Re: [PATCH] percpu: Fix hint invariant breakage

[Joonwon Kang]

> On Fri, 20 Mar 2026 11:52:14 +0000 Joonwon Kang <joonwonkang@xxxxxxxxxx> wrote:
> 
> > The invariant "scan_hint_start > contig_hint_start if and only if
> > scan_hint == contig_hint" should be kept for hint management. However,
> > it could be broken in some cases:
> > 
> >   - if (new contig == contig_hint == scan_hint) && (contig_hint_start <
> >     scan_hint_start < new contig start) && the new contig is to become a
> >     new contig_hint due to its better alignment, then scan_hint should
> >     be invalidated instead of keeping it.
> > 
> >   - if (new contig == contig_hint > scan_hint) && (start <
> >     contig_hint_start) && the new contig is not to become a new
> >     contig_hint, then scan_hint should be invalidated instead of being
> >     updated to the new contig.
> > 
> > This commit fixes this invariant breakage and also optimizes scan_hint
> > by keeping it or updating it when acceptable:
> > 
> >   - if (new contig > contig_hint > scan_hint) && (scan_hint_start < new
> >     contig start < contig_hint_start), then keep scan_hint instead of
> >     invalidating it.
> > 
> >   - if (new contig > contig_hint == scan_hint) && (contig_hint_start <
> >     new contig start < scan_hint_start), then update scan_hint to the
> >     old contig_hint instead of invalidating it.
> > 
> >   - if (new contig == contig_hint > scan_hint) && (new contig start <
> >     contig_hint_start) && the new contig is to become a new contig_hint
> >     due to its better alignment, then update scan_hint to the old
> >     contig_hint instead of invalidating or keeping it.
> > 
> 
> Thanks.  Does the change have any observable runtime effects?
> 
> Was this patch inspired by code inspection?

Yes, this change basically comes from manual code review. While there is
no benchmark result or others backing up the effects of this change,
Dennis may help on that later as he suggested in another review. The main
point of this patch should rather be fixing the "theoretical" invariant
breakage cases.

Thanks.