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Date: Thu, 12 Nov 2009 00:53:46 -0800
From: "Justin P. Mattock" <justinmattock@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
CC: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Mathieu Desnoyers <compudj@...stal.dyndns.org>
Subject: Re: [PATCH 3/3][RFC] tracing: Separate out x86 time stamp reading
and ns conversion
Steven Rostedt wrote:
> From: Steven Rostedt<srostedt@...hat.com>
>
> This patch separates the trace_clock_local time stamp reading from
> the conversion to nanoseconds for x86 by overriding the trace_clock_local
> and trace_noramlize_local furctions. It uses the time stamp normalize
> feature of the ring buffer to allow the ring buffer to record
> the raw cycles and have the read side convert it to nanoseconds.
>
> Before this separation, the cost of a trace was 179 ns, after it is
> 149 ns (30 ns performance boost 17%).
>
> perf top before separation:
>
> ------------------------------------------------------------------------------
> PerfTop: 1002 irqs/sec kernel:100.0% [1000Hz cpu-clock-msecs], (all, 4 CPUs)
> ------------------------------------------------------------------------------
>
> samples pcnt kernel function
> _______ _____ _______________
>
> 1653.00 - 25.0% : sched_clock
> 1147.00 - 17.4% : rb_reserve_next_event
> 865.00 - 13.1% : ring_buffer_lock_reserve
> 628.00 - 9.5% : rb_end_commit
> 521.00 - 7.9% : ring_buffer_unlock_commit
> 481.00 - 7.3% : __rb_reserve_next
> 392.00 - 5.9% : debug_smp_processor_id
> 284.00 - 4.3% : trace_clock_local
> 270.00 - 4.1% : ring_buffer_producer_thread [ring_buffer_benchmark]
> 108.00 - 1.6% : ring_buffer_event_data
> 100.00 - 1.5% : trace_recursive_unlock
> 70.00 - 1.1% : _spin_unlock_irq
> 30.00 - 0.5% : do_gettimeofday
> 21.00 - 0.3% : tick_nohz_stop_sched_tick
> 18.00 - 0.3% : read_tsc
>
> and after:
>
> ------------------------------------------------------------------------------
> PerfTop: 1024 irqs/sec kernel:100.0% [1000Hz cpu-clock-msecs], (all, 4 CPUs)
> ------------------------------------------------------------------------------
>
> samples pcnt kernel function
> _______ _____ _______________
>
> 1595.00 - 19.9% : rb_reserve_next_event
> 1521.00 - 18.9% : trace_clock_local
> 1393.00 - 17.3% : ring_buffer_lock_reserve
> 864.00 - 10.8% : __rb_reserve_next
> 745.00 - 9.3% : rb_end_commit
> 736.00 - 9.2% : ring_buffer_unlock_commit
> 395.00 - 4.9% : ring_buffer_producer_thread [ring_buffer_benchmark]
> 256.00 - 3.2% : debug_smp_processor_id
> 188.00 - 2.3% : ring_buffer_event_data
> 179.00 - 2.2% : trace_recursive_unlock
> 52.00 - 0.6% : _spin_unlock_irq
> 38.00 - 0.5% : read_tsc
> 34.00 - 0.4% : do_gettimeofday
> 27.00 - 0.3% : getnstimeofday
>
> Signed-off-by: Steven Rostedt<rostedt@...dmis.org>
> ---
> arch/x86/kernel/tsc.c | 35 +++++++++++++++++++++++++++++++++++
> 1 files changed, 35 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index cd982f4..c6576f2 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -37,6 +37,7 @@ static int __read_mostly tsc_unstable;
> static int __read_mostly tsc_disabled = -1;
>
> static int tsc_clocksource_reliable;
> +
> /*
> * Scheduler clock - returns current time in nanosec units.
> */
> @@ -64,6 +65,40 @@ u64 native_sched_clock(void)
> return __cycles_2_ns(this_offset);
> }
>
> +u64 trace_clock_local(void)
> +{
> + u64 this_offset;
> +
> + /*
> + * Fall back to jiffies if there's no TSC available:
> + * ( But note that we still use it if the TSC is marked
> + * unstable. We do this because unlike Time Of Day,
> + * the scheduler clock tolerates small errors and it's
> + * very important for it to be as fast as the platform
> + * can achive it. )
> + */
> + if (unlikely(tsc_disabled))
> + /* No locking but a rare wrong value is not a big deal: */
> + return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
> +
> + /* read the Time Stamp Counter: */
> + rdtscll(this_offset);
> +
> + return this_offset;
> +}
> +
> +void trace_normalize_local(int cpu, u64 *ts)
> +{
> + unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
> + unsigned long long cyc = *ts;
> +
> + if (unlikely(tsc_disabled))
> + return;
> +
> + ns += cyc * per_cpu(cyc2ns, cpu)>> CYC2NS_SCALE_FACTOR;
> + *ts = ns;
> +}
> +
> /* We need to define a real function for sched_clock, to override the
> weak default version */
> #ifdef CONFIG_PARAVIRT
>
Well, since I seem to have English as a second language
(according to the University, even though it's my only language)
my guess would be to correct this:
* ( But note that we still use it if the TSC is marked
+ * unstable. We do this because unlike Time Of Day,
+ * the scheduler clock tolerates small errors and it's
+ * very important for it to be as fast as the platform
+ * can achive it. )
to:
* But note that we still use it if the TSC is marked
+ * unstable. We do this because unlike Time Of Day,
+ * the scheduler clock tolerates small errors and it's
+ * very important for it to be as fast as the platform
+ * can achieve it.
(achive could be a word like pak da kah(boston))
but this is just cosmetic.
Justin P. Mattock
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