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Subject[PATCH 1/2] trace: profile likely and unlikely annotations
Impact: new unlikely/likely profiler

Andrew Morton recently suggested having an in-kernel way to profile
likely and unlikely macros. This patch achieves that goal.

When configured, every(*) likely and unlikely macro gets a counter attached
to it. When the condition is hit, the hit and misses of that condition
are recorded. These numbers can later be retrieved by:

/debugfs/tracing/profile_likely - All likely markers
/debugfs/tracing/profile_unlikely - All unlikely markers.

# cat /debug/tracing/profile_unlikely | head
correct incorrect % Function File Line
------- --------- - -------- ---- ----
2167 0 0 do_arch_prctl process_64.c 832
0 0 0 do_arch_prctl process_64.c 804
2670 0 0 IS_ERR err.h 34
71230 5693 7 __switch_to process_64.c 673
76919 0 0 __switch_to process_64.c 639
43184 33743 43 __switch_to process_64.c 624
12740 64181 83 __switch_to process_64.c 594
12740 64174 83 __switch_to process_64.c 590
# cat /debug/tracing/profile_unlikely | \
awk '{ if ($3 > 25) print $0; }' |head -20
44963 35259 43 __switch_to process_64.c 624
12762 67454 84 __switch_to process_64.c 594
12762 67447 84 __switch_to process_64.c 590
1478 595 28 syscall_get_error syscall.h 51
0 2821 100 syscall_trace_leave ptrace.c 1567
0 1 100 native_smp_prepare_cpus smpboot.c 1237
86338 265881 75 calc_delta_fair sched_fair.c 408
210410 108540 34 calc_delta_mine sched.c 1267
0 54550 100 sched_info_queued sched_stats.h 222
51899 66435 56 pick_next_task_fair sched_fair.c 1422
6 10 62 yield_task_fair sched_fair.c 982
7325 2692 26 rt_policy sched.c 144
0 1270 100 pre_schedule_rt sched_rt.c 1261
1268 48073 97 pick_next_task_rt sched_rt.c 884
0 45181 100 sched_info_dequeued sched_stats.h 177
0 15 100 sched_move_task sched.c 8700
0 15 100 sched_move_task sched.c 8690
53167 33217 38 schedule sched.c 4457
0 80208 100 sched_info_switch sched_stats.h 270
30585 49631 61 context_switch sched.c 2619
# cat /debug/tracing/profile_likely | awk '{ if ($3 > 25) print $0; }'
39900 36577 47 pick_next_task sched.c 4397
20824 15233 42 switch_mm mmu_context_64.h 18
0 7 100 __cancel_work_timer workqueue.c 560
617 66484 99 clocksource_adjust timekeeping.c 456
0 346340 100 audit_syscall_exit auditsc.c 1570
38 347350 99 audit_get_context auditsc.c 732
0 345244 100 audit_syscall_entry auditsc.c 1541
38 1017 96 audit_free auditsc.c 1446
0 1090 100 audit_alloc auditsc.c 862
2618 1090 29 audit_alloc auditsc.c 858
0 6 100 move_masked_irq migration.c 9
1 198 99 probe_sched_wakeup trace_sched_switch.c 58
2 2 50 probe_wakeup trace_sched_wakeup.c 227
0 2 100 probe_wakeup_sched_switch trace_sched_wakeup.c 144
4514 2090 31 __grab_cache_page filemap.c 2149
12882 228786 94 mapping_unevictable pagemap.h 50
4 11 73 __flush_cpu_slab slub.c 1466
627757 330451 34 slab_free slub.c 1731
2959 61245 95 dentry_lru_del_init dcache.c 153
946 1217 56 load_elf_binary binfmt_elf.c 904
102 82 44 disk_put_part genhd.h 206
1 1 50 dst_gc_task dst.c 82
0 19 100 tcp_mss_split_point tcp_output.c 1126
As you can see by the above, there's a bit of work to do in rethinking
the use of some unlikelys and likelys. Note: the unlikely case had 71 hits
that were more than 25%.

Note: After submitting my first version of this patch, Andrew Morton
showed me a version written by Daniel Walker, where I picked up
the following ideas from:

1) Using __builtin_constant_p to avoid profiling fixed values.
2) Using __FILE__ instead of instruction pointers.
3) Using the preprocessor to stop all profiling of likely
annotations from vsyscall_64.c.

Thanks to Andrew Morton, Arjan van de Ven, Theodore Tso and Ingo Molnar
for their feed back on this patch.

(*) Not ever unlikely is recorded, those that are used by vsyscalls
(a few of them) had to have profiling disabled.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
---
arch/x86/kernel/vsyscall_64.c | 8 ++
include/asm-generic/vmlinux.lds.h | 14 +++-
include/linux/compiler.h | 61 +++++++++++++-
kernel/trace/Kconfig | 16 ++++
kernel/trace/Makefile | 1 +
kernel/trace/trace_unlikely.c | 164 +++++++++++++++++++++++++++++++++++++
6 files changed, 261 insertions(+), 3 deletions(-)
create mode 100644 kernel/trace/trace_unlikely.c
diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index ebf2f12..1f2e7c6 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -17,6 +17,14 @@
* want per guest time just set the kernel.vsyscall64 sysctl to 0.
*/

+/* Protect userspace from profiling */
+#ifdef CONFIG_TRACE_UNLIKELY_PROFILE
+# undef likely
+# undef unlikely
+# define likely(x) likely_notrace(x)
+# define unlikely(x) unlikely_notrace(x)
+#endif
+
#include <linux/time.h>
#include <linux/init.h>
#include <linux/kernel.h>
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 63709a5..2110312 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -45,6 +45,17 @@
#define MCOUNT_REC()
#endif

+#ifdef CONFIG_TRACE_UNLIKELY_PROFILE
+#define LIKELY_PROFILE() VMLINUX_SYMBOL(__start_likely_profile) = .; \
+ *(_ftrace_likely) \
+ VMLINUX_SYMBOL(__stop_likely_profile) = .; \
+ VMLINUX_SYMBOL(__start_unlikely_profile) = .; \
+ *(_ftrace_unlikely) \
+ VMLINUX_SYMBOL(__stop_unlikely_profile) = .;
+#else
+#define LIKELY_PROFILE()
+#endif
+
/* .data section */
#define DATA_DATA \
*(.data) \
@@ -62,7 +73,8 @@
VMLINUX_SYMBOL(__stop___markers) = .; \
VMLINUX_SYMBOL(__start___tracepoints) = .; \
*(__tracepoints) \
- VMLINUX_SYMBOL(__stop___tracepoints) = .;
+ VMLINUX_SYMBOL(__stop___tracepoints) = .; \
+ LIKELY_PROFILE()

#define RO_DATA(align) \
. = ALIGN((align)); \
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 98115d9..935e30c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -59,8 +59,65 @@ extern void __chk_io_ptr(const volatile void __iomem *);
* specific implementations come from the above header files
*/

-#define likely(x) __builtin_expect(!!(x), 1)
-#define unlikely(x) __builtin_expect(!!(x), 0)
+#ifdef CONFIG_TRACE_UNLIKELY_PROFILE
+struct ftrace_likely_data {
+ const char *func;
+ const char *file;
+ unsigned line;
+ unsigned long correct;
+ unsigned long incorrect;
+};
+void ftrace_likely_update(struct ftrace_likely_data *f, int val, int expect);
+
+#define likely_notrace(x) __builtin_expect(!!(x), 1)
+#define unlikely_notrace(x) __builtin_expect(!!(x), 0)
+
+#define likely_check(x) ({ \
+ int ______r; \
+ static struct ftrace_likely_data \
+ __attribute__((__aligned__(4))) \
+ __attribute__((section("_ftrace_likely"))) \
+ ______f = { \
+ .func = __func__, \
+ .file = __FILE__, \
+ .line = __LINE__, \
+ }; \
+ ______f.line = __LINE__; \
+ ______r = likely_notrace(x); \
+ ftrace_likely_update(&______f, ______r, 1); \
+ ______r; \
+ })
+#define unlikely_check(x) ({ \
+ int ______r; \
+ static struct ftrace_likely_data \
+ __attribute__((__aligned__(4))) \
+ __attribute__((section("_ftrace_unlikely"))) \
+ ______f = { \
+ .func = __func__, \
+ .file = __FILE__, \
+ .line = __LINE__, \
+ }; \
+ ______f.line = __LINE__; \
+ ______r = unlikely_notrace(x); \
+ ftrace_likely_update(&______f, ______r, 0); \
+ ______r; \
+ })
+
+/*
+ * Using __builtin_constant_p(x) to ignore cases where the return
+ * value is always the same. This idea is taken from a similar patch
+ * written by Daniel Walker.
+ */
+# ifndef likely
+# define likely(x) (__builtin_constant_p(x) ? !!(x) : likely_check(x))
+# endif
+# ifndef unlikely
+# define unlikely(x) (__builtin_constant_p(x) ? !!(x) : unlikely_check(x))
+# endif
+#else
+# define likely(x) __builtin_expect(!!(x), 1)
+# define unlikely(x) __builtin_expect(!!(x), 0)
+#endif

/* Optimization barrier */
#ifndef barrier
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index d986216..a604f24 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -159,6 +159,22 @@ config BOOT_TRACER
selected, because the self-tests are an initcall as well and that
would invalidate the boot trace. )

+config TRACE_UNLIKELY_PROFILE
+ bool "Trace likely/unlikely profiler"
+ depends on DEBUG_KERNEL
+ select TRACING
+ help
+ This tracer profiles all the the likely and unlikely macros
+ in the kernel. It will display the results in:
+
+ /debugfs/tracing/profile_likely
+ /debugfs/tracing/profile_unlikely
+
+ Note: this will add a significant overhead, only turn this
+ on if you need to profile the system's use of these macros.
+
+ Say N if unsure.
+
config STACK_TRACER
bool "Trace max stack"
depends on HAVE_FUNCTION_TRACER
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 3e1f361..98e70ee 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -25,5 +25,6 @@ obj-$(CONFIG_STACK_TRACER) += trace_stack.o
obj-$(CONFIG_MMIOTRACE) += trace_mmiotrace.o
obj-$(CONFIG_BOOT_TRACER) += trace_boot.o
obj-$(CONFIG_FUNCTION_RET_TRACER) += trace_functions_return.o
+obj-$(CONFIG_TRACE_UNLIKELY_PROFILE) += trace_unlikely.o

libftrace-y := ftrace.o
diff --git a/kernel/trace/trace_unlikely.c b/kernel/trace/trace_unlikely.c
new file mode 100644
index 0000000..9493269
--- /dev/null
+++ b/kernel/trace/trace_unlikely.c
@@ -0,0 +1,164 @@
+/*
+ * unlikely profiler
+ *
+ * Copyright (C) 2008 Steven Rostedt <srostedt@redhat.com>
+ */
+#include <linux/kallsyms.h>
+#include <linux/seq_file.h>
+#include <linux/spinlock.h>
+#include <linux/debugfs.h>
+#include <linux/uaccess.h>
+#include <linux/module.h>
+#include <linux/ftrace.h>
+#include <linux/hash.h>
+#include <linux/fs.h>
+#include <asm/local.h>
+#include "trace.h"
+
+void ftrace_likely_update(struct ftrace_likely_data *f, int val, int expect)
+{
+ /* FIXME: Make this atomic! */
+ if (val == expect)
+ f->correct++;
+ else
+ f->incorrect++;
+}
+EXPORT_SYMBOL(ftrace_likely_update);
+
+struct ftrace_pointer {
+ void *start;
+ void *stop;
+};
+
+static void *
+t_next(struct seq_file *m, void *v, loff_t *pos)
+{
+ struct ftrace_pointer *f = m->private;
+ struct ftrace_likely_data *p = v;
+
+ (*pos)++;
+
+ if (v == (void *)1)
+ return f->start;
+
+ ++p;
+
+ if ((void *)p >= (void *)f->stop)
+ return NULL;
+
+ return p;
+}
+
+static void *t_start(struct seq_file *m, loff_t *pos)
+{
+ void *t = (void *)1;
+ loff_t l = 0;
+
+ for (; t && l < *pos; t = t_next(m, t, &l))
+ ;
+
+ return t;
+}
+
+static void t_stop(struct seq_file *m, void *p)
+{
+}
+
+static int t_show(struct seq_file *m, void *v)
+{
+ struct ftrace_likely_data *p = v;
+ const char *f;
+ unsigned long percent;
+
+ if (v == (void *)1) {
+ seq_printf(m, " correct incorrect %% "
+ " Function "
+ " File Line\n"
+ " ------- --------- - "
+ " -------- "
+ " ---- ----\n");
+ return 0;
+ }
+
+ /* Only print the file, not the path */
+ f = p->file + strlen(p->file);
+ while (f >= p->file && *f != '/')
+ f--;
+ f++;
+
+ if (p->correct) {
+ percent = p->incorrect * 100;
+ percent /= p->correct + p->incorrect;
+ } else
+ percent = p->incorrect ? 100 : 0;
+
+ seq_printf(m, "%8lu %8lu %3lu ", p->correct, p->incorrect, percent);
+ seq_printf(m, "%-30.30s %-20.20s %d\n", p->func, f, p->line);
+ return 0;
+}
+
+static struct seq_operations tracing_likely_seq_ops = {
+ .start = t_start,
+ .next = t_next,
+ .stop = t_stop,
+ .show = t_show,
+};
+
+static int tracing_likely_open(struct inode *inode, struct file *file)
+{
+ int ret;
+
+ ret = seq_open(file, &tracing_likely_seq_ops);
+ if (!ret) {
+ struct seq_file *m = file->private_data;
+ m->private = (void *)inode->i_private;
+ }
+
+ return ret;
+}
+
+static struct file_operations tracing_likely_fops = {
+ .open = tracing_likely_open,
+ .read = seq_read,
+ .llseek = seq_lseek,
+};
+
+extern unsigned long __start_likely_profile[];
+extern unsigned long __stop_likely_profile[];
+extern unsigned long __start_unlikely_profile[];
+extern unsigned long __stop_unlikely_profile[];
+
+static struct ftrace_pointer ftrace_likely_pos = {
+ .start = __start_likely_profile,
+ .stop = __stop_likely_profile,
+};
+
+static struct ftrace_pointer ftrace_unlikely_pos = {
+ .start = __start_unlikely_profile,
+ .stop = __stop_unlikely_profile,
+};
+
+static __init int ftrace_unlikely_init(void)
+{
+ struct dentry *d_tracer;
+ struct dentry *entry;
+
+ d_tracer = tracing_init_dentry();
+
+ entry = debugfs_create_file("profile_likely", 0444, d_tracer,
+ &ftrace_likely_pos,
+ &tracing_likely_fops);
+ if (!entry)
+ pr_warning("Could not create debugfs 'profile_likely' entry\n");
+
+ entry = debugfs_create_file("profile_unlikely", 0444, d_tracer,
+ &ftrace_unlikely_pos,
+ &tracing_likely_fops);
+ if (!entry)
+ pr_warning("Could not create debugfs"
+ " 'profile_unlikely' entry\n");
+
+ return 0;
+}
+
+device_initcall(ftrace_unlikely_init);
--
1.5.6.5
--


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