[Patch 1/3] prefetch the mmap_sem in the fault path

[Arjan van de Ven <arjan@intel.linux.com>]

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@linux.intel.com>

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

Index: linux-work/arch/x86_64/mm/fault.c
===================================================================
--- linux-work.orig/arch/x86_64/mm/fault.c
+++ linux-work/arch/x86_64/mm/fault.c
@@ -312,6 +312,10 @@ 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));
 	if (notify_die(DIE_PAGE_FAULT, "page fault", regs, error_code, 14,
@@ -325,8 +329,6 @@ asmlinkage void __kprobes do_page_fault(
 		printk("pagefault rip:%lx rsp:%lx cs:%lu ss:%lu address %lx error %lx\n",
 		       regs->rip,regs->rsp,regs->cs,regs->ss,address,error_code); 
 
-	tsk = current;
-	mm = tsk->mm;
 	info.si_code = SEGV_MAPERR;