Re: [PATCH v7 0/2] skip redundant sync IPIs when TLB flush sent them

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  9 Mar 2026 10:07:09 +0800 Lance Yang <lance.yang@linux.dev> wrote:

> Hi all,
> 
> 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.
> 
> Two-step plan as David suggested[4]:
> 
> Step 1 (this series): Skip redundant sync when we're 100% certain the TLB
> flush sent IPIs. INVLPGB is excluded because when supported, we cannot
> guarantee IPIs were sent, keeping it clean and simple.
> 
> Step 2 (future work): Send targeted IPIs only to CPUs actually doing
> software/lockless page table walks, benefiting all architectures.
> 
> Regarding Step 2, it obviously only applies to setups where Step 1 does not
> apply: like x86 with INVLPGB or arm64. Step 2 work is ongoing; early
> attempts showed ~3% GUP-fast overhead. Reducing the overhead requires more
> work and tuning; it will be submitted separately once ready.
> 
> ...
> 
>  arch/x86/include/asm/tlb.h      | 17 ++++++++++++++++-
>  arch/x86/include/asm/tlbflush.h |  2 ++
>  arch/x86/kernel/smpboot.c       |  1 +
>  arch/x86/mm/tlb.c               | 15 +++++++++++++++
>  include/asm-generic/tlb.h       | 17 +++++++++++++++++
>  mm/mmu_gather.c                 | 15 +++++++++++++++
>  6 files changed, 66 insertions(+), 1 deletion(-)

Kinda straddles both MM and x86.

I expect a v8 based on David's comments.

One merge path is for the x86 people to take this, noting David's acks.

The other merge path is via mm.git, if the x86 people can please
perform review.

And...  mm.git is basically full (overflowing) for this cycle and
review/test has some catching up to do.  So I'd prefer to only take the
important things.  This patchset is a performance improvement but
contains no measurements to demonstrate the benefit, so I'm not able to
determine its importance!