Re: [PATCH v5] perf: Use monotonic clock as a source for timestamps

[Adrian Hunter]

On 11/02/15 18:12, Peter Zijlstra wrote:
> 
> How about something like the below? I _think_ it should mostly work for
> x86, where the tsc is a 64bit wide cycle counter.

It would have to be based on CLOCK_MONOTONIC_RAW not CLOCK_MONOTONIC and you
would have to check the clocksource is TSC.

Why is CLOCK_MONOTONIC preferred anyway - I would have thought any
adjustment would skew performance timings?

> 
> I suppose we should extend the perf userpage time data with
> time_last_cycle and time_mask if/when we want to make this work on
> something with a short counter.
> 
> Of course, at that time we also need to somehow deal with that counter
> wrapping, its hardly practical to go iterate all possible userpg
> instances from a timer handler.
> 
> 
> ---
>  Documentation/kernel-parameters.txt |  9 +++++++
>  arch/x86/kernel/cpu/perf_event.c    | 44 ++++++++++++++++++++++++---------
>  include/linux/perf_event.h          |  6 +++++
>  kernel/events/core.c                | 49 ++++++++++++++++++++++++++++++++++---
>  kernel/time/timekeeping.c           | 30 +++++++++++++++++++++++
>  5 files changed, 123 insertions(+), 15 deletions(-)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 176d4fe4f076..52255676b6e2 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -91,6 +91,7 @@ the beginning of each description states the restrictions within which a
>  	NUMA	NUMA support is enabled.
>  	NFS	Appropriate NFS support is enabled.
>  	OSS	OSS sound support is enabled.
> +	PERF	Performance events and counters support is enabled.
>  	PV_OPS	A paravirtualized kernel is enabled.
>  	PARIDE	The ParIDE (parallel port IDE) subsystem is enabled.
>  	PARISC	The PA-RISC architecture is enabled.
> @@ -2796,6 +2797,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  			allocator.  This parameter is primarily	for debugging
>  			and performance comparison.
>  
> +	perf_use_local_clock
> +			[PERF]
> +			Use local_clock() as a source for perf timestamps
> +			generation. This was be the default behaviour and
> +			this parameter can be used to maintain backward
> +			compatibility or on older hardware with expensive
> +			monotonic clock source.
> +
>  	pf.		[PARIDE]
>  			See Documentation/blockdev/paride.txt.
>  
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index b71a7f86d68a..436a66632f76 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -1952,6 +1952,35 @@ static struct pmu pmu = {
>  	.flush_branch_stack	= x86_pmu_flush_branch_stack,
>  };
>  
> +static void local_clock_user_time(struct perf_event_mmap_page *userpg, u64 now)
> +{
> +	data = cyc2ns_read_begin();
> +
> +	userpg->cap_user_time = 1;
> +	userpg->time_mult = data->cyc2ns_mul;
> +	userpg->time_shift = data->cyc2ns_shift;
> +	userpg->time_offset = data->cyc2ns_offset - now;
> +
> +	userpg->cap_user_time_zero = 1;
> +	userpg->time_zero = data->cyc2ns_offset;
> +
> +	cyc2ns_read_end(data);
> +}
> +
> +extern void notrace __ktime_get_mono_fast(u64 *offset, u32 *mult, u16 *shift);
> +
> +static void ktime_fast_mono_user_time(struct perf_event_mmap_page *userpg, u64 now)
> +{
> +	userpg->cap_user_time = 1;
> +	userpg->cap_user_time_zero = 1;
> +
> +	__ktime_get_mono_fast(&userpg->time_zero,
> +			      &userpg->time_mult,
> +			      &userpg->time_shift);
> +
> +	userpg->offset = userpg->time_zero - now;
> +}
> +
>  void arch_perf_update_userpage(struct perf_event *event,
>  			       struct perf_event_mmap_page *userpg, u64 now)
>  {
> @@ -1966,17 +1995,10 @@ void arch_perf_update_userpage(struct perf_event *event,
>  	if (!sched_clock_stable())
>  		return;
>  
> -	data = cyc2ns_read_begin();
> -
> -	userpg->cap_user_time = 1;
> -	userpg->time_mult = data->cyc2ns_mul;
> -	userpg->time_shift = data->cyc2ns_shift;
> -	userpg->time_offset = data->cyc2ns_offset - now;
> -
> -	userpg->cap_user_time_zero = 1;
> -	userpg->time_zero = data->cyc2ns_offset;
> -
> -	cyc2ns_read_end(data);
> +	if (static_key_false(&perf_use_local_clock_key))
> +		local_clock_user_time(userpg, now);
> +	else
> +		ktime_fast_mono_user_time(userpg, now);
>  }
>  
>  /*
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 33262004c310..1d61f968113a 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -561,6 +561,12 @@ extern void perf_pmu_enable(struct pmu *pmu);
>  extern int perf_event_task_disable(void);
>  extern int perf_event_task_enable(void);
>  extern int perf_event_refresh(struct perf_event *event, int refresh);
> +
> +extern struct static_key perf_use_local_clock_key = STATIC_KEY_INIT_FALSE;
> +extern void __weak
> +arch_perf_update_userpage(struct perf_event *event,
> +			  struct perf_event_mmap_page *userpg, u64 now);
> +
>  extern void perf_event_update_userpage(struct perf_event *event);
>  extern int perf_event_release_kernel(struct perf_event *event);
>  extern struct perf_event *
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 13209a90b751..7bad385103ea 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -42,6 +42,8 @@
>  #include <linux/module.h>
>  #include <linux/mman.h>
>  #include <linux/compat.h>
> +#include <linux/sysctl.h>
> +#include <linux/jump_label.h>
>  
>  #include "internal.h"
>  
> @@ -322,9 +324,43 @@ extern __weak const char *perf_pmu_name(void)
>  	return "pmu";
>  }
>  
> +struct static_key perf_use_local_clock_key = STATIC_KEY_INIT_FALSE;
> +static bool perf_use_local_clock_param __initdata;
> +static int __init perf_use_local_clock_setup(char *__unused)
> +{
> +	perf_use_local_clock_param = true;
> +	return 1;
> +}
> +__setup("perf_use_local_clock", perf_use_local_clock_setup);
> +
> +static int sysctl_perf_sample_time_clk_id = CLOCK_MONOTONIC;
> +
> +static struct ctl_table perf_sample_time_kern_table[] = {
> +	{
> +		.procname       = "perf_sample_time_clk_id",
> +		.data           = &sysctl_perf_sample_time_clk_id,
> +		.maxlen         = sizeof(int),
> +		.mode           = 0444,
> +		.proc_handler   = proc_dointvec,
> +	},
> +	{}
> +};
> +
> +static struct ctl_table perf_sample_time_root_table[] = {
> +	{
> +		.procname	= "kernel",
> +		.mode		= 0555,
> +		.child		= perf_sample_time_kern_table,
> +	},
> +	{}
> +};
> +
>  static inline u64 perf_clock(void)
>  {
> -	return local_clock();
> +	if (static_key_false(&perf_use_local_clock_key))
> +		return local_clock();
> +	else
> +		return ktime_get_mono_fast_ns();
>  }
>  
>  static inline struct perf_cpu_context *
> @@ -4101,8 +4137,8 @@ static void perf_event_init_userpage(struct perf_event *event)
>  	rcu_read_unlock();
>  }
>  
> -void __weak arch_perf_update_userpage(
> -	struct perf_event *event, struct perf_event_mmap_page *userpg, u64 now)
> +void __weak arch_perf_update_userpage(struct perf_event *event,
> +				      struct perf_event_mmap_page *userpg, u64 now)
>  {
>  }
>  
> @@ -4487,7 +4523,7 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
>  	if (vma->vm_flags & VM_WRITE)
>  		flags |= RING_BUFFER_WRITABLE;
>  
> -	rb = rb_alloc(nr_pages, 
> +	rb = rb_alloc(nr_pages,
>  		event->attr.watermark ? event->attr.wakeup_watermark : 0,
>  		event->cpu, flags);
>  
> @@ -8516,6 +8552,11 @@ void __init perf_event_init(void)
>  	 */
>  	BUILD_BUG_ON((offsetof(struct perf_event_mmap_page, data_head))
>  		     != 1024);
> +
> +	if (perf_use_local_clock_param)
> +		static_key_slow_inc(&perf_use_local_clock_key);
> +	else
> +		register_sysctl_table(perf_sample_time_root_table);
>  }
>  
>  static int __init perf_event_sysfs_init(void)
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index b124af259800..37bed5931a91 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -334,6 +334,36 @@ u64 notrace ktime_get_mono_fast_ns(void)
>  }
>  EXPORT_SYMBOL_GPL(ktime_get_mono_fast_ns);
>  
> +void notrace __ktime_get_mono_fast(u64 *offset, u32 *mult, u16 *shift)
> +{
> +	struct tk_read_base *tkr;
> +	unsigned int seq;
> +	cycle_t cycle_now, delta;
> +	u64 nsecs, now;
> +
> +	do {
> +		seq = raw_read_seqcount(&tk_fast_mono.seq);
> +		tkr = tk_fast_mono.base + (seq & 0x01);
> +
> +		cycle_now = tkr->read(tkr->clock);
> +		delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask);
> +
> +		nsec = delta * tkr->mult + tkr->xtime_nsec;
> +		nsec >>= tkr->shift;
> +		nsec += arch_gettimeoffset();
> +
> +		now = ktime_to_ns(tkr->base_mono) + nsec;
> +
> +		*mult = tkr->mult;
> +		*shift = tkr->shift;
> +
> +		nsec = mul_u64_u32_shr(cycle_now, tkr->mult, tkr->shift);
> +
> +		*offset = now - nsec;
> +
> +	} while (read_seqcount_retry(&tk_fast_mono.seq, seq));
> +}
> +
>  #ifdef CONFIG_GENERIC_TIME_VSYSCALL_OLD
>  
>  static inline void update_vsyscall(struct timekeeper *tk)
> 
> 

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