Re: [PATCH v2] rcu/kvfree: Add kvfree_rcu_barrier() API

[Paul E. McKenney]

On Thu, Sep 12, 2024 at 06:16:56PM +0200, Uladzislau Rezki wrote:
> On Wed, Sep 11, 2024 at 03:39:19AM -0700, Paul E. McKenney wrote:
> > On Wed, Sep 11, 2024 at 11:43:54AM +0200, Uladzislau Rezki wrote:
> > > On Tue, Sep 10, 2024 at 08:42:54AM -0700, Paul E. McKenney wrote:
> > > > On Tue, Aug 20, 2024 at 05:59:35PM +0200, Uladzislau Rezki (Sony) wrote:
> > > > > Add a kvfree_rcu_barrier() function. It waits until all
> > > > > in-flight pointers are freed over RCU machinery. It does
> > > > > not wait any GP completion and it is within its right to
> > > > > return immediately if there are no outstanding pointers.
> > > > > 
> > > > > This function is useful when there is a need to guarantee
> > > > > that a memory is fully freed before destroying memory caches.
> > > > > For example, during unloading a kernel module.
> > > > > 
> > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > > > > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> > > > 
> > > > As a follow-on patch, once kvfree_rcu_barrier() is accepted into
> > > > mainline, should we add a call to kvfree_rcu_barrier() to the
> > > > rcu_barrier_throttled() function in kernel/rcu/tree.c?
> > > > 
> > > > This would allow the do_rcu_barrier module parameter to be used to clear
> > > > out kfree_rcu() as well as call_rcu() work.  This would be useful to
> > > > people running userspace benchmarks that cause the kernel to do a lot
> > > > of kfree_rcu() calls.  Always good to avoid messing up the results from
> > > > the current run due to deferred work from the previous run.  Even better
> > > > would be to actually account for the deferred work, but do_rcu_barrier
> > > > can help with that as well.  ;-)
> > > > 
> > > > Thoughts?
> > > >
> > > Makes sense. To be make sure that all objects are flushed. And as you
> > > mentioned it is good to have it for benchmarking as a return to a baseline
> > > point.
> > > 
> > > One issue is probably a "name" which would be common for both:
> > > 
> > > rcu_barrier()
> > > kvfree_rcu_barrier()
> > > 
> > > i mean /sys/module/rcutree/parameters/do_rcu_barrier. From how i
> > > would see it, it is supposed to trigger just rcu_barrier() API.
> > 
> > One approach would be to keep the old functionality, but create
> > a new sysfs variable that does both.  Except that to avoid code
> > duplication, we would likely end up with both actually doing
> > both.
> > 
> > Another approach is to rename the sysfs variable.  This might
> > work if there are not too many people using it.  Might.  ;-)
> > 
> > Other approaches?
> > 
> Maybe just rename from/to: do_rcu_barrier -> do_barrier? Probably this
> would be the best, but as you noted, there might be users :)
> 
> To be safe, we can add kvfree_rcu_barrier() to the rcu_barrier_throttled() and  
> document that it does both now!

That does sound safest to me.  We just might find that our users (if any)
expected that it already did both.  ;-)

							Thanx, Paul