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SubjectRe: [PATCH] atmel_tc clocksource/clockevent code
Date
On Wednesday 05 March 2008, Thomas Gleixner wrote:
> > So, hires timestamps -> really really welcome.
> > hires timers -> there should be a (configurable) minimal resolution
> > that fits the hardware to not overload the CPU.
>
> clockevents let you set a minimum delta already. This can be set at
> runtime before registering the device.

But the overhead isn't specific to any given clockevent
device. The same issue was reported on other ARMs.

The acceptable overhead is a function of system-specific
factors including load and CPU clock rate, not of a given
clockevent device.

I wouldn't think teaching multiple such drivers about such
issues could be better than having clockevents_program_event()
consider a system-specific lower bound ...

- Dave



==============
Add a "min_delta_ns" kernel parameter to help systems avoid excess
scheduling and timer overheads. As with init_timer_deferrable(),
this batches timer IRQs to reduce those overheads.

For example, on one ARM9 platform the timer IRQ takes 2 usec, but the
system then needs over 150 usec to handle scheduler-related tasks.
On such a system, applications using many high resolution timers can
work better if min_delta_ns is used to avoid scheduling timer IRQs
so often that no other work gets done.

---
Documentation/kernel-parameters.txt | 7 +++++++
kernel/time/clockevents.c | 15 +++++++++++++++
2 files changed, 22 insertions(+)

--- a/Documentation/kernel-parameters.txt 2008-02-23 11:36:13.000000000 -0800
+++ b/Documentation/kernel-parameters.txt 2008-03-05 12:58:20.000000000 -0800
@@ -1146,6 +1146,13 @@ and is between 256 and 4096 characters.

mga= [HW,DRM]

+ min_delta_ns=n [GENERIC_TIME] Defines the minimum amount of time
+ that oneshot clockevent sources will be asked to
+ sleep. This is measured in nanoseconds, and may be
+ overridden by the clockevent device. You probably
+ want this number to be more than your system's timer
+ IRQ overhead.
+
mousedev.tap_time=
[MOUSE] Maximum time between finger touching and
leaving touchpad surface for touch to be considered
--- a/kernel/time/clockevents.c 2008-02-10 15:48:29.000000000 -0800
+++ b/kernel/time/clockevents.c 2008-03-05 12:38:14.000000000 -0800
@@ -29,6 +29,18 @@ static RAW_NOTIFIER_HEAD(clockevents_cha
/* Protection for the above */
static DEFINE_SPINLOCK(clockevents_lock);

+/* This is the lower bound on oneshot timer intervals; setting its
+ * value too low can maintain HRT overhead at objectionable levels.
+ */
+static unsigned long min_delta_ns;
+
+static int __init set_min_delta_ns(char *str)
+{
+ simple_strtoul(str, NULL, 0);
+ return 1;
+}
+__setup("min_delta_ns", set_min_delta_ns);
+
/**
* clockevents_delta2ns - Convert a latch value (device ticks) to nanoseconds
* @latch: value to convert
@@ -98,6 +110,9 @@ int clockevents_program_event(struct clo
if (dev->mode == CLOCK_EVT_MODE_SHUTDOWN)
return 0;

+ if (delta < min_delta_ns)
+ delta = min_delta_ns;
+
if (delta > dev->max_delta_ns)
delta = dev->max_delta_ns;
if (delta < dev->min_delta_ns)

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