Re: [PATCH] ARC: Explicitly set ARCH_SLAB_MINALIGN = 8

[Peter Zijlstra]

On Thu, Feb 14, 2019 at 10:44:49AM +0000, Alexey Brodkin wrote:
> > On Wed, Feb 13, 2019 at 03:23:36PM -0800, Vineet Gupta wrote:
> > > On 2/13/19 4:56 AM, Peter Zijlstra wrote:
> > > >
> > > > Personally I think u64 and company should already force natural
> > > > alignment; but alas.
> > >
> > > But there is an ISA/ABI angle here too. e.g. On 32-bit ARC, LDD (load double) is
> > > allowed to take a 32-bit aligned address to load a register pair. Thus all u64
> > > need not be 64-bit aligned (unless attribute aligned 8 etc) hence the relaxation
> > > in ABI (alignment of long long is 4). You could certainly argue that we end up
> > > undoing some of it anyways by defining things like ARCH_KMALLOC_MINALIGN to 8, but
> > > still...
> > 
> > So what happens if the data is then split across two cachelines; will a
> > STD vs LDD still be single-copy-atomic? I don't _think_ we rely on that
> > for > sizeof(unsigned long), with the obvious exception of atomic64_t,
> > but yuck...
> 
> STD & LDD are simple store/load instructions so there's no problem for
> their 64-bit data to be from 2 subsequent cache lines as well as 2 pages
> (if we're that unlucky). Or you mean something else?

u64 x;

WRITE_ONCE(x, 0x1111111100000000);
WRITE_ONCE(x, 0x0000000011111111);

vs

t = READ_ONCE(x);

is t allowed to be 0x1111111111111111 ?

If the data is split between two cachelines, the hardware must do
something very funny to avoid that.

single-copy-atomicity requires that to never happen; IOW no load or
store tearing. You must observe 'whole' values, no mixing.

Linux requires READ_ONCE()/WRITE_ONCE() to be single-copy-atomic for
<=sizeof(unsigned long) and atomic*_read()/atomic*_set() for all atomic
types. Your atomic64_t alignment should ensure this is so.

So while I think we're fine, I do find hardware instructions that tear
yuck (yah, I know, x86...)

> > So even though it is allowed by the chip; does it really make sense to
> > use this?
> 
> It gives performance benefits when dealing with either 64-bit or even
> larger buffers, see how we use it in our string routines like here [1].
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arc/lib/memset-archs.S#n81

That doesn't require the ABI alignment crud.