so that the value is now one, waiting
> > > > > > 	for the value-zero readers, and freeing up the old elements.
> > 
> > Your (3) and (4).
> > 
> > >       o	CPU 1 releases gc_mutex.
> > > > [1]
> > 
> > Your (5).
> > 
> > > > > > o	CPU 0 continues execution, first atomically incrementing
> > > > > > 	users_counter[0], then traversing the list, possibly sleeping.
> > 
> > Now the trick here is that CPU 0 has the old value of users_counter_idx.
> > So the reader and the garbage collector now disagree on which interval
> > they are operating in.
> > 
> > And CPU 0 might now be holding an element that will be deleted by the
> > next round of GC.
> > 
> > >       o	CPU 2 takes gc_mutex.
> > 
> > Your (1) again.  Presumably your single kernel thread migrated from
> > CPU 1 to CPU 2, which could really happen.
> > 
> > > > > > o	CPU 2 starts a new round of garbage collection, again finding
> > > > > > 	some elements to delete, and thus again setting
> > > > > > 	"elements_deleted" to true.  One of the elements deleted
> > > > > > 	is the one that CPU 0 is currently referencing while asleep.
> > 
> > Your (2) again.
> > 
> > > > > No. CPU 2 can't start a new round of GC because GC function is exclusively
> > > > > executed because of gc_mutex mutex.
> > > > 
> > > > But CPU 1 would have released gc_mutex back at time [1], right?
> > > > 
> > > Yes, CPU 1 will release gc_mutex after freeing up elements (which were removed
> > > from the list after gc_mutex was taken).
> > > 
> > > If CPU 0 sleeps between "idx = atomic_read(&users_counter_idx)" and
> > > "atomic_inc(&users_counter[idx])", CPU 0 will not see the element
> > > removed by CPU 1 because CPU 0 has not started list traversal.
> > > Same result for CPU 0 sleeping between "atomic_inc(&users_counter[idx])"
> > > and "list_for_each_rcu() {".
> > 
> > No, CPU 0 really did start list traversal three bullets ago.  The
> > problem is that the reader and gc disagree on what interval they are in.
> > 
> > > > > > o	CPU 2 continues garbage collection, inverting the value of
> > > > > > 	users_counter_idx, so that the value is now zero, waiting
> > > > > > 	for the value-one readers, and freeing up the old elements.
> > > > > > 	Note that CPU 0 is a value-zero reader, so that CPU 2 will
> > > > > > 	not wait on it.
> > > > > > 
> > > > > > 	CPU 2 therefore kfree()s the element that CPU 0 is currently
> > > > > > 	referencing.
> > 
> > Your (3) and (4) again.  Note that the reader has incremented
> > users_counter[0], while the GC is waiting only for users_counter[1].
> > So the GC is not going to wait for the reader.
> > 
> > > > > CPU 2 won't continue GC, for CPU 2 can't start a new round of GC.
> > > > 
> > > > I still don't see why CPU 0 would not have released gc_mutex back
> > > > at point [1].
> > > > 
> > > CPU 1 has released gc_mutex at point [1].
> > > In that case, CPU 2 can take gc_mutex and start a new round.
> > > Nobody can start a new round before previous round finishes.
> > > 
> > > CPU 2 can start a new round, but by that time, CPU 0 finished list traversal
> > > and atomically decremented users_counter[0] . CPU 1 won't finish a GC round
> > > before CPU 0 decrements users_counter[0], and thus CPU 2 won't start
> > > a new GC round before CPU 0 finishes list traversal.
> > 
> > No, because CPU 2 is waiting on users_counter[1] to reach zero, but
> > the reader has incremented users_counter[0].  GC will thus -not- wait
> > on the reader.
> > 
> Ah, I understood. You are right.
> CPU 2 has to wait for not only users_counter[1] but also users_counter[0].

This sort of algorithm is indeed subtle and difficult to get right.
Let's just say that I have made this same mistake in the past, as has
everyone else that I know of who has tried something similar.  ;-)

> > Modern CPUs are quite complex.  There is a multi-cycle penalty for the
> > instruction being atomic in the first place, and there can be many tens
> > or even hundreds of cycles penalty if the variable to be manipulated
> > resides in some other CPU's cache.
> > 
> I thought atomic_t is a handy and lightweight counter. But atomic_t may
> cause big penalty. I see.

The two exceptions are atomic_read() and atomic_write(), which, despite
the names, do not involve atomic instructions.  They are instead for
type safety.

> > These penalties were larger in older SMP hardware.  Also, in general,
> > the larger the system, the worse the penalties.  Getting data on and off
> > a chip is quite expensive.  See slide 11 of:
> > 
> > http://www.rdrop.com/users/paulmck/scalability/paper/TMevalSlides.2008.10.19a.pdf
> > 
> > for measurements on a few-years-old system.  Newer multi-core systems
> > are about a factor of six faster, but only if you keep everything on a
> > single die.  If you go to multiple sockets, there is still improvement,
> > but only a factor of two or so in terms of clock period.
> > 
> Wow, what a large difference.

Yeah, little problems with the finite speed of light, much less electrons.
And the atomic nature of matter.

> > > Another keyword which is worrisome for me is NUMA.
> > > My understanding is that NUMA splits RAM into nodes and tries to use RAM
> > > in current node.
> > > In NUMA environment, (for example) "mov eax, [ebx]" takes three CPU cycles
> > > if ebx refers current node and hundred CPU cycles if ebx refers other node?
> > > Then, is it preferable to place copy of ACL information to every node
> > > rather than sharing one ACL information?
> > 
> > Even without NUMA, a load that misses all caches and comes from DRAM
> > costs many tens or even a few hundred cycles.  NUMA increases the pain,
> > normally by a small multiple.  The exact numbers will depend on the
> > hardware, of course.
> > 
> I see. NUMA's pain is smaller than I thought.
> I don't need to worry about NUMA for the foreseeable future.

Indeed, you usually only need to worry about NUMA after you have solved
the SMP problems.

> > > Subject: [PATCH] SRCU: Allow longer timeout for non-urgent reclaimer.
> > > 
> > > Currently synchronize_srcu() checks for readers for every jiffies.
> > > But if reader sleeps for long, we don't need to check so frequently.
> > > 
> > > This patch allows non-urgent SRCU reclaimers (e.g. checking for every second
> > > is sufficient) to use longer timeout.
> > 
> > Looks good to me!  Of course, if it turns out that you don't actually
> > need it, then not much benefit in including it, but:
> > 
> > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> I see. Regarding my environment (VMware on Core2Duo PC), it seems no problem
> because the GC thread does not appear on /usr/bin/top .
> But if somebody noticed (maybe embedded/realtime/huge systems),
> let's apply this.

Fair enough!!!

						Thanx, Paul
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