[RFC PATCH] arm64: cpuinfo: reduce cache contention on update_{feature}_support

[Suzuki.Poulose@arm.com (Suzuki K. Poulose)]

On 04/09/15 17:04, Yury Norov wrote:
> This patch is on top of https://lkml.org/lkml/2015/9/2/413
>
> In master, there's only a single function -
> 	update_mixed_endian_el0_support
> And similar function is on review mentioned above.
>
> The algorithm for them is like this:
>   - there's system-wide boolean marker for the feature that is
>     initially enabled;
>   - there's also updater for the feature that may disable it
>     system-widely if feature is not supported on current CPU.
>   - updater is called for each CPU on bootup.
>
> The problem is the way updater does its work. On each CPU, it
> unconditionally updates system-wide marker. For multi-core
> system it makes CPU issue invalidate message for a cache
> line containing marker. This invalidate increases cache
> contention for nothing, because there's a single marker reset
> that is really needed, and the others are useless.
>
> If the number of system-wide markers of this sort will grow,
> it may become a trouble on large-scale SOCs. The fix is trivial,
> though: do system-wide marker update conditionally, and preserve
> corresponding cache line in shared state for all update() calls,
> except, probably, one.

As I have mentioned already, this patch (and the per feature functions)
won't be needed once we merge my series (which is waiting for the merge
window to see the public lights)

Cheers
Suzuki