Re: [PATCH] perf, x86: Optimize intel_pmu_pebs_fixup_ip()

[Peter Zijlstra]

On Wed, Oct 23, 2013 at 10:48:38PM +0200, Peter Zijlstra wrote:
> I'll also make sure to test we actually hit the fault path
> by concurrently running something like:
> 
>  while :; echo 1 > /proc/sys/vm/drop_caches ; done
> 
> while doing perf top or so.. 

So the below appears to work; I've ran:

  while :; do echo 1 > /proc/sys/vm/drop_caches; sleep 1; done &
  while :; do make O=defconfig-build/ clean; perf record -a -g fp -e cycles:pp make O=defconfig-build/ -s -j64; done

And verified that the if (in_nmi()) trace_printk() was visible in the
trace output verifying we indeed took the fault from the NMI code.

I've had this running for ~ 30 minutes or so and the machine is still
healthy.

Don, can you give this stuff a spin on your system?

---
 arch/x86/lib/usercopy.c | 43 +++++++++++++++----------------------------
 arch/x86/mm/fault.c     | 43 +++++++++++++++++++++++--------------------
 2 files changed, 38 insertions(+), 48 deletions(-)

diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c
index 4f74d94c8d97..5465b8613944 100644
--- a/arch/x86/lib/usercopy.c
+++ b/arch/x86/lib/usercopy.c
@@ -11,39 +11,26 @@
 #include <linux/sched.h>
 
 /*
- * best effort, GUP based copy_from_user() that is NMI-safe
+ * We rely on the nested NMI work to allow atomic faults from the NMI path; the
+ * nested NMI paths are careful to preserve CR2.
  */
 unsigned long
 copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
 {
-	unsigned long offset, addr = (unsigned long)from;
-	unsigned long size, len = 0;
-	struct page *page;
-	void *map;
-	int ret;
+	unsigned long ret;
 
 	if (__range_not_ok(from, n, TASK_SIZE))
-		return len;
-
-	do {
-		ret = __get_user_pages_fast(addr, 1, 0, &page);
-		if (!ret)
-			break;
-
-		offset = addr & (PAGE_SIZE - 1);
-		size = min(PAGE_SIZE - offset, n - len);
-
-		map = kmap_atomic(page);
-		memcpy(to, map+offset, size);
-		kunmap_atomic(map);
-		put_page(page);
-
-		len  += size;
-		to   += size;
-		addr += size;
-
-	} while (len < n);
-
-	return len;
+		return 0;
+
+	/*
+	 * Even though this function is typically called from NMI/IRQ context
+	 * disable pagefaults so that its behaviour is consistent even when
+	 * called form other contexts.
+	 */
+	pagefault_disable();
+	ret = __copy_from_user_inatomic(to, from, n);
+	pagefault_enable();
+
+	return n - ret;
 }
 EXPORT_SYMBOL_GPL(copy_from_user_nmi);
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 3aaeffcfd67a..506564b13ba7 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -51,7 +51,7 @@ kmmio_fault(struct pt_regs *regs, unsigned long addr)
 	return 0;
 }
 
-static inline int __kprobes notify_page_fault(struct pt_regs *regs)
+static inline int __kprobes kprobes_fault(struct pt_regs *regs)
 {
 	int ret = 0;
 
@@ -1048,7 +1048,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code)
 			return;
 
 		/* kprobes don't want to hook the spurious faults: */
-		if (notify_page_fault(regs))
+		if (kprobes_fault(regs))
 			return;
 		/*
 		 * Don't take the mm semaphore here. If we fixup a prefetch
@@ -1060,23 +1060,8 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code)
 	}
 
 	/* kprobes don't want to hook the spurious faults: */
-	if (unlikely(notify_page_fault(regs)))
+	if (unlikely(kprobes_fault(regs)))
 		return;
-	/*
-	 * It's safe to allow irq's after cr2 has been saved and the
-	 * vmalloc fault has been handled.
-	 *
-	 * User-mode registers count as a user access even for any
-	 * potential system fault or CPU buglet:
-	 */
-	if (user_mode_vm(regs)) {
-		local_irq_enable();
-		error_code |= PF_USER;
-		flags |= FAULT_FLAG_USER;
-	} else {
-		if (regs->flags & X86_EFLAGS_IF)
-			local_irq_enable();
-	}
 
 	if (unlikely(error_code & PF_RSVD))
 		pgtable_bad(regs, error_code, address);
@@ -1088,17 +1073,35 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code)
 		}
 	}
 
-	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
-
 	/*
 	 * If we're in an interrupt, have no user context or are running
 	 * in an atomic region then we must not take the fault:
 	 */
 	if (unlikely(in_atomic() || !mm)) {
+		if (in_nmi())
+			trace_printk("YIPEE!!!\n");
 		bad_area_nosemaphore(regs, error_code, address);
 		return;
 	}
 
+	/*
+	 * It's safe to allow irq's after cr2 has been saved and the
+	 * vmalloc fault has been handled.
+	 *
+	 * User-mode registers count as a user access even for any
+	 * potential system fault or CPU buglet:
+	 */
+	if (user_mode_vm(regs)) {
+		local_irq_enable();
+		error_code |= PF_USER;
+		flags |= FAULT_FLAG_USER;
+	} else {
+		if (regs->flags & X86_EFLAGS_IF)
+			local_irq_enable();
+	}
+
+	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
+
 	if (error_code & PF_WRITE)
 		flags |= FAULT_FLAG_WRITE;
 
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