Re: [PATCH] hugetlb: add cond_resched() to __unmap_hugepage_range()

[Michal Hocko]

On Tue 18-08-26 16:59:38, Andrew Morton wrote:
> On Wed, 19 Aug 2026 00:46:09 +0100 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> 
> > On Tue, Aug 18, 2026 at 04:24:30PM -0700, Andrew Morton wrote:
> > > On Wed, 19 Aug 2026 06:18:58 +0800 Lance Yang <lance.yang@xxxxxxxxx> wrote:
> > > 
> > > > >> > 
> > > > >> > That's pretty bad behavior and we might want to fix it in earlier
> > > > >> > kernels.  Is PREEMPT_NONE effectively dead in 6.18.x and its
> > > > >> > existing users?
> > > > >> > 
> > > > >> > If yes, we do want to fix older kernels then we should merge this.
> > > > >> > 
> > > > >> 
> > > > >> Okay, but that would be stable-only fixes?
> > > > >
> > > > >Not understanding.
> > > > >
> > > > >Maybe you refer to adding a patch to -stable but not to -linus?  That's
> > > > >against the -stable rules
> > > > >(Documentation/process/stable-kernel-rules.rst).
> > > > 
> > > > Since cond_resched() is a scheduling no-op under LAZY/FULL anyway (only
> > > > the __might_resched() debug check remains), why not take this upstream
> > > > with Cc: stable?
> > > 
> > > I think so - as long as PREEPMT_NONE exists we should support it as
> > > well as we can.  If the day comes that PREEMPT_NONE is removed, then we
> > > get to remove lots of cond_resched()s.  mm/ has 200 of the things.
> > > 
> > > > Mainline scheduling stays unchanged, and stable can pick it up for old
> > > > PREEMPT_NONE kernels. wdyt?
> > > 
> > > Addressing this issue in older kernels is another reason.
> > 
> > That is not the direction that the scheduler developers wish us to take.
> 
> Well back luck.
> 
> "Packet receive timeouts were traced to sparse HugeTLB unmapping in
> production".  Do sched developers have a proposal to fix that in
> year-old kernels?

Yes, do not use PREEMPT_NONE in anything that is latency sensitive.
It makes very little sense to add more cond_resched, just to be
removed later.
-- 
Michal Hocko
SUSE Labs