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SubjectRe: [Patch] Re: Nasty suprise with uptime
On Thu, 1 Nov 2001, Tim Schmielau wrote:

> On Thu, 1 Nov 2001, george anzinger wrote:
>
[...]
> > Doesn't this need to be protected on SMP machines? What if two cpus
> > call get_jiffies64() at the same time... Seems like jiffies_hi could
> > get bumped twice instead of once.
> >
> > George
> >
>
> Yes, it does, my race protection is bogus. Petr Vandrovec also pointed out
> that. So we do need either to
> a) stuff jiffies_hi and jiffies_last into one atomic type
> (16 bits is enough for each) or
> b) use locking.
> My next patch will use b), but I won't do it until I have resolved the
> most annoying stability issues. I won't have time to do this before the
> weekend, and don't want to bother the list too much either.
>
> Maybe the lockups are just due to my setting of INITIAL_JIFFIES instead of
> waiting 471 days. The time adjustment routines are good candidates for
> this kind of mistakes. Any ideas anyone where else I might have forgotten
> to introduce INITIAL_JIFFIES ?
>


Well, I did the next patch without waiting for progress on the stability
front (fsck still in heavy use here). As an excercise I added proper
locking to get_jiffies64().

Actually, it is not decided yet whether jiffies_hi should be incremented
in get_jiffies64() or in do_timer(), and whether jiffies and jiffies_hi
should be glued together to form a 64 bit value, so the innards of
get_jiffies64() might change again.

However, no objections have been raised on the interface, so I started to
add some uses of it. I bumped up the start_time field of struct task_struct
to 64 bits, so that ps output will still be ok after the 32 bit
wraparound. This change might break userland applications that expect the
/proc/#/stat fields to fit in 32 bit variables. Yet this is not a
regression as the kernel was broken for these cases until now.

As a side effect, the problem of the current get_jiffies64()
implementation loosing jiffie wraparounds is weakened further.
With the current patch wraparounds can only be lost if no processes are
started or finished for 497 days, a condition that I can only imagine to
be met by embedded devices.

Tim


--- linux-2.4.14-pre6/fs/proc/proc_misc.c Thu Oct 11 19:46:57 2001
+++ linux-2.4.14-pre6-jiffies64/fs/proc/proc_misc.c Wed Oct 31 22:53:55 2001
@@ -39,6 +39,7 @@
#include <asm/uaccess.h>
#include <asm/pgtable.h>
#include <asm/io.h>
+#include <asm/div64.h>


#define LOAD_INT(x) ((x) >> FSHIFT)
@@ -103,15 +104,19 @@
static int uptime_read_proc(char *page, char **start, off_t off,
int count, int *eof, void *data)
{
- unsigned long uptime;
+ u64 uptime;
+ unsigned long remainder;
unsigned long idle;
int len;

- uptime = jiffies;
+ uptime = get_jiffies64() - INITIAL_JIFFIES;
+ remainder = (unsigned long) do_div(uptime, HZ);
+
idle = init_tasks[0]->times.tms_utime + init_tasks[0]->times.tms_stime;

- /* The formula for the fraction parts really is ((t * 100) / HZ) % 100, but
- that would overflow about every five days at HZ == 100.
+ /* The formula for the fraction part of the idle time really is
+ ((t * 100) / HZ) % 100, but that would overflow about
+ every five days at HZ == 100.
Therefore the identity a = (a / b) * b + a % b is used so that it is
calculated as (((t / HZ) * 100) + ((t % HZ) * 100) / HZ) % 100.
The part in front of the '+' always evaluates as 0 (mod 100). All divisions
@@ -121,14 +126,14 @@
*/
#if HZ!=100
len = sprintf(page,"%lu.%02lu %lu.%02lu\n",
- uptime / HZ,
- (((uptime % HZ) * 100) / HZ) % 100,
+ (unsigned long) uptime,
+ (remainder * 100) / HZ,
idle / HZ,
(((idle % HZ) * 100) / HZ) % 100);
#else
len = sprintf(page,"%lu.%02lu %lu.%02lu\n",
- uptime / HZ,
- uptime % HZ,
+ (unsigned long) uptime,
+ remainder,
idle / HZ,
idle % HZ);
#endif
--- linux-2.4.14-pre6/kernel/timer.c Mon Oct 8 19:41:41 2001
+++ linux-2.4.14-pre6-jiffies64/kernel/timer.c Thu Nov 1 23:05:38 2001
@@ -65,7 +65,7 @@

extern int do_setitimer(int, struct itimerval *, struct itimerval *);

-unsigned long volatile jiffies;
+unsigned long volatile jiffies = INITIAL_JIFFIES;

unsigned int * prof_buffer;
unsigned long prof_len;
@@ -117,7 +117,7 @@
INIT_LIST_HEAD(tv1.vec + i);
}

-static unsigned long timer_jiffies;
+static unsigned long timer_jiffies = INITIAL_JIFFIES;

static inline void internal_add_timer(struct timer_list *timer)
{
@@ -638,7 +638,7 @@
}

/* jiffies at the most recent update of wall time */
-unsigned long wall_jiffies;
+unsigned long wall_jiffies = INITIAL_JIFFIES;

/*
* This spinlock protect us from races in SMP while playing with xtime. -arca
@@ -683,6 +683,37 @@
if (TQ_ACTIVE(tq_timer))
mark_bh(TQUEUE_BH);
}
+
+
+#if BITS_PER_LONG < 48
+
+u64 get_jiffies64(void)
+{
+ static unsigned long jiffies_hi = 0;
+ static unsigned long jiffies_last = INITIAL_JIFFIES;
+ static spinlock_t jiffies64_lock = SPIN_LOCK_UNLOCKED;
+ unsigned long jiffies_tmp;
+
+ spin_lock(&jiffies64_lock);
+ jiffies_tmp = jiffies; /* avoid races */
+ if (jiffies_tmp < jiffies_last) /* We have a wrap */
+ jiffies_hi++;
+ jiffies_last = jiffies_tmp;
+ spin_unlock(&jiffies64_lock);
+
+ return (jiffies_tmp | ((u64)jiffies_hi) << BITS_PER_LONG);
+}
+
+#else
+ /* jiffies is wide enough to not wrap for 8716 years at HZ==1024 */
+
+static inline u64 get_jiffies64(void)
+{
+ return (u64)jiffies;
+}
+
+#endif /* BITS_PER_LONG < 48 */
+

#if !defined(__alpha__) && !defined(__ia64__)

--- linux-2.4.14-pre6/kernel/info.c Sat Apr 21 01:15:40 2001
+++ linux-2.4.14-pre6-jiffies64/kernel/info.c Wed Oct 31 23:15:26 2001
@@ -12,15 +12,19 @@
#include <linux/smp_lock.h>

#include <asm/uaccess.h>
+#include <asm/div64.h>

asmlinkage long sys_sysinfo(struct sysinfo *info)
{
struct sysinfo val;
+ u64 uptime;

memset((char *)&val, 0, sizeof(struct sysinfo));

cli();
- val.uptime = jiffies / HZ;
+ uptime = get_jiffies64() - INITIAL_JIFFIES;
+ do_div(uptime, HZ);
+ val.uptime = (unsigned long) uptime;

val.loads[0] = avenrun[0] << (SI_LOAD_SHIFT - FSHIFT);
val.loads[1] = avenrun[1] << (SI_LOAD_SHIFT - FSHIFT);
--- linux-2.4.14-pre6/include/linux/sched.h Wed Oct 31 23:06:23 2001
+++ linux-2.4.14-pre6-jiffies64/include/linux/sched.h Fri Nov 2 00:46:39 2001
@@ -351,7 +351,7 @@
unsigned long it_real_incr, it_prof_incr, it_virt_incr;
struct timer_list real_timer;
struct tms times;
- unsigned long start_time;
+ u64 start_time;
long per_cpu_utime[NR_CPUS], per_cpu_stime[NR_CPUS];
/* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */
unsigned long min_flt, maj_flt, nswap, cmin_flt, cmaj_flt, cnswap;
@@ -543,7 +543,10 @@

#include <asm/current.h>

+#define INITIAL_JIFFIES ((unsigned long)(-120*HZ))
extern unsigned long volatile jiffies;
+extern u64 get_jiffies64(void);
+
extern unsigned long itimer_ticks;
extern unsigned long itimer_next;
extern struct timeval xtime;
--- linux-2.4.14-pre6/kernel/fork.c Wed Oct 24 02:44:15 2001
+++ linux-2.4.14-pre6-jiffies64/kernel/fork.c Thu Nov 1 22:57:20 2001
@@ -647,7 +647,7 @@
}
#endif
p->lock_depth = -1; /* -1 = no lock */
- p->start_time = jiffies;
+ p->start_time = get_jiffies64();

INIT_LIST_HEAD(&p->local_pages);

--- linux-2.4.14-pre6/kernel/acct.c Mon Mar 19 21:35:08 2001
+++ linux-2.4.14-pre6-jiffies64/kernel/acct.c Fri Nov 2 00:13:37 2001
@@ -56,6 +56,7 @@
#include <linux/tty.h>

#include <asm/uaccess.h>
+#include <asm/div64.h>

/*
* These constants control the amount of freespace that suspend and
@@ -227,20 +228,24 @@
* This routine has been adopted from the encode_comp_t() function in
* the kern_acct.c file of the FreeBSD operating system. The encoding
* is a 13-bit fraction with a 3-bit (base 8) exponent.
+ *
+ * Bumped up to encode 64 bit values. Unfortunately the result may
+ * overflow now.
*/

#define MANTSIZE 13 /* 13 bit mantissa. */
-#define EXPSIZE 3 /* Base 8 (3 bit) exponent. */
+#define EXPSIZE 3 /* 3 bit exponent. */
+#define EXPBASE 3 /* Base 8 (3 bit) exponent. */
#define MAXFRACT ((1 << MANTSIZE) - 1) /* Maximum fractional value. */

-static comp_t encode_comp_t(unsigned long value)
+static comp_t encode_comp_t(u64 value)
{
int exp, rnd;

exp = rnd = 0;
while (value > MAXFRACT) {
- rnd = value & (1 << (EXPSIZE - 1)); /* Round up? */
- value >>= EXPSIZE; /* Base 8 exponent == 3 bit shift. */
+ rnd = value & (1 << (EXPBASE - 1)); /* Round up? */
+ value >>= EXPBASE; /* Base 8 exponent == 3 bit shift. */
exp++;
}

@@ -248,16 +253,21 @@
* If we need to round up, do it (and handle overflow correctly).
*/
if (rnd && (++value > MAXFRACT)) {
- value >>= EXPSIZE;
+ value >>= EXPBASE;
exp++;
}

/*
* Clean it up and polish it off.
*/
- exp <<= MANTSIZE; /* Shift the exponent into place */
- exp += value; /* and add on the mantissa. */
- return exp;
+ if (exp >= (1 << EXPSIZE)) {
+ /* Overflow. Return largest representable number instead */
+ return (1 << (MANTSIZE + EXPSIZE)) - 1;
+ } else {
+ exp <<= MANTSIZE; /* Shift the exponent into place */
+ exp += value; /* and add on the mantissa. */
+ return exp;
+ }
}

/*
@@ -277,6 +287,7 @@
struct acct ac;
mm_segment_t fs;
unsigned long vsize;
+ u64 elapsed;

/*
* First check to see if there is enough free_space to continue
@@ -294,8 +305,10 @@
strncpy(ac.ac_comm, current->comm, ACCT_COMM);
ac.ac_comm[ACCT_COMM - 1] = '\0';

- ac.ac_btime = CT_TO_SECS(current->start_time) + (xtime.tv_sec - (jiffies / HZ));
- ac.ac_etime = encode_comp_t(jiffies - current->start_time);
+ elapsed = get_jiffies64() - current->start_time;
+ ac.ac_etime = encode_comp_t(elapsed);
+ do_div(elapsed, HZ);
+ ac.ac_btime = xtime.tv_sec - elapsed;
ac.ac_utime = encode_comp_t(current->times.tms_utime);
ac.ac_stime = encode_comp_t(current->times.tms_stime);
ac.ac_uid = current->uid;
--- linux-2.4.14-pre6/fs/proc/array.c Thu Oct 11 18:00:01 2001
+++ linux-2.4.14-pre6-jiffies64/fs/proc/array.c Thu Nov 1 23:04:02 2001
@@ -343,7 +343,7 @@
ppid = task->pid ? task->p_opptr->pid : 0;
read_unlock(&tasklist_lock);
res = sprintf(buffer,"%d (%s) %c %d %d %d %d %d %lu %lu \
-%lu %lu %lu %lu %lu %ld %ld %ld %ld %ld %ld %lu %lu %ld %lu %lu %lu %lu %lu \
+%lu %lu %lu %lu %lu %ld %ld %ld %ld %ld %ld %llu %lu %ld %lu %lu %lu %lu %lu \
%lu %lu %lu %lu %lu %lu %lu %lu %d %d\n",
task->pid,
task->comm,
@@ -366,7 +366,7 @@
nice,
0UL /* removed */,
task->it_real_value,
- task->start_time,
+ (unsigned long long)(task->start_time - INITIAL_JIFFIES),
vsize,
mm ? mm->rss : 0, /* you might want to shift this left 3 */
task->rlim[RLIMIT_RSS].rlim_cur,
-
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