Re: [PATCH v2 2/5] mm: move per-vma lock into vm_area_struct

[Lorenzo Stoakes]

On Wed, Nov 13, 2024 at 03:45:24PM +0100, Vlastimil Babka wrote:
> On 11/13/24 15:28, Lorenzo Stoakes wrote:
> > On Tue, Nov 12, 2024 at 11:46:32AM -0800, Suren Baghdasaryan wrote:
> >> Back when per-vma locks were introduces, vm_lock was moved out of
> >> vm_area_struct in [1] because of the performance regression caused by
> >> false cacheline sharing. Recent investigation [2] revealed that the
> >> regressions is limited to a rather old Broadwell microarchitecture and
> >> even there it can be mitigated by disabling adjacent cacheline
> >> prefetching, see [3].
> >
> > I don't see a motivating reason as to why we want to do this? We increase
> > memory usage here which is not good, but later lock optimisation mitigates
>
> I'd say we don't normally split logically single structures into multiple
> structures just because they might pack better in multiple slabs vs single
> slab. Because that means more complicated management, extra pointer
> dereferences etc. And if that split away part is a lock, it even complicates
> things further. So the only motivation for doing that split was that it
> appeared to perform better, but that was found to be misleading.
>
> But sure it can be described better, and include the new
> SLAB_TYPESAFE_BY_RCU conversion part as the motivation - that would be
> likely impossible to do with a split away lock.

Right, my point is that there is no justification given here at all, and we
should give one. I understand the provenance of why we split the lock, but
there has to be a motivating reason if everything is working fine right
now.

The SLAB_TYPESAFE_BY_RCU one seems to be the key one, but also something
along the lines of complicated management, concern about ordering of
allocating/freeing things, etc.

Just needs some extra explanation here.

>
> > it, but why wouldn't we just do the lock optimisations and use less memory
> > overall?
>
> If the lock is made much smaller then the packing benefit by split might
> disappear, as is the case here.
>

Yeah, but the lock would be smaller so we'd save space still right?


> >> This patchset moves vm_lock back into vm_area_struct, aligning it at the
> >> cacheline boundary and changing the cache to be cache-aligned as well.
> >> This causes VMA memory consumption to grow from 160 (vm_area_struct) + 40
> >> (vm_lock) bytes to 256 bytes:
> >>
> >>     slabinfo before:
> >>      <name>           ... <objsize> <objperslab> <pagesperslab> : ...
> >>      vma_lock         ...     40  102    1 : ...
> >>      vm_area_struct   ...    160   51    2 : ...
> >
> > Pedantry, but it might be worth mentioning how much this can vary by config.
> >
> > For instance, on my machine:
> >
> > vm_area_struct    125238 138820    184   44
> >
> >>
> >>     slabinfo after moving vm_lock:
> >>      <name>           ... <objsize> <objperslab> <pagesperslab> : ...
> >>      vm_area_struct   ...    256   32    2 : ...
> >>
> >> Aggregate VMA memory consumption per 1000 VMAs grows from 50 to 64 pages,
> >> which is 5.5MB per 100000 VMAs. This memory consumption growth can be
> >> addressed later by optimizing the vm_lock.
> >
> > Yes grabbing this back is of critical importance I'd say! :)
>
> I doubt it's that critical. We'll have to weight that against introducing
> another non-standard locking primitive.

Avoiding unnecessary VMA overhead is important, and a strong part of the
motivation for the guard pages series for instance, so I don't think we
should be unconcerned about unnecessary extra memory usage.

I'm guessing from what you say you're not in favour of the subsequent
'non-standard' locking changes...

>
> > Functionally it looks ok to me but would like to see a stronger
> > justification in the commit msg! :)
> >
> >>
> >> [1] https://lore.kernel.org/all/20230227173632.3292573-34-surenb@xxxxxxxxxx/
> >> [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/
> >> [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@xxxxxxxxxxxxxx/
> >>
> >> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> >> ---
> >>  include/linux/mm.h       | 28 +++++++++++++----------
> >>  include/linux/mm_types.h |  6 +++--
> >>  kernel/fork.c            | 49 ++++------------------------------------
> >>  3 files changed, 25 insertions(+), 58 deletions(-)
> >>