Re: [RFC PATCH] arm64: cpuinfo: reduce cache contention on update_{feature}_support
From: Suzuki K. Poulose
Date: Wed Sep 16 2015 - 10:49:21 EST
On 04/09/15 20:52, Yury Norov wrote:
On Fri, Sep 04, 2015 at 05:40:57PM +0100, Suzuki K. Poulose wrote:
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)
OK. Than waiting for your patchset.
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-September/370386.html
Cheers
Suzuki
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