Re: [PATCH 07/12] s390: add pte_free_defer(), with use of mmdrop_async()

Hugh Dickins
2023-06-15T21:09:36Z

On Thu, 15 Jun 2023, Jason Gunthorpe wrote:
> On Wed, Jun 14, 2023 at 02:59:33PM -0700, Hugh Dickins wrote:
>
> > I guess the best thing would be to modify kernel/fork.c to allow the
> > architecture to override free_mm(), and arch/s390 call_rcu to free mm.
> > But as a quick and dirty s390-end workaround, how about:
>
> RCU callbacks are not ordered so that doesn't seem like it helps..

Thanks, that's an interesting and important point, which I need to knock
into my head better.

But can you show me where that's handled in the existing mm/mmu_gather.c
include/asm-generic/tlb.h framework? I don't see any rcu_barrier()s
there, yet don't the pmd_huge_pte pointers point into pud page tables
freed shortly afterwards also by RCU?

>
> synchronize_rcu would do the job since it is ordered, but I think the
> performance cost is too great to just call it from mmdrop

Yes, on x86 it proved to be a non-starter; maybe s390 doesn't have the
same limitation, but it was clear I was naive to hope that a slowdown
on the exit mm path might not be noticeable.

>
> rcu_barrier() followed by call_rcu on the mm struct might work, but I
> don't know the cost

SLAB_TYPESAFE_BY_RCU handling has the rcu_barrier() built in,
when the slab is destroyed.

>
> A per-cpu refcount scheme might also do the job reasonably
>
> Making the page frag pool global (per-cpu global I guess) would also
> remove the need to reach back to the freeable mm_struct and reduce the
> need for struct page memory. This views it as a special kind of
> kmemcache.

I haven't thought in that direction at all. Hmm. Or did I think of
it once, but discarded for accounting reasons - IIRC (haven't rechecked)
page table pages are charged to memcg, and counted for meminfo and other(?)
purposes: if the fragments are all lumped into a global pool, we lose that.
I think I decided: maybe a good idea, but not a change I should make to
get me out of this particular hole.

>
> Another approach is to not use a rcu_head in the ptdesc at all.
>
> With a global kmemcache-like-thing we could probably also organize
> something where you don't use a rcu_head in the ptdesc, but instead
> just a naked 'next' pointer. This would give enough space to have two
> next pointers and the next pointers can be re-used for the normal free
> list as well.
>
> In this flow you'd thread the free'd frags onto a waterfall of global
> per-cpu lists:
> - RCU free the next cycle
> - RCU free this cycle
> - Actually free
>
> Where a single rcu_head and single call_rcu frees the entire 2nd list
> to the 3rd list and then schedules the 1st list to be RCU'd next. This
> eliminates the need to store a function pointer in the ptdesc at
> all.
>
> It requires some global per-cpu lock on the free/alloc paths however,
> but this is basically what every other arch does as it frees the page
> back to the page allocator.
>
> I suspect that two next pointers would also eliminate pt_frag_refcount
> entirely as we can encode that information in the low bits of the next
> pointers.

This scheme is clearer in your head than it is in mine. It may be the
best solution, but I don't see it clearly enough to judge. I'll carry
on with my way, then you can replace it later on.
>
> > (Funnily enough, there's no problem when the stored mm gets re-used for
> > a different mm, once past its spin_lock_init(&mm->context.lock);
> > because
>
> We do that have really weird "type safe by rcu" thing in the
> allocators, but I don't quite know how it works.

I'm quite familiar with it, since I invented it (SLAB_DESTROY_BY_RCU
in 2.6.9 to solve the locking for anon_vma): so it does tend to be my
tool of choice when appropriate. It's easy: but you cannot reinitialize
the structure on each kmem_cache_alloc(), in particular the spinlocks of
the new allocation may have to serve a tail of use from a previous
allocation at the same address.

>
> > Powerpc is like that. I have no idea how much gets wasted that way.
> > I was keen not to degrade what s390 does: which is definitely superior,
> > but possibly not worth the effort.
>
> Yeah, it would be good to understand if this is really sufficiently
> beneficial..
>
> > I'll look into it, once I understand c2c224932fd0. But may have to write
> > to Vishal first, or get the v2 of my series out: if only I could work out
> > a safe and easy way of unbreaking s390...

My latest notion is, just for getting v2 series out, a global spinlock:
to be replaced before reaching an actual release.

>
> Can arches opt in to RCU freeing page table support and still keep
> your series sane?

Yes, or perhaps we mean different things: I thought most architectures
are already freeing page tables by RCU. s390 included.
"git grep MMU_GATHER_RCU_TABLE_FREE" shows plenty of selects.

>
> Honestly, I feel like trying to RCU enable page tables should be its
> own series. It is a sufficiently tricky subject on its own right.

Puzzled,
Hugh