Commit ac742d37 authored Sep 09, 2015 by Rasmus Villemoes Committed by Thomas Gleixner Sep 22, 2015

futex: Force hot variables into a single cache line



futex_hash() references two global variables: the base pointer
futex_queues and the size of the array futex_hashsize. The latter is
marked __read_mostly, while the former is not, so they are likely to
end up very far from each other. This means that futex_hash() is
likely to encounter two cache misses.

We could mark futex_queues as __read_mostly as well, but that doesn't
guarantee they'll end up next to each other (and even if they do, they
may still end up in different cache lines). So put the two variables
in a small singleton struct with sufficient alignment and mark that as
__read_mostly.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: kbuild test robot <fengguang.wu@intel.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: http://lkml.kernel.org/r/1441834601-13633-1-git-send-email-linux@rasmusvillemoes.dk


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent 93edc8bd

kernel/futex.c

+11 −2

Original line number	Diff line number	Diff line
		@@ -255,9 +255,18 @@ struct futex_hash_bucket {
		struct plist_head chain;
		} ____cacheline_aligned_in_smp;

		static unsigned long __read_mostly futex_hashsize;
		/*
		* The base of the bucket array and its size are always used together
		* (after initialization only in hash_futex()), so ensure that they
		* reside in the same cacheline.
		*/
		static struct {
		struct futex_hash_bucket *queues;
		unsigned long hashsize;
		} __futex_data __read_mostly __aligned(2*sizeof(long));
		#define futex_queues (__futex_data.queues)
		#define futex_hashsize (__futex_data.hashsize)

		static struct futex_hash_bucket *futex_queues;

		/*
		* Fault injections for futexes.

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