Messages in this thread | | | Date | Wed, 21 Oct 2009 10:53:15 -0400 | From | Mathieu Desnoyers <> | Subject | Re: Kernel RCU: shrink the size of the struct rcu_head |
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* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > On Sun, Oct 18, 2009 at 07:29:18PM -0400, Mathieu Desnoyers wrote: > > Hi Paul, > > > > I noticed that you already discussed the possibility of shrinking the > > struct rcu_head by removing the function pointer. > > (http://kernel.org/pub/linux/kernel/people/paulmck/rcutodo.html) > > > > The ideas brought in so far require having per-callback lists, which > > involves a bit of management overhead and don't permit keeping the > > call_rcu() in cpu order. > > But please note that this is on the "Possibly Dubious Changes" list. ;-) > > > You might want to look into the Userspace RCU urcu-defer.c > > implementation, where I perform pointer encoding to compact the usual > > case, expected to be the same callback passed as parameter multiple > > times in a row to call_rcu(). This is very typical with multiple free() > > calls for different data structures next to each other. > > > > This typically keeps the size of the information to encode per callback > > down to a minimum: the size of a single pointer. It would be good to > > trace the kernel usage of call_rcu() to see if my assumption holds. > > > > I just thought I should tell you before you start looking at this > > issue further. > > So the idea is to maintain a per-CPU queue of function pointers, but > with the pointers on this queue encoded to save space, correct?
Yes, exactly.
> If I > understand correctly, the user-level rcu-defer implementation relies on > the following: > > 1. It is illegal to call _rcu_defer_queue() within an RCU read-side > critical section (due to the call to rcu_defer_barrier_thread() > which in turn calls synchronize_rcu(). This is necessary to > handle queue overflow. (Which appears to be why you introduce > a new API, as it is legal to invoke call_rcu() from within an > RCU read-side critical section.)
When dealing with queue overflow, I figured we have 4 alternatives. Either:
1, 2, 3) We proceed to execution of {the single, all, thread local} callback(s) on the spot after a synchronize_rcu().
4) We expand the queue by allocating more memory.
The idea of pointer encoding to save space could be used with any of 1, 2, 3, or 4. As you say, call_rcu() requires (4), because it tolerates being called from an rcu read-side C.S.. 1, 2, 3 are incompatible with read-side C.S. context because they require to use synchronize_rcu() within the C.S., which would deadlock on its calling context.
Now, there is a rationale for the choice of (3) in my urcu-defer implementation:
* It's how I can deal with memory full (-ENOMEM) without letting the system die with exit(). How does the kernel call_rcu() deal with this currently ? BUG_ON, WARN_ON ?
* It acts as a rate limiter for urcu_defer_queue(). Basically, if a thread starts enqueuing callbacks too fast, it will eventually fill its queue and have to empty it itself. AFAIK, It's not possible to do that if you allow call_rcu() to be called from read-side C.S..
I could even extend rcu_defer_queue() to take a second rate-limiter callback, which would check if the thread went over some threshold and give a more precise limit (e.g. amount of memory to be freed) on the rate than the "4096 callbacks in flight max", which have been chosen by benchmarks, but is a bit arbitrary in terms of overall callback effect.
How important is it to permit enqueuing callbacks from within rcu read-side C.S. in terms of real-life usage ? If it is really that important to fill this use-case, then I could have a mode for call_rcu() that expands the RCU callback queue upon overflow. But as I argue above, I really prefer the control we have with a fixed-sized queue.
> > 2. It is OK to wait for a grace period when a thread calls > rcu_defer_unregister_thread() while exiting. In the kernel, > this is roughly equivalent to the CPU_DYING notifier, which > cannot block, thus cannot wait for a grace period. > > I could imagine copying the per-CPU buffer somewhere, though > my experience with the RCU/CPU-hotplug interface does not > encourage me in this direction. ;-)
As you say, we don't _have_ to empty the queue before putting a thread/cpu offline. We could simply copy the unplugged cpu queue to an orphan queue, as you currently do in your implementation. I agree that it would be more suitable to the cpu hotplug CPU_DYING execution context, due to its inherent trickiness.
Thanks,
Mathieu
> > Thanx, Paul
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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