Re: [PATCH] group xtime, xtime_lock, wall_to_monotonic, avenrun, calc_load_count fields together in ktimed

Andrew Morton
2006-12-09T05:46:38Z

On Fri, 8 Dec 2006 17:52:09 +0100
Eric Dumazet <dada1@cosmosbay.com> wrote:

> This patch introduces a new structure called ktimed (Kernel Time Data), where
> some time keeping related variables are put together to share as few cache
> lines as possible. This avoid some false sharing, (since linker could put
> calc_load_count in a *random* cache line for example)
>
> I also optimized calc_load() to not always call count_active_tasks() :
> It should call it only once every 5 seconds (LOAD_FREQ=5*HZ)
>
> Note : x86_64 was using an arch specific placement of __xtime and __xtime_lock
> (see arch/x86_64/kernel/vmlinux.lds.S). (vsyscall stuff)
> It is now using a specific placement of __ktimed, since xtime and xtime_lock
> are now fields from __ktimed.
>
> Note : I failed to move jiffies64 as well in ktimed : too many changes needed
> because of jiffies aliasing (and endianess), but it could be done.
>

Sounds like you have about three patches there.

<save attachment, read from file, s/^/> />

>
> -extern struct timespec xtime;
> -extern struct timespec wall_to_monotonic;
> -extern seqlock_t xtime_lock;
> +/*
> + * define a structure to keep all fields close to each others.
> + */
> +struct ktimed_struct {
> + struct timespec _xtime;
> + struct timespec wall_to_monotonic;
> + seqlock_t lock;
> + unsigned long avenrun[3];
> + int calc_load_count;
> +};

crappy name, but I guess it doesn't matter as nobody will use it at this
stage. But...

> +extern struct ktimed_struct ktimed;
> +#define xtime ktimed._xtime
> +#define wall_to_monotonic ktimed.wall_to_monotonic
> +#define xtime_lock ktimed.lock
> +#define avenrun ktimed.avenrun

They'll use these instead.

Frankly, I think we'd be better off removing these macros and, longer-term,
use

write_seqlock(time_data.xtime_lock);

The approach you have here would be a good transition-period thing.

> void timekeeping_init(void);
>
> --- linux-2.6.19/kernel/timer.c 2006-12-08 11:50:11.000000000 +0100
> +++ linux-2.6.19-ed/kernel/timer.c 2006-12-08 18:13:24.000000000 +0100
> @@ -570,11 +570,13 @@ found:
> * however, we will ALWAYS keep the tv_nsec part positive so we can use
> * the usual normalization.
> */
> -struct timespec xtime __attribute__ ((aligned (16)));
> -struct timespec wall_to_monotonic __attribute__ ((aligned (16)));
> -
> -EXPORT_SYMBOL(xtime);
> -
> +#ifndef ARCH_HAVE_KTIMED

argh, another ARCH_HAVE_MESS. Due to the x86_64 vsyscall code.

Could you please see if we can nuke this by making
kernel/timer.c:xtime_lock use attribute(weak)? In a separate patch ;)

> +struct ktimed_struct ktimed __cacheline_aligned = {
> + .lock = __SEQLOCK_UNLOCKED(ktimed.lock),
> + .calc_load_count = LOAD_FREQ,
> +};
> +EXPORT_SYMBOL(ktimed);
> +#endif
>
> /* XXX - all of this timekeeping code should be later moved to time.c */
> #include <linux/clocksource.h>
> @@ -995,9 +997,6 @@ static unsigned long count_active_tasks(
> *
> * Requires xtime_lock to access.
> */
> -unsigned long avenrun[3];
> -
> -EXPORT_SYMBOL(avenrun);
>
> /*
> * calc_load - given tick count, update the avenrun load estimates.
> @@ -1006,27 +1005,21 @@ EXPORT_SYMBOL(avenrun);
> static inline void calc_load(unsigned long ticks)
> {
> unsigned long active_tasks; /* fixed-point */
> - static int count = LOAD_FREQ;
>
> - active_tasks = count_active_tasks();
> - for (count -= ticks; count < 0; count += LOAD_FREQ) {
> - CALC_LOAD(avenrun[0], EXP_1, active_tasks);
> - CALC_LOAD(avenrun[1], EXP_5, active_tasks);
> - CALC_LOAD(avenrun[2], EXP_15, active_tasks);
> + ktimed.calc_load_count -= ticks;
> +
> + if (unlikely(ktimed.calc_load_count < 0)) {
> + active_tasks = count_active_tasks();
> + do {
> + ktimed.calc_load_count += LOAD_FREQ;
> + CALC_LOAD(avenrun[0], EXP_1, active_tasks);
> + CALC_LOAD(avenrun[1], EXP_5, active_tasks);
> + CALC_LOAD(avenrun[2], EXP_15, active_tasks);
> + } while (ktimed.calc_load_count < 0);
> }
> }
>
> ...
>
> +extern struct ktimed_struct __ktimed;
> +#define __xtime_lock __ktimed.lock
> +#define __xtime __ktimed._xtime
>
> /* kernel space (writeable) */
> extern struct vxtime_data vxtime;
> extern int vgetcpu_mode;
> extern struct timezone sys_tz;
> extern int sysctl_vsyscall;
> -extern seqlock_t xtime_lock;
>
> -extern int sysctl_vsyscall;
> -
> -#define ARCH_HAVE_XTIME_LOCK 1
> +#define ARCH_HAVE_KTIMED 1
>

hm, the patch seems to transform a mess into a mess. I guess it's a messy
problem.

I agree that aggregating all the time-related things into a struct like
this makes some sense. As does aggregating them all into a similar-looking
namespace, but that'd probably be too intrusive - too late for that.