Re: BPF memory model

[Barret Rhoden <brho@google.com>]

On 9/8/23 04:42, Paul E. McKenney wrote:
> But what BPF programs are you running that are seeing excessive 
> synchronization overhead? That will tell us which operations to start 
> with. (Or maybe it is time to just add the full Linux-kernel 
> atomic-operations kitchen sink, but that would not normally be the way 
> to bet.)

Here's what I use in BPF, (also for writing parallel schedulers):
- READ_ONCE/WRITE_ONCE
- compiler atomic builtins, like CAS, swap/exchange, fetch_and_add, etc.
- smp_store_release, __atomic_load_n, etc.
- at one point, i was sprinkling asm volatile ("" ::: "memory") around 
too, though not in any active code at the moment.

My mental model, right or wrong, is that I am operating under something 
like the LKMM, and that I need to convince the compiler to spit out the 
right code (sort of like writing shared memory code to talk to a device 
or userspace) and hope the JIT does the right thing.

Barret