perf/x86: Add ability to calculate TSC from perf sample timestamps

For modern CPUs, perf clock is directly related to TSC.  TSC
can be calculated from perf clock and vice versa using a simple
calculation.  Two of the three componenets of that calculation
are already exported in struct perf_event_mmap_page.  This patch
exports the third.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: http://lkml.kernel.org/r/1372425741-1676-3-git-send-email-adrian.hunter@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

4 files changed, 33 insertions, 2 deletions

diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index c91e8b9d588b..235be70d5bb4 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -49,6 +49,7 @@ extern void tsc_init(void);
 extern void mark_tsc_unstable(char *reason);
 extern int unsynchronized_tsc(void);
 extern int check_tsc_unstable(void);
+extern int check_tsc_disabled(void);
 extern unsigned long native_calibrate_tsc(void);
 
 extern int tsc_clocksource_reliable;
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index a7c7305030cc..8355c84b9729 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1884,6 +1884,7 @@ static struct pmu pmu = {
 void arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)
 {
         userpg->cap_usr_time = 0;
+        userpg->cap_usr_time_zero = 0;
         userpg->cap_usr_rdpmc = x86_pmu.attr_rdpmc;
         userpg->pmc_width = x86_pmu.cntval_bits;
 
@@ -1897,6 +1898,11 @@ void arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)
         userpg->time_mult = this_cpu_read(cyc2ns);
         userpg->time_shift = CYC2NS_SCALE_FACTOR;
         userpg->time_offset = this_cpu_read(cyc2ns_offset) - now;
+
+        if (sched_clock_stable && !check_tsc_disabled()) {
+                userpg->cap_usr_time_zero = 1;
+                userpg->time_zero = this_cpu_read(cyc2ns_offset);
+        }
 }
 
 /*
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 6ff49247edf8..930e5d48f560 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -89,6 +89,12 @@ int check_tsc_unstable(void)
 }
 EXPORT_SYMBOL_GPL(check_tsc_unstable);
 
+int check_tsc_disabled(void)
+{
+        return tsc_disabled;
+}
+EXPORT_SYMBOL_GPL(check_tsc_disabled);
+
 #ifdef CONFIG_X86_TSC
 int __init notsc_setup(char *str)
 {
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 0041aedf2297..efef1d37a371 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -378,7 +378,8 @@ struct perf_event_mmap_page {
                 struct {
                         __u64   cap_usr_time            : 1,
                                 cap_usr_rdpmc           : 1,
-                                cap_____res             : 62;
+                                cap_usr_time_zero       : 1,
+                                cap_____res             : 61;
                 };
         };
 
@@ -420,12 +421,29 @@ struct perf_event_mmap_page {
         __u16   time_shift;
         __u32   time_mult;
         __u64   time_offset;
+        /*
+         * If cap_usr_time_zero, the hardware clock (e.g. TSC) can be calculated
+         * from sample timestamps.
+         *
+         *   time = timestamp - time_zero;
+         *   quot = time / time_mult;
+         *   rem  = time % time_mult;
+         *   cyc = (quot << time_shift) + (rem << time_shift) / time_mult;
+         *
+         * And vice versa:
+         *
+         *   quot = cyc >> time_shift;
+         *   rem  = cyc & ((1 << time_shift) - 1);
+         *   timestamp = time_zero + quot * time_mult +
+         *               ((rem * time_mult) >> time_shift);
+         */
+        __u64   time_zero;
 
                 /*
                  * Hole for extension of the self monitor capabilities
                  */
 
-        __u64   __reserved[120];        /* align to 1k */
+        __u64   __reserved[119];        /* align to 1k */
 
         /*
          * Control data for the mmap() data buffer.