Messages in this thread | | | Date | Mon, 7 Sep 2015 09:56:36 +0100 | From | Catalin Marinas <> | Subject | Re: [RFC PATCH] arm64: cpuinfo: reduce cache contention on update_{feature}_support |
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On Fri, Sep 04, 2015 at 09:36:06AM -0700, David Daney wrote: > On 09/04/2015 09:04 AM, 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. > > > >Signed-off-by: Yury Norov <ynorov@caviumnetworks.com> > >--- > > arch/arm64/kernel/cpuinfo.c | 6 ++++-- > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > >diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c > >index 4a6ae31..9972c1e 100644 > >--- a/arch/arm64/kernel/cpuinfo.c > >+++ b/arch/arm64/kernel/cpuinfo.c > >@@ -87,12 +87,14 @@ bool system_supports_aarch32_el0(void) > > > > static void update_mixed_endian_el0_support(struct cpuinfo_arm64 *info) > > { > >- mixed_endian_el0 &= id_aa64mmfr0_mixed_endian_el0(info->reg_id_aa64mmfr0); > >+ if (mixed_endian_el0 && !id_aa64mmfr0_mixed_endian_el0(info->reg_id_aa64mmfr0)) > >+ mixed_endian_el0 = false; > > } > > > > static void update_aarch32_el0_support(struct cpuinfo_arm64 *info) > > { > >- aarch32_el0 &= id_aa64pfr0_aarch32_el0(info->reg_id_aa64pfr0); > >+ if (aarch32_el0 && !id_aa64pfr0_aarch32_el0(info->reg_id_aa64pfr0)) > >+ aarch32_el0 = false; > > } > > How many times in the lifetime of the kernel are these functions called? > > If it is just done at startup, then there is no "steady state" performance > impact, and the burden of complicating the code may not be worthwhile.
I fully agree. Unless the code is on some hot path, I really don't care about few cycles potentially saved during boot.
And in general, with any such micro optimisations, I want to see benchmark results to prove it worth.
-- Catalin
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