Re: [PATCH 00/11] assorted replacement of x[mz]alloc_bytes()

[Jan Beulich <jbeulich@suse.com>]

On 08.04.2021 14:57, Andrew Cooper wrote:
> On 08/04/2021 13:13, Jan Beulich wrote:
>> In the long run I think we want to do away with these type-unsafe
>> interfaces, the more that they also request (typically) excess
>> alignment. This series of entirely independent patches is
>> eliminating the instances where it's relatively clear that they're
>> not just "blob" allocations.
>>
>>
>> 03: x86/MCE: avoid effectively open-coding xmalloc_array()
>> 04: x86/HVM: avoid effectively open-coding xmalloc_array()
>> 05: x86/oprofile: avoid effectively open-coding xmalloc_array()
>> 06: x86/IRQ: avoid over-alignment in alloc_pirq_struct()
>> 07: EFI/runtime: avoid effectively open-coding xmalloc_array()
>> 08: hypfs: avoid effectively open-coding xzalloc_array()
>> 10: video/lfb: avoid effectively open-coding xzalloc_array()
> 
> The flex conversions are fine, but I am unconvinced by argument for
> interchanging array() and bytes().
> 
> The cacheline size is 64 bytes, and the minimum allocation size is 16,
> plus a bhdr overhead of 32 bytes, so you're already at most of a
> cacheline for a nominally-zero sized allocation.

But you're aware that the alignment x[mz]alloc_bytes() forces is
128 bytes? Plus, while sizeof(struct bhdr) is indeed 32, the
overhead on allocated blocks is

#define BHDR_OVERHEAD   (sizeof(struct bhdr) - MIN_BLOCK_SIZE)

i.e. 16 (i.e. the other half of the 32 is already the minimum
block size of 16 that you also mention). IOW a cacheline sized
block would yield 48 bytes of usable space. Specifically a
meaningful change in the PV case from what patch 06 does, where
we only need 40 bytes.

> There can however be a severe penalty from cacheline sharing, which is
> why the bytes() form does have a minimum alignment.  There is one
> xmalloc heap shared across the entire system, so you've got no idea what
> might be sharing the same cache line for sub-line allocations.

This would call for distinguishing short-lived allocations (and
ones to be used mainly from a single CPU) from long-lived ones
having system wide use. I.e. a request to gain further allocation
function flavors, when already the introduction of the one new
xv[mz]alloc() has caused long-winded discussions with (so far) no
real outcome.

> We should not support sub-line allocations IMO.  The savings is a
> handful of bytes at best, and some horrible performance cliffs to
> avoid.  People running virtualisation are not going to be ram
> constrained to the order of a few bytes.
> 
> For small allocations which don't require specific alignment, then
> putting bhdr and the allocation in the same line is fine (if we don't do
> this already), but we shouldn't be in the position of having two bhdr's
> in the same cache line, even if there are plenty of single-byte
> allocations in the theoretical worst case.

That's a request to tweak allocator internals then, not an argument
against the conversions done here.

Jan