Re: Loadavg accounting error on arm64

[Peter Zijlstra]

On Mon, Nov 16, 2020 at 09:10:54AM +0000, Mel Gorman wrote:
> Similarly, it's not clear why the arm64 implementation
> does not call smp_acquire__after_ctrl_dep in the smp_load_acquire
> implementation. Even when it was introduced, the arm64 implementation
> differed significantly from the arm implementation in terms of what
> barriers it used for non-obvious reasons.

This is because ARM64's smp_cond_load_acquire() implementation uses
smp_load_aquire() directly, as opposed to the generic version that uses
READ_ONCE().

This is because ARM64 has a load-acquire instruction, which is highly
optimized, and generally considered cheaper than the smp_rmb() from
smp_acquire__after_ctrl_dep().

Or so I've been led to believe.