Re: [PATCH v6 RESEND 0/2] use static key to optimize pgtable_l4_enabled

[Andrew Jones <ajones@ventanamicro.com>]

On Thu, Jan 12, 2023 at 01:28:40AM +0800, Jisheng Zhang wrote:
> On Sun, Dec 11, 2022 at 06:44:04PM +0800, Jisheng Zhang wrote:
> > On Sat, Oct 08, 2022 at 10:28:35PM +0800, Jisheng Zhang wrote:
> > > On Thu, Oct 06, 2022 at 04:37:57PM +0800, Jisheng Zhang wrote:
> > > > On Wed, Oct 05, 2022 at 06:05:28PM -0700, Palmer Dabbelt wrote:
> > > > > On Sun, 21 Aug 2022 07:09:16 PDT (-0700), jszhang@kernel.org wrote:
> > > > > > The pgtable_l4|[l5]_enabled check sits at hot code path, performance
> > > > > > is impacted a lot. Since pgtable_l4|[l5]_enabled isn't changed after
> > > > > > boot, so static key can be used to solve the performance issue[1].
> > > > > > 
> > > > > > An unified way static key was introduced in [2], but it only targets
> > > > > > riscv isa extension. We dunno whether SV48 and SV57 will be considered
> > > > > > as isa extension, so the unified solution isn't used for
> > > > > > pgtable_l4[l5]_enabled now.
> > > > > > 
> > > > > > patch1 fixes a NULL pointer deference if static key is used a bit earlier.
> > > > > > patch2 uses the static key to optimize pgtable_l4|[l5]_enabled.
> > > > > > 
> > > > > > [1] http://lists.infradead.org/pipermail/linux-riscv/2021-December/011164.html
> > > > > > [2] https://lore.kernel.org/linux-riscv/20220517184453.3558-1-jszhang@kernel.org/T/#t
> > > > > > 
> > > > > > Since v5:
> > > > > >  - Use DECLARE_STATIC_KEY_FALSE
> > > > > > 
> > > > > > Since v4:
> > > > > >  - rebased on v5.19-rcN
> > > > > >  - collect Reviewed-by tags
> > > > > >  - Fix kernel panic issue if SPARSEMEM is enabled by moving the
> > > > > >    riscv_finalise_pgtable_lx() after sparse_init()
> > > > > > 
> > > > > > Since v3:
> > > > > >  - fix W=1 call to undeclared function 'static_branch_likely' error
> > > > > > 
> > > > > > Since v2:
> > > > > >  - move the W=1 warning fix to a separate patch
> > > > > >  - move the unified way to use static key to a new patch series.
> > > > > > 
> > > > > > Since v1:
> > > > > >  - Add a W=1 warning fix
> > > > > >  - Fix W=1 error
> > > > > >  - Based on v5.18-rcN, since SV57 support is added, so convert
> > > > > >    pgtable_l5_enabled as well.
> > > > > > 
> > > > > > 
> > > > > > Jisheng Zhang (2):
> > > > > >   riscv: move sbi_init() earlier before jump_label_init()
> > > > > >   riscv: turn pgtable_l4|[l5]_enabled to static key for RV64
> > > > > > 
> > > > > >  arch/riscv/include/asm/pgalloc.h    | 16 ++++----
> > > > > >  arch/riscv/include/asm/pgtable-32.h |  3 ++
> > > > > >  arch/riscv/include/asm/pgtable-64.h | 60 ++++++++++++++++++---------
> > > > > >  arch/riscv/include/asm/pgtable.h    |  5 +--
> > > > > >  arch/riscv/kernel/cpu.c             |  4 +-
> > > > > >  arch/riscv/kernel/setup.c           |  2 +-
> > > > > >  arch/riscv/mm/init.c                | 64 ++++++++++++++++++-----------
> > > > > >  arch/riscv/mm/kasan_init.c          | 16 ++++----
> > > > > >  8 files changed, 104 insertions(+), 66 deletions(-)
> > > > > 
> > > > > Sorry for being slow here, but it looks like this still causes some early
> > > > > boot hangs.  Specifically kasan+sparsemem is failing.  As you can probably
> > > > > see from the latency I'm still a bit buried right now so I'm not sure when
> > > > > I'll have a chance to take more of a look.
> > > > 
> > > > Hi Palmer,
> > > > 
> > > > Before V4, there is a bug which can cause kernel panic when SPARSEMEM
> > > > is enabled, V4 have fixed it by moving the riscv_finalise_pgtable_lx()
> > > > after sparse_init(). And I just tested the riscv-pgtable_static_key
> > > > branch in your tree, enabling KASAN and SPARSEMEM, system booted fine.
> > > > I'm not sure what happened. Could you please send me your kernel
> > > > config file? I want to fix any issue which can block this series being
> > > > merged in 6.1-rc1.
> > > 
> > > Hi Palmer,
> > > 
> > > I know you are busy ;) Do you have time to send me your test kernel
> > > config file so that I can reproduce the "early boot hang"?
> > > 
> > > Thanks
> > 
> > Hi Palmer,
> > 
> > I think the early boot hangs maybe the same as the one which has been
> > fixed by commit 9f2ac64d6ca6 ("riscv: mm: add missing memcpy in
> > kasan_init"). Will you give this series another try for v6.2-rc1? If
> > the boot hang can still be reproduced, could you please send me your
> > .config file?
> > 
> > Thanks in advance
> Hi all,
> 
> Just request to comment what to do with this patch, I think there
> are two independent points to consult:
> 
> 1. IIRC, Palmer gave this patch two chances to merge in early versions
> but he found boot hangs if enable KASAN and SPARSEMEM, while I can't
> reproduce the boot hang. And I also expect the hang should be fixed by
> commit 9f2ac64d6ca6 ("riscv: mm: add missing memcpy in kasan_init")
> 
> 2. Now we know alternative is preferred than static branch for ISA
> extensions dynamic code patching. So we also need to switch static
> branch usage here to alternative mechanism, but the problem is
> SV48 and SV57 are not ISA extensions, so we can't directly make use
> of the recently introduced riscv_has_extension_likely|unlikely()[1] 
> which is based on alternative mechanism.

We could rename the "has_extension" framework to "has_cpufeature" and
then lump extensions and features such as sv48 and sv57 together. Or,
if it's best to keep extensions separate, then duplicate the framework
to create a "has_non_extension_feature" version where features like
sv48 and sv57 live.

Thanks,
drew

> 
> Any comments are appreciated.
> 
> Thanks in advance
> 
> [1] https://lore.kernel.org/linux-riscv/20230111171027.2392-1-jszhang@kernel.org/T/#t
> 
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