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

[Alexey Brodkin]

Hi Vineet,

> -----Original Message-----
> From: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> Sent: Friday, February 15, 2019 4:34 AM
> To: Alexey Brodkin <alexey.brodkin@xxxxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: David Laight <David.Laight@xxxxxxxxxx>; linux-snps-arc@xxxxxxxxxxxxxxxxxxx; Arnd Bergmann
> <arnd.bergmann@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx; Mark Rutland
> <mark.rutland@xxxxxxx>
> Subject: Re: [PATCH] ARC: Explicitly set ARCH_SLAB_MINALIGN = 8
> 
> On 2/14/19 12:50 AM, Alexey Brodkin wrote:
> 
> >>>>> I suspect the slab allocator should be returning 8 byte aligned addresses
> >>>>> on all systems....
> >>>>
> >>>> why ? As I understand it is still not fool proof against the expected alignment of
> >>>> inner members. There ought to be a better way to enforce all this.
> >>>
> >>> I agree that for ARC ARCH_SLAB_MINALIGN should be at least 8.
> >>
> >> This issue aside, are there other reasons ? Because making it 8 on ARC is just
> >> pending the eventuality for later.
> >
> > But that's pretty much the same for other 32-bit arches that have 64-bit atomics
> > like ARM etc. From what I may see from ARM's documentation for LDREXD/SRREXD they
> > require double-word alignment of data as well.
> 
> Right LLOCKD/SCONDD (64-bit exclusive load/store) needs 64-bit aligned effective
> addresses for micro-arch reasons (1 vs 2 cache lines) etc.
> 
> So lets try to unpack this for me. Say we had.
> 
>    struct foo {
> 	int        a;
> 	atomic64_t b;
>    };
> 
> The atomic64_t (which for ARC and most others is u64 __attribute__((aligned(8))
> *already ensures* that there a 4 b padding is generated by gcc (I just confirmed
> with a simple test case).
> 
>    #ifdef DOALIGN__
>    #define my_u64	__u64 __attribute__((aligned(8)))
>    #else
>    #define my_u64	__u64
>    #endif
> 
>   struct foo on_heap;
> 
>   printf(%d", &on_heap.b)
> 
> $ arc-linux-gcc -O2 test.c -DDOALIGN__ -c --save-temps
> 
>    main:
> 	mov_s r1,@on_heap+8   <----
> 	mov_s r0,@.LC0
> 	b @printf
> 
> W/o the alignment attribute (say normal LDD/STD)
> 
> $ arc-linux-gcc -O2 test.c -c --save-temps
> 
>    main:
> 	mov_s r1,@on_heap+4
> 	mov_s r0,@.LC0
> 	b @printf
> 
> So indeed your patch aligns dynamic structs to 64-bit, ensuring any embedded
> aligned_u64 to be 64-bit aligned as well. Phew !
> 
> 
> > That said if for some reason atomic64_t variable is unaligned execution on
> > any (or at least most) 32-bit architectures will lead to run-time failure,
> > i.e. we'll know about it and this will be fixed.
> >
> > And what I'm doing by that change (ARCH_SLAB_MINALIGN=8 for ARC) I'm just
> > working-around peculiarity of ARC ABI.
> 
> Right.

So are you OK with this patch or something should be done before applying?

> >
> > Out of curiosity I checked if there're any other occurrences of "alingof(long long)"
> > and there seems to be a couple of more:
> > ----------------------------------->8-----------------------------
> > # git grep alignof | grep "long long"
> >
> > ...
> >
> > kernel/workqueue.c:5693:        WARN_ON(__alignof__(struct pool_workqueue) < __alignof__(long
> long));
> > mm/slab.c:155:#define   REDZONE_ALIGN           max(BYTES_PER_WORD, __alignof__(unsigned long long))
> 
> For ARC, it will be max(4,4) so 4
> for others 32-bit,it will be max(4,8)
> 
> So indeed it makes sense to change it.

I guess that's still a separate change for generic code, right?
I.e. I'll do it in a separate patch.

-Alexey