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

From: Suzuki K. Poulose
Date: Fri Sep 04 2015 - 12:41:05 EST

On 04/09/15 17:04, Yury Norov wrote:
This patch is on top of

In master, there's only a single function -
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)


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