Re: [PATCH 7.2 v10 0/2] skip redundant sync IPIs when TLB flush sent them

Andrew Morton
2026-04-24T14:15:39Z

On Fri, 24 Apr 2026 13:37:04 +0000 Pasha Tatashin <pasha.tatashin@soleen.com> wrote:

> On 04-24 06:30, Andrew Morton wrote:
> > On Fri, 24 Apr 2026 14:25:26 +0800 Lance Yang <lance.yang@linux.dev> wrote:
> >
> > > When page table operations require synchronization with software/lockless
> > > walkers, they call tlb_remove_table_sync_{one,rcu}() after flushing the
> > > TLB (tlb->freed_tables or tlb->unshared_tables).
> > >
> > > On architectures where the TLB flush already sends IPIs to all target CPUs,
> > > the subsequent sync IPI broadcast is redundant. This is not only costly on
> > > large systems where it disrupts all CPUs even for single-process page table
> > > operations, but has also been reported to hurt RT workloads[1].
> > >
> > > This series introduces tlb_table_flush_implies_ipi_broadcast() to check if
> > > the prior TLB flush already provided the necessary synchronization. When
> > > true, the sync calls can early-return.
> > >
> > > A few cases rely on this synchronization:
> > >
> > > 1) hugetlb PMD unshare[2]: The problem is not the freeing but the reuse
> > > of the PMD table for other purposes in the last remaining user after
> > > unsharing.
> > >
> > > 2) khugepaged collapse[3]: Ensure no concurrent GUP-fast before collapsing
> > > and (possibly) freeing the page table / re-depositing it.
> >
> > Sashiko questions:
> > https://sashiko.dev/#/patchset/20260424062528.71951-1-lance.yang@linux.dev
> >
> > (I never know if my Sashiko emails are welcome/useful. Maybe Sashiko
> > said the same stuff about v9 and it's all wrong. But better safe than
> > sorry!)
>
> These emails are helpful; but, I do not believe you should have to
> manually follow up with a link to every new patch series.
>
> Perhaps Sashiko could automatically send a summary email in response to
> the cover letter, or provide a link once the reviews are complete. For
> the kexec ML, we opted-in with Roman to receive automated emails from
> sashiko.

Yep. I'd be OK with an automatic reply-to-all. Maybe some won't like
that.

An alternative I've discussed with Roman is an automated
reply-to-author with a cc to a dedicated list (we could use mm-commits
for now).

Preferences?

(I'd suggest automated-reply-to-all, see who complains, then figure out
why they can't figure out mail filtering ;))