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

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On Mon, Feb 26, 2024 at 07:29:04PM -0500, Kent Overstreet wrote:
> 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.

You might well be right, but let's please try it before adding overhead to
kfree_rcu() and friends.  I bet it will prove to be good and sufficient.

> 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.

On __ksize():

 * This should only be used internally to query the true size of allocations.
 * It is not meant to be a way to discover the usable size of an allocation
 * after the fact. Instead, use kmalloc_size_roundup().

Except that kmalloc_size_roundup() doesn't look like it is meant for
this use case.  On __ksize() being used only internally, I would not be
at all averse to kfree_rcu() and friends moving to mm.

The idea is for kfree_rcu() to invoke __ksize() when given slab memory
and folio_size() when given vmalloc() memory?

							Thanx, Paul




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