LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Ingo Molnar <mingo@elte.hu>
To: Alexey Zaytsev <alexey.zaytsev@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	lkml <linux-kernel@vger.kernel.org>
Subject: Re: printk timestamps jumping back and forth in 2.6.25-rc.
Date: Thu, 28 Feb 2008 21:16:13 +0100	[thread overview]
Message-ID: <20080228201613.GA30999@elte.hu> (raw)
In-Reply-To: <20080228201315.GA30736@elte.hu>


* Ingo Molnar <mingo@elte.hu> wrote:

> > Anything else I can do?
> 
> yes, please test the patch below - does it fix the problem?

(assuming you prefer patches that build, here's an updated one below.)

	Ingo

-------------->
Subject: sched: make cpu_clock() globally synchronous
From: Ingo Molnar <mingo@elte.hu>
Date: Thu Feb 28 21:00:21 CET 2008

Alexey Zaytsev reported (and bisected) that the introduction of
cpu_clock() in printk made the timestamps jump back and forth.

Make cpu_clock() more reliable while still keeping it fast when it's
called frequently.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/sched.c |   52 +++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 49 insertions(+), 3 deletions(-)

Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -710,11 +710,39 @@ static inline u64 global_rt_runtime(void
 	return (u64)sysctl_sched_rt_runtime * NSEC_PER_USEC;
 }
 
+static const unsigned long long time_sync_thresh = 100000;
+
+static DEFINE_PER_CPU(unsigned long long, time_offset);
+static DEFINE_PER_CPU(unsigned long long, prev_cpu_time);
+
 /*
- * For kernel-internal use: high-speed (but slightly incorrect) per-cpu
- * clock constructed from sched_clock():
+ * Global lock which we take every now and then to synchronize
+ * the CPUs time. This method is not warp-safe, but it's good
+ * enough to synchronize slowly diverging time sources and thus
+ * it's good enough for tracing:
  */
-unsigned long long cpu_clock(int cpu)
+static DEFINE_SPINLOCK(time_sync_lock);
+static unsigned long long prev_global_time;
+
+static unsigned long long __sync_cpu_clock(cycles_t time, int cpu)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&time_sync_lock, flags);
+
+	if (time < prev_global_time) {
+		per_cpu(time_offset, cpu) += prev_global_time - time;
+		time = prev_global_time;
+	} else {
+		prev_global_time = time;
+	}
+
+	spin_unlock_irqrestore(&time_sync_lock, flags);
+
+	return time;
+}
+
+static unsigned long long __cpu_clock(int cpu)
 {
 	unsigned long long now;
 	unsigned long flags;
@@ -735,6 +763,24 @@ unsigned long long cpu_clock(int cpu)
 
 	return now;
 }
+
+/*
+ * For kernel-internal use: high-speed (but slightly incorrect) per-cpu
+ * clock constructed from sched_clock():
+ */
+unsigned long long cpu_clock(int cpu)
+{
+	unsigned long long prev_cpu_time, time, delta_time;
+
+	prev_cpu_time = per_cpu(prev_cpu_time, cpu);
+	time = __cpu_clock(cpu) + per_cpu(time_offset, cpu);
+	delta_time = time-prev_cpu_time;
+
+	if (unlikely(delta_time > time_sync_thresh))
+		time = __sync_cpu_clock(time, cpu);
+
+	return time;
+}
 EXPORT_SYMBOL_GPL(cpu_clock);
 
 #ifndef prepare_arch_switch

  reply	other threads:[~2008-02-28 20:16 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-28 14:26 Alexey Zaytsev
2008-02-28 19:21 ` David Miller
2008-02-28 20:13 ` Ingo Molnar
2008-02-28 20:16   ` Ingo Molnar [this message]
2008-02-28 21:24     ` Alexey Zaytsev
2008-02-29 19:45       ` Ingo Molnar
2008-03-01  0:42         ` Alexey Zaytsev
2008-03-01  0:59           ` Alexey Zaytsev
2008-03-03 10:45             ` Ingo Molnar
2008-03-03 22:16               ` Alexey Zaytsev
2008-03-04 12:44                 ` Ingo Molnar
2008-02-29 11:11     ` Paul Mackerras
2008-02-29 18:42       ` David Miller
2008-02-29 19:44         ` Ingo Molnar
2008-03-21 18:55     ` Alexey Zaytsev

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20080228201613.GA30999@elte.hu \
    --to=mingo@elte.hu \
    --cc=alexey.zaytsev@gmail.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=tglx@linutronix.de \
    --subject='Re: printk timestamps jumping back and forth in 2.6.25-rc.' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).