Re: [PATCH bpf-next v2 1/3] bpf: Support inlining bpf_kptr_xchg() helper

[Eduard Zingerman <eddyz87@gmail.com>]

On Sat, 2023-12-23 at 18:40 +0800, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
> 
> The motivation of inlining bpf_kptr_xchg() comes from the performance
> profiling of bpf memory allocator benchmark. The benchmark uses
> bpf_kptr_xchg() to stash the allocated objects and to pop the stashed
> objects for free. After inling bpf_kptr_xchg(), the performance for
> object free on 8-CPUs VM increases about 2%~10%. The inline also has
> downside: both the kasan and kcsan checks on the pointer will be
> unavailable.
> 
> bpf_kptr_xchg() can be inlined by converting the calling of
> bpf_kptr_xchg() into an atomic_xchg() instruction. But the conversion
> depends on two conditions:
> 1) JIT backend supports atomic_xchg() on pointer-sized word
> 2) For the specific arch, the implementation of xchg is the same as
>    atomic_xchg() on pointer-sized words.
> 
> It seems most 64-bit JIT backends satisfies these two conditions. But
> as a precaution, defining a weak function bpf_jit_supports_ptr_xchg()
> to state whether such conversion is safe and only supporting inline for
> 64-bit host.
> 
> For x86-64, it supports BPF_XCHG atomic operation and both xchg() and
> atomic_xchg() use arch_xchg() to implement the exchange, so enabling the
> inline of bpf_kptr_xchg() on x86-64 first.
> 
> Signed-off-by: Hou Tao <houtao1@huawei.com>

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>