Re: [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO

[Kent Overstreet <kent.overstreet@linux.dev>]

On Mon, Feb 26, 2024 at 04:05:37PM -0800, Paul E. McKenney wrote:
> On Mon, Feb 26, 2024 at 06:29:43PM -0500, Kent Overstreet wrote:
> > Well, we won't want it getting hammered on continuously - we should be
> > able to tune reclaim so that doesn't happen.
> > 
> > I think getting numbers on the amount of memory stranded waiting for RCU
> > is probably first order of business - minor tweak to kfree_rcu() et all
> > for that; there's APIs they can query to maintain that counter.
> 
> We can easily tell you the number of blocks of memory waiting to be freed.
> But RCU does not know their size.  Yes, we could ferret this on each
> call to kmem_free_rcu(), but that might not be great for performance.
> We could traverse the lists at runtime, but such traversal must be done
> with interrupts disabled, which is also not great.
> 
> > then, we can add a heuristic threshhold somewhere, something like 
> > 
> > if (rcu_stranded * multiplier > reclaimable_memory)
> > 	kick_rcu()
> 
> If it is a heuristic anyway, it sounds best to base the heuristic on
> the number of objects rather than their aggregate size.

I don't think that'll really work given that object size can very from <
100 bytes all the way up to 2MB hugepages. The shrinker API works that
way and I positively hate it; it's really helpful for introspection and
debugability later to give good human understandable units to this
stuff.

And __ksize() is pretty cheap, and I think there might be room in struct
slab to stick the object size there instead of getting it from the slab
cache - and folio_size() is cheaper still.