Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a separate file

[Will Deacon <will@kernel.org>]

On Thu, Jun 20, 2019 at 05:33:03PM +0800, Guo Ren wrote:
> On Wed, Jun 19, 2019 at 8:39 PM Will Deacon <will.deacon@arm.com> wrote:
> >
> > On Wed, Jun 19, 2019 at 08:18:04PM +0800, Guo Ren wrote:
> > > On Wed, Jun 19, 2019 at 5:12 PM Will Deacon <will.deacon@arm.com> wrote:
> > > > This is one place where I'd actually prefer not to go down the route of
> > > > making the code generic. Context-switching and low-level TLB management
> > > > is deeply architecture-specific and I worry that by trying to make this
> > > > code common, we run the real risk of introducing subtle bugs on some
> > > > architecture every time it is changed.
> > > "Add generic asid code" and "move arm's into generic" are two things.
> > > We could do
> > > first and let architecture's maintainer to choose.
> >
> > If I understand the proposal being discussed, it involves basing that
> > generic ASID allocation code around the arm64 implementation which I don't
> > necessarily think is a good starting point.
> ...
> >
> > > > Furthermore, the algorithm we use
> > > > on arm64 is designed to scale to large systems using DVM and may well be
> > > > too complex and/or sub-optimal for architectures with different system
> > > > topologies or TLB invalidation mechanisms.
> > > It's just a asid algorithm not very complex and there is a callback
> > > for architecture to define their
> > > own local hart tlb flush. Seems it has nothing with DVM or tlb
> > > broadcast mechanism.
> >
> > I'm pleased that you think the algorithm is not very complex, but I'm also
> > worried that you might not have fully understood some of its finer details.
> I understand your concern about my less understanding of asid
> technology. Here is
> my short-description of arm64 asid allocator: (If you find anything
> wrong, please
> correct me directly, thx :)

The complexity mainly comes from the fact that this thing runs concurrently
with itself without synchronization on the fast-path. Coupled with the
need to use the same ASID for all threads of a task, you end up in fiddly
situations where rollover can occur on one CPU whilst another CPU is trying
to schedule a thread of a task that already has threads running in
userspace.

However, it's architecture-specific whether or not you care about that
scenario.

> > The reason I mention DVM and TLB broadcasting is because, depending on
> > the mechanisms in your architecture relating to those, it may be strictly
> > required that all concurrently running threads of a process have the same
> > ASID at any given point in time, or it may be that you really don't care.
> >
> > If you don't care, then the arm64 allocator is over-engineered and likely
> > inefficient for your system. If you do care, then it's worth considering
> > whether a lock is sufficient around the allocator if you don't expect high
> > core counts. Another possibility is that you end up using only one ASID and
> > invalidating the local TLB on every context switch. Yet another design
> > would be to manage per-cpu ASID pools.
> I'll keep my system use the same ASID for SMP + IOMMU :P

You will want a separate allocator for that:

https://lkml.kernel.org/r/20190610184714.6786-2-jean-philippe.brucker@arm.com

> Yes, there are two styles of asid allocator: per-cpu ASID (MIPS) or
> same ASID (ARM).
> If the CPU couldn't support cache/tlb coherency maintian in hardware,
> it should use
> per-cpu ASID style because IPI is expensive and per-cpu ASID style
> need more software
> mechanism to improve performance (eg: delay cache flush). From software view the
> same ASID is clearer and easier to build bigger system with more TLB caches.
> 
> I think the same ASID style is a more sensible choice for modern
> processor and let it be
> one of generic is reasonable.

I'm not sure I agree. x86, for example, is better off using a different
algorithm for allocating its PCIDs.

> > So rather than blindly copying the arm64 code, I suggest sitting down and
> > designing something that fits to your architecture instead. You may end up
> > with something that is both simpler and more efficient.
> In fact, riscv folks have discussed a lot about arm's asid allocator
> and I learned
> a lot from the discussion:
> https://lore.kernel.org/linux-riscv/20190327100201.32220-1-anup.patel@wdc.com/

If you require all threads of the same process to have the same ASID, then
that patch looks broken to me.

Will

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