Messages in this thread Patch in this message | | | From | David Brownell <> | Subject | Re: [PATCH] atmel_tc clocksource/clockevent code | Date | Wed, 5 Mar 2008 13:14:38 -0800 |
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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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