On Thu, Oct 28, 2021 at 2:22 AM Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> wrote: > > > I found some atomic_add/dec are replaced with atomic_add/dec_return, > > I am going to replace -return variants with -fetch variants, potentially -fetch > > > those helpers with return value imply a full memory barrier around it, but > > others without return value do not. Do you have any numbers to show > > the impact? Maybe atomic_add/dec_return_relaxed can help this. > > The generic variant uses arch_cmpxchg() for all atomic variants > without any extra barriers. Therefore, on platforms that use generic > implementations there won't be performance differences except for an > extra branch that checks results when VM_BUG_ON is enabled. > > On x86 the difference between the two is the following > > atomic_add: > lock add %eax,(%rsi) > > atomic_fetch_add: > lock xadd %eax,(%rsi) > > atomic_fetch_add_relaxed: > lock xadd %eax,(%rsi) > > No differences between relaxed and non relaxed variants. However, we Right. There is no difference on x86. Maybe there are differences in other architectures. > used lock xadd instead of lock add. I am not sure if the performance > difference is going to be different. > > Pasha
