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Message-ID: <s5h1wf3gjz3.wl%tiwai@suse.de>
Date: Fri, 20 Jul 2007 17:42:24 +0200
From: Takashi Iwai <tiwai@...e.de>
To: Ingo Molnar <mingo@...e.hu>
Cc: linux-kernel@...r.kernel.org
Subject: Re: Regression with sys_time() speedup patch
At Fri, 20 Jul 2007 17:12:13 +0200,
Ingo Molnar wrote:
>
>
> * Takashi Iwai <tiwai@...e.de> wrote:
>
> > Hi Ingo,
> >
> > 2.6.22-git as of today caused some regression on my P4 machine. For
> > example, automount process takes 10-30% CPU time constantly, and
> > sometimes vim couldn't write files properly.
>
> does the patch below fix it?
Yep, through a short run, it seems working fine.
But, is it safe to introduce another variable different from
xtime.tv_sec? Since xtime.tv_sec is referred in many other places,
I'm afaid this may bring confusion...
thanks,
Takashi
>
> Ingo
>
> --------------->
> Subject: time: introduce xtime_seconds
> From: Ingo Molnar <mingo@...e.hu>
>
> introduce the xtime_seconds optimization. This is a read-mostly
> low-resolution time source available to sys_time() and kernel-internal
> use. This variable is kept uptodate atomically, and it's monotically
> increased, every time some time interface constructs an xtime-alike time
> result that overflows the seconds value. (it's updated from the timer
> interrupt as well)
>
> this way high-resolution time results update their seconds component at
> the same time sys_time() does it:
>
> 1184858832999989000
> 1184858832000000000
> 1184858832999992000
> 1184858832000000000
> 1184858832999996000
> 1184858832000000000
> 1184858832999999000
> 1184858832000000000
> 1184858833000003000
> 1184858833000000000
> 1184858833000006000
> 1184858833000000000
> 1184858833000009000
> 1184858833000000000
>
> [ these are nsec time results from alternating calls to sys_time() and
> sys_gettimeofday(), recorded at the seconds boundary. ]
>
> instead of the previous (non-coherent) behavior:
>
> 1184848950999987000
> 1184848950000000000
> 1184848950999990000
> 1184848950000000000
> 1184848950999994000
> 1184848950000000000
> 1184848950999997000
> 1184848950000000000
> 1184848951000001000
> 1184848950000000000
> 1184848951000005000
> 1184848950000000000
> 1184848951000008000
> 1184848950000000000
> 1184848951000011000
> 1184848950000000000
> 1184848951000015000
>
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> ---
> include/linux/time.h | 13 +++++++++++--
> kernel/time.c | 25 ++++++-------------------
> kernel/time/timekeeping.c | 28 ++++++++++++++++++++++++----
> 3 files changed, 41 insertions(+), 25 deletions(-)
>
> Index: linux/include/linux/time.h
> ===================================================================
> --- linux.orig/include/linux/time.h
> +++ linux/include/linux/time.h
> @@ -91,19 +91,28 @@ static inline struct timespec timespec_s
> extern struct timespec xtime;
> extern struct timespec wall_to_monotonic;
> extern seqlock_t xtime_lock __attribute__((weak));
> +extern unsigned long xtime_seconds;
>
> extern unsigned long read_persistent_clock(void);
> void timekeeping_init(void);
>
> +extern void __update_xtime_seconds(unsigned long new_xtime_seconds);
> +
> +static inline void update_xtime_seconds(unsigned long new_xtime_seconds)
> +{
> + if (unlikely((long)(new_xtime_seconds - xtime_seconds) > 0))
> + __update_xtime_seconds(new_xtime_seconds);
> +}
> +
> static inline unsigned long get_seconds(void)
> {
> - return xtime.tv_sec;
> + return xtime_seconds;
> }
>
> struct timespec current_kernel_time(void);
>
> #define CURRENT_TIME (current_kernel_time())
> -#define CURRENT_TIME_SEC ((struct timespec) { xtime.tv_sec, 0 })
> +#define CURRENT_TIME_SEC ((struct timespec) { xtime_seconds, 0 })
>
> extern void do_gettimeofday(struct timeval *tv);
> extern int do_settimeofday(struct timespec *tv);
> Index: linux/kernel/time.c
> ===================================================================
> --- linux.orig/kernel/time.c
> +++ linux/kernel/time.c
> @@ -58,11 +58,10 @@ EXPORT_SYMBOL(sys_tz);
> asmlinkage long sys_time(time_t __user * tloc)
> {
> /*
> - * We read xtime.tv_sec atomically - it's updated
> - * atomically by update_wall_time(), so no need to
> - * even read-lock the xtime seqlock:
> + * We read xtime_seconds atomically - it's updated
> + * atomically by update_xtime_seconds():
> */
> - time_t i = xtime.tv_sec;
> + time_t i = xtime_seconds;
>
> smp_rmb(); /* sys_time() results are coherent */
>
> @@ -226,11 +225,11 @@ inline struct timespec current_kernel_ti
>
> do {
> seq = read_seqbegin(&xtime_lock);
> -
> +
> now = xtime;
> } while (read_seqretry(&xtime_lock, seq));
>
> - return now;
> + return now;
> }
>
> EXPORT_SYMBOL(current_kernel_time);
> @@ -377,19 +376,7 @@ void do_gettimeofday (struct timeval *tv
> tv->tv_sec = sec;
> tv->tv_usec = usec;
>
> - /*
> - * Make sure xtime.tv_sec [returned by sys_time()] always
> - * follows the gettimeofday() result precisely. This
> - * condition is extremely unlikely, it can hit at most
> - * once per second:
> - */
> - if (unlikely(xtime.tv_sec != tv->tv_sec)) {
> - unsigned long flags;
> -
> - write_seqlock_irqsave(&xtime_lock, flags);
> - update_wall_time();
> - write_sequnlock_irqrestore(&xtime_lock, flags);
> - }
> + update_xtime_seconds(sec);
> }
> EXPORT_SYMBOL(do_gettimeofday);
>
> Index: linux/kernel/time/timekeeping.c
> ===================================================================
> --- linux.orig/kernel/time/timekeeping.c
> +++ linux/kernel/time/timekeeping.c
> @@ -45,14 +45,28 @@ EXPORT_SYMBOL(xtime_lock);
> * used instead.
> */
> struct timespec xtime __attribute__ ((aligned (16)));
> -struct timespec wall_to_monotonic __attribute__ ((aligned (16)));
> -static unsigned long total_sleep_time; /* seconds */
> -
> EXPORT_SYMBOL(xtime);
>
> +struct timespec wall_to_monotonic __attribute__ ((aligned (16))) __read_mostly;
> +static unsigned long total_sleep_time __read_mostly; /* seconds */
> +
> +unsigned long xtime_seconds __read_mostly;
> +EXPORT_SYMBOL(xtime_seconds);
> +
> +/* pointer to current clocksource: */
> +static struct clocksource *clock __read_mostly;
>
> -static struct clocksource *clock; /* pointer to current clocksource */
> +/*
> + * Called when either xtime or any xtime-alike result back to
> + * user-space overflows the xtime_seconds field:
> + */
> +void __update_xtime_seconds(unsigned long new_xtime_seconds)
> +{
> + unsigned long old_xtime_seconds = xtime_seconds;
>
> + if ((long)(new_xtime_seconds - old_xtime_seconds) > 0)
> + cmpxchg(&xtime_seconds, old_xtime_seconds, new_xtime_seconds);
> +}
>
> #ifdef CONFIG_GENERIC_TIME
> /**
> @@ -100,6 +113,8 @@ static inline void __get_realtime_clock_
> } while (read_seqretry(&xtime_lock, seq));
>
> timespec_add_ns(ts, nsecs);
> +
> + update_xtime_seconds(ts->tv_sec);
> }
>
> /**
> @@ -256,6 +271,8 @@ void __init timekeeping_init(void)
> clock->cycle_last = clocksource_read(clock);
>
> xtime.tv_sec = sec;
> + update_xtime_seconds(sec);
> +
> xtime.tv_nsec = 0;
> set_normalized_timespec(&wall_to_monotonic,
> -xtime.tv_sec, -xtime.tv_nsec);
> @@ -290,6 +307,8 @@ static int timekeeping_resume(struct sys
> unsigned long sleep_length = now - timekeeping_suspend_time;
>
> xtime.tv_sec += sleep_length;
> + update_xtime_seconds(xtime.tv_sec);
> +
> wall_to_monotonic.tv_sec -= sleep_length;
> total_sleep_time += sleep_length;
> }
> @@ -464,6 +483,7 @@ void update_wall_time(void)
> clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
> xtime.tv_sec++;
> second_overflow();
> + update_xtime_seconds(xtime.tv_sec);
> }
>
> /* interpolator bits */
>
-
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