Re: [PATCH 1/4] rcu/kvfree: Support dynamic rcu_head for single argument objects

[Paul E. McKenney]

On Wed, Aug 28, 2024 at 07:00:11PM +0200, Uladzislau Rezki wrote:
> On Wed, Aug 28, 2024 at 04:58:48PM +0200, Vlastimil Babka wrote:
> > On 8/28/24 13:09, Uladzislau Rezki (Sony) wrote:
> > > Add a support of dynamically attaching an rcu_head to an object
> > > which gets freed via the single argument of kvfree_rcu(). This is
> > > used in the path, when a page allocation fails due to a high memory
> > > pressure.
> > > 
> > > The basic idea behind of this is to minimize a hit of slow path
> > > which requires a caller to wait until a grace period is passed.
> > > 
> > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > 
> > So IIUC it's a situation where we can't allocate a page, but we hope the
> > kmalloc-32 slab has still free objects to give us dyn_rcu_head's before it
> > would have to also make a page allocation?
> > 
> Yes, you understood it correctly :)
> 
> >
> > So that may really be possible and there might potentially be many such
> > objects, but I wonder if there's really a benefit. The system is struggling
> > for memory and the single-argument caller specifically is _mightsleep so it
> > could e.g. instead go direct reclaim a page rather than start depleting the
> > kmalloc-32 slab, no?
> > 
> This is a good question about benefit and i need to say that i do not
> have a strong opinion here. I post this patch to get some opinions about
> it. This "dynamic attaching" we discussed with RCU folk a few years ago
> and decided not to go with it. I have not found an information why.

If I remember correctly, I asked "How are you testing this?", which
was then taken as a criticism rather than a question.  ;-)

No one has reported an OOM-related problem with the code in its current
form, for what little that is worth.

							Thanx, Paul

> The page request path, which is "normal/fast", can lead to a "light"
> direct reclaim, if still fails, then we are in a high pressure situation.
> Depleting a slab is probably not worth it, especially that the patch in
> this series:
> 
> [PATCH 4/4] rcu/kvfree: Switch to expedited version in slow path
> 
> switches to more faster synchronize_rcu() version to speedup a reclaim.
> 
> + this [PATCH 3/4] rcu/kvfree: Use polled API in a slow path
> which also improves a slow path in terms of that a GP might be already
> passed for the object being freed.
> 
> I am totally OK to drop this patch. This is fine to me.
> 
> --
> Uladzislau Rezki