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

Kent Overstreet
2024-02-27T01:08:28Z

On Mon, Feb 26, 2024 at 04:55:29PM -0800, Paul E. McKenney wrote:
> 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.

__ksize() is the right helper to use for this; ksize() is "how much
usable memory", __ksize() is "how much does this occupy".

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

__ksize() for slab memory, but folio_size() would be for page
allocations - actually, I think compound_order() is more appropriate
here, but that's willy's area. IOW, for free_pages_rcu(), which AFAIK we
don't have yet but it looks like we're going to need.

I'm scanning through vmalloc.c and I don't think we have a helper yet to
query the allocation size - I can write one tomorrow, giving my brain a
rest today :)