[PATCH] [26/66] x86_64: prefetch the mmap_sem in the fault path

["Andi Kleen" <ak@xxxxxxx>]

From: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
In a micro-benchmark that stresses the pagefault path, the down_read_trylock
on the mmap_sem showed up quite high on the profile. Turns out this lock is
bouncing between cpus quite a bit and thus is cache-cold a lot. This patch
prefetches the lock (for write) as early as possible (and before some other
somewhat expensive operations). With this patch, the down_read_trylock
basically fell out of the top of profile.

Signed-off-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Signed-off-by: Andi Kleen <ak@xxxxxxx>

---
 arch/x86_64/mm/fault.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

Index: linux/arch/x86_64/mm/fault.c
===================================================================
--- linux.orig/arch/x86_64/mm/fault.c
+++ linux/arch/x86_64/mm/fault.c
@@ -314,11 +314,13 @@ asmlinkage void __kprobes do_page_fault(
 	unsigned long flags;
 	siginfo_t info;
 
+	tsk = current;
+	mm = tsk->mm;
+	prefetchw(&mm->mmap_sem);
+
 	/* get the address */
 	__asm__("movq %%cr2,%0":"=r" (address));
 
-	tsk = current;
-	mm = tsk->mm;
 	info.si_code = SEGV_MAPERR;
 
 
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