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sched/cputime: Drop now-stale mul_u64_u64_div_u64() over-approximation guard

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Commit 77baa5bafc ("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 b29a62d87c ("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
This commit is contained in:
Nicolas Pitre 2026-05-14 16:26:29 -04:00 committed by Peter Zijlstra
parent eecd5e117c
commit 95f44886af
1 changed files with 0 additions and 6 deletions
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6
kernel/sched/cputime.c
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@ -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:
/*