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 <[email protected]>
---
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;
-
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