Re: [PATCH] ARM: tlb: Prevent flushing insane large ranges one by one

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Wed, May 24, 2023 at 11:18:12AM +0100, Robin Murphy wrote:
> On 2023-05-24 10:32, Thomas Gleixner wrote:
> > vmalloc uses lazy TLB flushes for unmapped ranges to avoid excessive TLB
> > flushing on every unmap. The lazy flushing coalesces unmapped ranges and
> > invokes flush_tlb_kernel_range() with the combined range.
> > 
> > The coalescing can result in ranges which spawn the full vmalloc address
> > range. In the case of flushing an executable mapping in the module address
> > space this range is extended to also flush the direct map alias.
> > 
> > flush_tlb_kernel_range() then walks insane large ranges, the worst case
> > observed was ~1.5GB.
> > 
> > The range is flushed page by page, which takes several milliseconds to
> > complete in the worst case and obviously affects all processes in the
> > system. In the worst case observed this causes the runtime of a realtime
> > task on an isolated CPU to be almost doubled over the normal worst
> > case, which makes it miss the deadline.
> > 
> > Cure this by sanity checking the range against a threshold and fall back to
> > tlb_flush_all() when the range is too large.
> > 
> > The default threshold is 32 pages, but for CPUs with CP15 this is evaluated
> > at boot time via read_cpuid(CPUID_TLBTYPE) and set to the half of the TLB
> > size.
> > 
> > The vmalloc range coalescing could be improved to provide a list or
> > array of ranges to flush, which allows to avoid overbroad flushing, but
> > that's a major surgery and does not solve the problem of actual
> > justified large range flushes which can happen due to the lazy flush
> > mechanics in vmalloc. The lazy flush results in batching which is biased
> > towards large range flushes by design.
> > 
> > Fixes: db64fe02258f ("mm: rewrite vmap layer")
> > Reported-by: John Ogness <john.ogness@linutronix.de>
> > Debugged-by: John Ogness <john.ogness@linutronix.de>
> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > Tested-by: John Ogness <john.ogness@linutronix.de>
> > Link: https://lore.kernel.org/all/87a5y5a6kj.ffs@tglx
> > ---
> >   arch/arm/include/asm/cputype.h  |    5 +++++
> >   arch/arm/include/asm/tlbflush.h |    2 ++
> >   arch/arm/kernel/setup.c         |   10 ++++++++++
> >   arch/arm/kernel/smp_tlb.c       |    4 ++++
> >   4 files changed, 21 insertions(+)
> > 
> > --- a/arch/arm/include/asm/cputype.h
> > +++ b/arch/arm/include/asm/cputype.h
> > @@ -196,6 +196,11 @@ static inline unsigned int __attribute_c
> >   	return read_cpuid(CPUID_MPUIR);
> >   }
> > +static inline unsigned int __attribute_const__ read_cpuid_tlbsize(void)
> > +{
> > +	return 64 << ((read_cpuid(CPUID_TLBTYPE) >> 1) & 0x03);
> > +}
> 
> This appears to be specific to Cortex-A9 - these bits are
> implementation-defined, and it looks like on on most other Arm Ltd. CPUs
> they have no meaning at all, e.g.[1][2][3], but they could still hold some
> wildly unrelated value on other implementations.

That sucks. I guess we'll need to decode the main CPU ID register and
have a table, except for Cortex-A9 where we can read the TLB size.

If that's not going to work either, then the MM layer needs to get
fixed not to be so utterly stupid to request a TLB flush over an
insanely large range - or people will just have to put up with
latency sucking on 32-bit ARM platforms.

-- 
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