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Date: Fri, 24 Jan 2020 16:42:15 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>
Cc: Andrew Fox <afox@...hat.com>,
Stephen Johnston <sjohnsto@...hat.com>,
linux-kernel@...r.kernel.org,
Stanislaw Gruszka <sgruszka@...hat.com>
Subject: Re: [PATCH] sched/cputime: make scale_stime() more precise
On 01/22, Oleg Nesterov wrote:
>
> To remind, scale_stime(stime, rtime, total) is not precise, to say at
> least. For example:
>
> stime = -1ul/33333; total = stime*3; rtime = total*5555555555;
>
> scale_stime() returns 9067034312525142184 while the correct result is
> 6148914688753325707.
>
> OK, these random numbers are not realistic, usually the relative error
> is small enough.
>
> However, even if the relative error is small, the absolute error can be
> huge. And this means that if you watch /proc/$pid/status incrementally
> to see how stime/utime grow, you can get the completely wrong numbers.
>
> Say, utime (or stime) can be frozen for unpredictably long time, as if
> the monitored application "hangs" in kernel mode, while the real split
> is 50/50.
See another test-case below. Arguments:
start_time start_utime_percent inc_time inc_utime_percent
For example,
$ ./test 8640000 50 600 50 | head
simulates process which runs 100 days 50/50 in user/kernel mode, then it
starts to check utime/stime every 600 seconds and print the difference.
The output:
old new
0:600000000000 300000000000:300000000000
0:600000000000 300000000000:300000000000
0:600000000000 300000000000:300000000000
600000000000:0 300000000000:300000000000
499469920248:100530079752 300000000000:300000000000
0:600000000000 300000000000:300000000000
0:600000000000 300000000000:300000000000
600000000000:0 300000000000:300000000000
499490181588:100509818412 300000000000:300000000000
it looks as if this process can spend 20 minutes entirely in kernel mode.
Oleg.
-------------------------------------------------------------------------------
#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#define noinline __attribute__((__noinline__))
typedef unsigned long long u64;
typedef unsigned int u32;
typedef unsigned __int128 u128;
static inline u64 div_u64_rem(u64 dividend, u32 divisor, u32 *remainder)
{
*remainder = dividend % divisor;
return dividend / divisor;
}
static inline u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
{
*remainder = dividend % divisor;
return dividend / divisor;
}
static inline u64 div64_u64(u64 dividend, u64 divisor)
{
return dividend / divisor;
}
static inline u64 div_u64(u64 dividend, u32 divisor)
{
u32 remainder;
return div_u64_rem(dividend, divisor, &remainder);
}
static inline int fls64(u64 x)
{
int bitpos = -1;
/*
* AMD64 says BSRQ won't clobber the dest reg if x==0; Intel64 says the
* dest reg is undefined if x==0, but their CPU architect says its
* value is written to set it to the same as before.
*/
asm("bsrq %1,%q0"
: "+r" (bitpos)
: "rm" (x));
return bitpos + 1;
}
static inline int ilog2(u64 n)
{
return fls64(n) - 1;
}
#define swap(a, b) \
do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
u64 scale_stime(u64 stime, u64 rtime, u64 total)
{
u64 scaled;
for (;;) {
/* Make sure "rtime" is the bigger of stime/rtime */
if (stime > rtime)
swap(rtime, stime);
/* Make sure 'total' fits in 32 bits */
if (total >> 32)
goto drop_precision;
/* Does rtime (and thus stime) fit in 32 bits? */
if (!(rtime >> 32))
break;
/* Can we just balance rtime/stime rather than dropping bits? */
if (stime >> 31)
goto drop_precision;
/* We can grow stime and shrink rtime and try to make them both fit */
stime <<= 1;
rtime >>= 1;
continue;
drop_precision:
/* We drop from rtime, it has more bits than stime */
rtime >>= 1;
total >>= 1;
}
/*
* Make sure gcc understands that this is a 32x32->64 multiply,
* followed by a 64/32->64 divide.
*/
scaled = div_u64((u64) (u32) stime * (u64) (u32) rtime, (u32)total);
return scaled;
}
u64 new_scale_stime(u64 stime, u64 rtime, u64 total)
{
u64 res = 0, div, rem;
if (ilog2(stime) + ilog2(rtime) > 62) {
div = div64_u64_rem(rtime, total, &rem);
res = div * stime;
rtime = rem;
int shift = ilog2(stime) + ilog2(rtime) - 62;
if (shift > 0) {
rtime >>= shift;
total >>= shift;
if (!total)
return res;
}
}
return res + div64_u64(stime * rtime, total);
}
struct task_cputime {
u64 stime;
u64 utime;
unsigned long long sum_exec_runtime;
};
struct prev_cputime {
u64 utime;
u64 stime;
};
void cputime_adjust(int new, struct task_cputime *curr, struct prev_cputime *prev,
u64 *ut, u64 *st)
{
u64 rtime, stime, utime;
rtime = curr->sum_exec_runtime;
if (prev->stime + prev->utime >= rtime)
goto out;
stime = curr->stime;
utime = curr->utime;
if (stime == 0) {
utime = rtime;
goto update;
}
if (utime == 0) {
stime = rtime;
goto update;
}
stime = (new ? new_scale_stime : scale_stime)(stime, rtime, stime + utime);
update:
if (stime < prev->stime)
stime = prev->stime;
utime = rtime - stime;
if (utime < prev->utime) {
utime = prev->utime;
stime = rtime - utime;
}
prev->stime = stime;
prev->utime = utime;
out:
*ut = prev->utime;
*st = prev->stime;
}
void prdiff(int new, struct task_cputime *curr, struct prev_cputime *prev)
{
struct prev_cputime __prev = *prev;
u64 ut, st, ud, sd;
cputime_adjust(new, curr, prev, &ut, &st);
ud = ut - __prev.utime;
sd = st - __prev.stime;
printf("%16llu:%-16llu", ud, sd);
}
#define SEC 1000000000ULL
void parse_cputime(struct task_cputime *t, char **argv)
{
double total = strtod(argv[0], NULL) * SEC;
double utime = strtod(argv[1], NULL) / 100;
utime *= total;
t->utime = utime;
t->stime = total - utime;
}
int main(int argc, char **argv)
{
struct prev_cputime old_prev = {};
struct prev_cputime new_prev = {};
struct task_cputime curr, diff;
u64 tmp;
if (argc != 5) {
printf("usage: %s start_time utime_percent inc_time utime_percent\n", argv[0]);
return 0;
}
parse_cputime(&curr, argv+1);
parse_cputime(&diff, argv+3);
curr.sum_exec_runtime = curr.utime + curr.stime;
cputime_adjust(0, &curr, &old_prev, &tmp, &tmp);
cputime_adjust(1, &curr, &new_prev, &tmp, &tmp);
printf("%18s%15s\t%18s\n", "old", "", "new");
for (;;) {
curr.utime += diff.utime;
curr.stime += diff.stime;
curr.sum_exec_runtime = curr.utime + curr.stime;
prdiff(0, &curr, &old_prev);
printf("\t");
prdiff(1, &curr, &new_prev);
printf("\n");
}
return 0;
}
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