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Date: Thu, 12 Feb 2015 12:04:54 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Peter Zijlstra <peterz@...radead.org>,
Pawel Moll <pawel.moll@....com>
CC: ajh mls <ajhmls@...il.com>,
Richard Cochran <richardcochran@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
Ingo Molnar <mingo@...hat.com>,
Paul Mackerras <paulus@...ba.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
John Stultz <john.stultz@...aro.org>,
Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
Christopher Covington <cov@...eaurora.org>,
Namhyung Kim <namhyung@...nel.org>,
David Ahern <dsahern@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Tomeu Vizoso <tomeu@...euvizoso.net>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v5] perf: Use monotonic clock as a source for timestamps
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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