diff options
| -rw-r--r-- | kernel/sched/cputime.c | 57 |
1 files changed, 35 insertions, 22 deletions
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index 33508dc78d0c..e9198abfca53 100644 --- a/kernel/sched/cputime.c +++ b/kernel/sched/cputime.c @@ -506,34 +506,47 @@ void account_idle_ticks(unsigned long ticks) } /* - * Perform (stime * rtime) / total with reduced chances - * of multiplication overflows by using smaller factors - * like quotient and remainders of divisions between - * rtime and total. + * Perform (stime * rtime) / total, but avoid multiplication overflow by + * loosing precision when the numbers are big. */ static cputime_t scale_stime(u64 stime, u64 rtime, u64 total) { - u64 rem, res, scaled; + u64 scaled; - if (rtime >= total) { - /* - * Scale up to rtime / total then add - * the remainder scaled to stime / total. - */ - res = div64_u64_rem(rtime, total, &rem); - scaled = stime * res; - scaled += div64_u64(stime * rem, total); - } else { - /* - * Same in reverse: scale down to total / rtime - * then substract that result scaled to - * to the remaining part. - */ - res = div64_u64_rem(total, rtime, &rem); - scaled = div64_u64(stime, res); - scaled -= div64_u64(scaled * rem, total); + for (;;) { + /* Make sure "rtime" is the bigger of stime/rtime */ + if (stime > rtime) { + u64 tmp = rtime; rtime = stime; stime = tmp; + } + + /* 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 (__force cputime_t) scaled; } |