tickless kernel works with PIT+TSC on the OLPC boards (2.6.20-rc1-rt4)

[Jim Gettys]

On Mon, 2006-12-25 at 22:01 -0200, Marcelo Tosatti wrote:
> Hi,
> 
> 2.6.20-rc1-rt4 works on the Geode with the PIT on aperiodic mode
> (CONFIG_NO_HZ, aka tickless).
> 

Cool.

> Thomas disabled usage of the TSC as a clock source for high resolution
> timers on the mainline kernel because of:
> 
> 1) frequency scaling 
> 2) TSC stops counting when CPU is halted
> 3) SMP synchronization issues?
> 
> >From what I can tell the Geode can safely use the TSC for highres, since:
> 
> 1) Does not support frequency scaling, 
> 2) The TSC _does_ count when the CPU is halted. Furthermore, the Geode
> supports a mode called "suspension on halt", where Suspend mode (which
> interacts with the power management states) is entered. TSC counting
> during suspend mode is controlled by bit 8 of the Bus Controller
> Configuration Register #0 (thanks Tom!).
> 3) no SMP :)
> 
> So the attached patch on top of 2.6.20-rc1-rt4 checks if "RTSC counts
> during suspension", enabling the "is_continuous" flag required by the
> highres timers subsystem. Comments?
> 
> Running a kernel with CONFIG_HZ=100, runlevel 3, I see an average of 50
> timer interrupts per second. 
> 
> There's a bunch of badly behaving timers which should be
> investigated/tuned (note that this is not a realistic workload). dhcdbd
> and some still unknown continuous USB URB submission show as the top
> offenders (along with the framebuffer console cursor, which should not
> show up on real scenarios).

Heh.  The blinking console cursor was one of the offenders that Dave
Jones pointed out at OLS last summer.  Would be good to fix it.

I have a memory we've also come across the 8042 driver doing something
very stupid as well, (for PS/2 hotplug), but maybe that has gotten fixed
(Andres?).  We certainly don't need PS/2 hot plug given the lack of a
PS/2 connector ;-).

I also have memories of someone saying a USB rewrite in progress should
clean up the USB timers, but don't remember the details.

> 
> -bash-3.1# echo 1 > timer_stats ; sleep 5s ; cat timer_stats
> Timer Stats Version: v0.1
> Sample period: 5.014 s
>    2,     0 swapper          hrtimer_restart_sched_tick (hrtimer_sched_tick)
>   25,     1 swapper          fbcon_add_cursor_timer (cursor_timer_handler)
>   84,     0 swapper          hrtimer_stop_sched_tick (hrtimer_sched_tick)
>   24,   935 dhcdbd           schedule_timeout (process_timeout)
>   19,     1 swapper          usb_hcd_submit_urb (rh_timer_func)
>    5,     1 swapper          init_tsc_clocksource (verify_tsc_freq)
>    1,     1 init             schedule_timeout (process_timeout)
>    2,     1 swapper          schedule_delayed_work_on (delayed_work_timer_fn)
>    1,     1 swapper          neigh_table_init_no_netlink (neigh_periodic_timer)
>    1,     0 swapper          page_writeback_init (wb_timer_fn)
>    1,  1128 sleep            do_nanosleep (hrtimer_wakeup)
> 165 total events, 33.32 events/sec
> 
> format: nr_events	pid taskname	setupfn (handlerfn)
> 
> Thomas, great work! 

Yup.  Wonderful stuff Thomas!


> 
> --- ./arch/i386/kernel/tsc.c.orig	2006-12-25 20:28:15.000000000 -0200
> +++ ./arch/i386/kernel/tsc.c	2006-12-25 21:04:40.000000000 -0200
> @@ -16,6 +16,7 @@
>  #include <asm/delay.h>
>  #include <asm/tsc.h>
>  #include <asm/io.h>
> +#include <asm/msr.h>
>  
>  #include "mach_timer.h"
>  
> @@ -478,6 +479,17 @@
>   	return num_possible_cpus() > 1;
>  }
>  
> +static void check_geode_tsc_reliable(void) 
> +{
> +	unsigned long val;
> +
> +#define RTSC_SUSP 0x100 /* RTSC counts during suspend */
> +
> +	rdmsrl(MSR_GEODE_BUSCONT_CONF0, val);
> +	if ((val & RTSC_SUSP))
> +		clocksource_tsc.is_continuous = 1;
> +}
> +
>  static int __init init_tsc_clocksource(void)
>  {
>  
> @@ -507,6 +519,8 @@
>  		if (!pmtmr_ioport || !clocksource_tsc.rating)
>  			clocksource_tsc.is_continuous = 0;
>  
> +		check_geode_tsc_reliable();
> +
>  		pm_multiplier = clocksource_hz2mult(PMTMR_TICKS_PER_SEC, 22);
>  
>  		init_timer(&verify_tsc_freq_timer);
> --- ./include/asm-i386/msr.h.orig	2006-12-25 20:15:34.000000000 -0200
> +++ ./include/asm-i386/msr.h	2006-12-25 20:20:44.000000000 -0200
> @@ -307,4 +307,7 @@
>  #define MSR_CORE_PERF_GLOBAL_CTRL	0x38f
>  #define MSR_CORE_PERF_GLOBAL_OVF_CTRL	0x390
>  
> +/* Geode GX defined MSRs */
> +#define MSR_GEODE_BUSCONT_CONF0		0x1900
> +
>  #endif /* __ASM_MSR_H */
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-- 
Jim Gettys
One Laptop Per Child