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| author | Nicolas Pitre <nico@fluxnic.net> | 2026-05-14 23:26:29 +0300 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2026-05-19 13:17:35 +0300 |
| commit | 95f44886afec7cbce0ff2a5ed8158fbe8aa6f2ec (patch) | |
| tree | e031ca20412288422a1ca7666f98ef21b97ff5fe | |
| parent | eecd5e117cfa63a353f4c69fdcea5d9b14af698e (diff) | |
| download | linux-95f44886afec7cbce0ff2a5ed8158fbe8aa6f2ec.tar.xz | |
sched/cputime: Drop now-stale mul_u64_u64_div_u64() over-approximation guard
Commit 77baa5bafcbe ("sched/cputime: Fix mul_u64_u64_div_u64() precision
for cputime") added a clamp in cputime_adjust():
if (unlikely(stime > rtime))
stime = rtime;
The justification was that mul_u64_u64_div_u64() could over-approximate
on some architectures (notably arm64 and the old 32-bit fallback), so
the mathematically impossible stime > rtime was nevertheless reachable
and would underflow utime = rtime - stime.
That premise no longer holds. Commit b29a62d87cc0 ("mul_u64_u64_div_u64:
make it precise always") replaced the fallback implementation with an
exact 128-bit long division, and the x86_64 inline asm already produced
exact results. The helper now returns the mathematically correct
floor(a*b/d) on every architecture, so stime <= rtime is guaranteed by
stime <= stime + utime and the clamp is dead code.
Remove it along with its stale comment.
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260514202629.673539-1-nico@fluxnic.net
| -rw-r--r-- | kernel/sched/cputime.c | 6 |
1 files changed, 0 insertions, 6 deletions
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index fbf31db0d2f3..6e85023a81ff 100644 --- a/kernel/sched/cputime.c +++ b/kernel/sched/cputime.c @@ -587,12 +587,6 @@ void cputime_adjust(struct task_cputime *curr, struct prev_cputime *prev, } stime = mul_u64_u64_div_u64(stime, rtime, stime + utime); - /* - * Because mul_u64_u64_div_u64() can approximate on some - * achitectures; enforce the constraint that: a*b/(b+c) <= a. - */ - if (unlikely(stime > rtime)) - stime = rtime; update: /* |