[tip:x86/mm] x86/mm: Optimize boot-time paging mode switching cost

From: tip-bot for Kirill A. Shutemov
Date: Wed Feb 21 2018 - 06:03:40 EST

Commit-ID: 39b9552281abfcdfc54162897018890dafe7ffef
Gitweb: https://git.kernel.org/tip/39b9552281abfcdfc54162897018890dafe7ffef
Author: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
AuthorDate: Fri, 16 Feb 2018 14:49:48 +0300
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 21 Feb 2018 10:19:18 +0100

x86/mm: Optimize boot-time paging mode switching cost

By this point we have functioning boot-time switching between 4- and
5-level paging mode. But naive approach comes with cost.

Numbers below are for kernel build, allmodconfig, 5 times.

CONFIG_X86_5LEVEL=n:

Performance counter stats for 'sh -c make -j100 -B -k >/dev/null' (5 runs):

17308719.892691 task-clock:u (msec) # 26.772 CPUs utilized ( +- 0.11% )
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
331,993,164 page-faults:u # 0.019 M/sec ( +- 0.01% )
43,614,978,867,455 cycles:u # 2.520 GHz ( +- 0.01% )
39,371,534,575,126 stalled-cycles-frontend:u # 90.27% frontend cycles idle ( +- 0.09% )
28,363,350,152,428 instructions:u # 0.65 insn per cycle
# 1.39 stalled cycles per insn ( +- 0.00% )
6,316,784,066,413 branches:u # 364.948 M/sec ( +- 0.00% )
250,808,144,781 branch-misses:u # 3.97% of all branches ( +- 0.01% )

646.531974142 seconds time elapsed ( +- 1.15% )

CONFIG_X86_5LEVEL=y:

Performance counter stats for 'sh -c make -j100 -B -k >/dev/null' (5 runs):

17411536.780625 task-clock:u (msec) # 26.426 CPUs utilized ( +- 0.10% )
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
331,868,663 page-faults:u # 0.019 M/sec ( +- 0.01% )
43,865,909,056,301 cycles:u # 2.519 GHz ( +- 0.01% )
39,740,130,365,581 stalled-cycles-frontend:u # 90.59% frontend cycles idle ( +- 0.05% )
28,363,358,997,959 instructions:u # 0.65 insn per cycle
# 1.40 stalled cycles per insn ( +- 0.00% )
6,316,784,937,460 branches:u # 362.793 M/sec ( +- 0.00% )
251,531,919,485 branch-misses:u # 3.98% of all branches ( +- 0.00% )

658.886307752 seconds time elapsed ( +- 0.92% )

The patch tries to fix the performance regression by using
cpu_feature_enabled(X86_FEATURE_LA57) instead of pgtable_l5_enabled in
all hot code paths. These will statically patch the target code for
additional performance.

CONFIG_X86_5LEVEL=y + the patch:

Performance counter stats for 'sh -c make -j100 -B -k >/dev/null' (5 runs):

17381990.268506 task-clock:u (msec) # 26.907 CPUs utilized ( +- 0.19% )
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
331,862,625 page-faults:u # 0.019 M/sec ( +- 0.01% )
43,697,726,320,051 cycles:u # 2.514 GHz ( +- 0.03% )
39,480,408,690,401 stalled-cycles-frontend:u # 90.35% frontend cycles idle ( +- 0.05% )
28,363,394,221,388 instructions:u # 0.65 insn per cycle
# 1.39 stalled cycles per insn ( +- 0.00% )
6,316,794,985,573 branches:u # 363.410 M/sec ( +- 0.00% )
251,013,232,547 branch-misses:u # 3.97% of all branches ( +- 0.01% )

645.991174661 seconds time elapsed ( +- 1.19% )

Unfortunately, this approach doesn't help with text size:

vmlinux.before .text size: 8190319
vmlinux.after .text size: 8200623

The .text section is increased by about 4k. Not sure if we can do anything
about this.

Signed-off-by: Kirill A. Shuemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Link: http://lkml.kernel.org/r/20180216114948.68868-4-kirill.shutemov@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/boot/compressed/misc.h | 5 +++++
arch/x86/entry/entry_64.S | 11 ++---------
arch/x86/include/asm/pgtable_64_types.h | 5 ++++-
arch/x86/kernel/head64.c | 9 +++++++--
arch/x86/kernel/head_64.S | 2 +-
arch/x86/mm/kasan_init_64.c | 6 ++++++
6 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 9d323dc..4d369c3 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -12,6 +12,11 @@
#undef CONFIG_PARAVIRT_SPINLOCKS
#undef CONFIG_KASAN

+#ifdef CONFIG_X86_5LEVEL
+/* cpu_feature_enabled() cannot be used that early */
+#define pgtable_l5_enabled __pgtable_l5_enabled
+#endif
+
#include <linux/linkage.h>
#include <linux/screen_info.h>
#include <linux/elf.h>
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 2c06348..b18acdf 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -275,15 +275,8 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
* depending on paging mode) in the address.
*/
#ifdef CONFIG_X86_5LEVEL
- testl $1, pgtable_l5_enabled(%rip)
- jz 1f
- shl $(64 - 57), %rcx
- sar $(64 - 57), %rcx
- jmp 2f
-1:
- shl $(64 - 48), %rcx
- sar $(64 - 48), %rcx
-2:
+ ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
+ "shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57
#else
shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index 68909a6..d5c21a3 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -21,7 +21,10 @@ typedef unsigned long pgprotval_t;
typedef struct { pteval_t pte; } pte_t;

#ifdef CONFIG_X86_5LEVEL
-extern unsigned int pgtable_l5_enabled;
+extern unsigned int __pgtable_l5_enabled;
+#ifndef pgtable_l5_enabled
+#define pgtable_l5_enabled cpu_feature_enabled(X86_FEATURE_LA57)
+#endif
#else
#define pgtable_l5_enabled 0
#endif
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 8161e71..0c855de 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -32,6 +32,11 @@
#include <asm/microcode.h>
#include <asm/kasan.h>

+#ifdef CONFIG_X86_5LEVEL
+#undef pgtable_l5_enabled
+#define pgtable_l5_enabled __pgtable_l5_enabled
+#endif
+
/*
* Manage page tables very early on.
*/
@@ -40,8 +45,8 @@ static unsigned int __initdata next_early_pgt;
pmdval_t early_pmd_flags = __PAGE_KERNEL_LARGE & ~(_PAGE_GLOBAL | _PAGE_NX);

#ifdef CONFIG_X86_5LEVEL
-unsigned int pgtable_l5_enabled __ro_after_init;
-EXPORT_SYMBOL(pgtable_l5_enabled);
+unsigned int __pgtable_l5_enabled __ro_after_init;
+EXPORT_SYMBOL(__pgtable_l5_enabled);
unsigned int pgdir_shift __ro_after_init = 39;
EXPORT_SYMBOL(pgdir_shift);
unsigned int ptrs_per_p4d __ro_after_init = 1;
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 3e9de0f..326c631 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -124,7 +124,7 @@ ENTRY(secondary_startup_64)
/* Enable PAE mode, PGE and LA57 */
movl $(X86_CR4_PAE | X86_CR4_PGE), %ecx
#ifdef CONFIG_X86_5LEVEL
- testl $1, pgtable_l5_enabled(%rip)
+ testl $1, __pgtable_l5_enabled(%rip)
jz 1f
orl $X86_CR4_LA57, %ecx
1:
diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
index 0df0dd1..d8ff013 100644
--- a/arch/x86/mm/kasan_init_64.c
+++ b/arch/x86/mm/kasan_init_64.c
@@ -1,6 +1,12 @@
// SPDX-License-Identifier: GPL-2.0
#define DISABLE_BRANCH_PROFILING
#define pr_fmt(fmt) "kasan: " fmt
+
+#ifdef CONFIG_X86_5LEVEL
+/* Too early to use cpu_feature_enabled() */
+#define pgtable_l5_enabled __pgtable_l5_enabled
+#endif
+
#include <linux/bootmem.h>
#include <linux/kasan.h>
#include <linux/kdebug.h>