[PATCH] perf: optimize clear page in Intel specified model with movq instruction

[Jinhua Wu]

Clear page is the most time-consuming procedure in page fault handling.
Kernel use fast-string instruction to clear page. We found that in specified
Intel model such as CPX and ICX, the movq instruction perform much better
than fast-string instruction when corresponding page is not in cache.
But when the page is in cache, fast string perform better. We show the test
result in the following:

machine: Intel CPX

Allocated memory size      Page fault latency per 4K byte
                          rep stosb             movq
--------------------    ----------------  ------------------
        8MB               2057.13ns           1338.38ns
        64MB              1850.71ns           1200.20ns
        512MB             1918.40ns           1196.91ns
        4096MB            1931.24ns           1189.41ns

We can find that there is 40% performance improvement. So we add a blacklist
for Intel specified model, in which we use movq instruction to clear page.

Signed-off-by: Jinhua Wu <wujinhua@xxxxxxxxxxxxxxxxx>
Signed-off-by: Jiayu Ni <jiayu.ni@xxxxxxxxxxxxxxxxx>
Signed-off-by: Artie Ding <artie.ding@xxxxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/page_64.h | 18 ++++++++++++------
 arch/x86/kernel/cpu/intel.c    | 22 ++++++++++++++++++++++
 arch/x86/mm/init.c             |  9 +++++++++
 3 files changed, 43 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
index 4bde0dc..1fedfbe 100644
--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -7,6 +7,8 @@
 #ifndef __ASSEMBLY__
 #include <asm/alternative.h>
 
+#include <linux/jump_label.h>
+
 /* duplicated to the one in bootmem.h */
 extern unsigned long max_pfn;
 extern unsigned long phys_base;
@@ -43,15 +45,19 @@ static inline unsigned long __phys_addr_nodebug(unsigned long x)
 void clear_page_orig(void *page);
 void clear_page_rep(void *page);
 void clear_page_erms(void *page);
+extern struct static_key_false clear_page_movq_key;
 
 static inline void clear_page(void *page)
 {
-	alternative_call_2(clear_page_orig,
-			   clear_page_rep, X86_FEATURE_REP_GOOD,
-			   clear_page_erms, X86_FEATURE_ERMS,
-			   "=D" (page),
-			   "0" (page)
-			   : "cc", "memory", "rax", "rcx");
+	if (static_branch_unlikely(&clear_page_movq_key))
+		clear_page_orig(page);
+	else
+		alternative_call_2(clear_page_orig,
+				clear_page_rep, X86_FEATURE_REP_GOOD,
+				clear_page_erms, X86_FEATURE_ERMS,
+				"=D" (page),
+				"0" (page)
+				: "cc", "memory", "rax", "rcx");
 }
 
 void copy_page(void *to, void *from);
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 8321c43..3366da0 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -38,6 +38,28 @@
 #include <asm/apic.h>
 #endif
 
+/* Optimize clear page with movq in specific Intel CPU */
+#include <asm/cpu_device_id.h>
+#include <linux/jump_label.h>
+
+DEFINE_STATIC_KEY_FALSE(clear_page_movq_key);
+EXPORT_SYMBOL_GPL(clear_page_movq_key);
+
+extern const struct x86_cpu_id *x86_match_cpu(const struct x86_cpu_id *match);
+
+const struct x86_cpu_id faststring_blacklist_match[] __initconst = {
+	X86_MATCH_INTEL_FAM6_MODEL(SKYLAKE_X, 0),
+	X86_MATCH_INTEL_FAM6_MODEL(ICELAKE_X, 0),
+	X86_MATCH_INTEL_FAM6_MODEL(ICELAKE_XEON_D, 0)
+};
+
+void enable_clear_page_movq(void)
+{
+    if (x86_match_cpu(faststring_blacklist_match))
+		static_branch_enable(&clear_page_movq_key);
+}
+
+
 enum split_lock_detect_state {
 	sld_off = 0,
 	sld_warn,
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 23a14d8..480c189 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -28,6 +28,12 @@
 #include <asm/memtype.h>
 
 /*
+ * Optimize clear page with movq in specific Intel CPU
+ * Definition in intel.c
+ */
+extern void enable_clear_page_movq(void);
+
+/*
  * We need to define the tracepoints somewhere, and tlb.c
  * is only compiled when SMP=y.
  */
@@ -775,6 +781,9 @@ void __init init_mem_mapping(void)
 
 	x86_init.hyper.init_mem_mapping();
 
+	/* Optimize clear page with mov in specific Intel CPU */
+	enable_clear_page_movq();
+
 	early_memtest(0, max_pfn_mapped << PAGE_SHIFT);
 }
 
-- 
1.8.3.1