Re: [PATCH v8 0/2] skip redundant sync IPIs when TLB flush sent them

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

On Tue, 24 Mar 2026 16:52:36 +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.
> 
> On a 64-core Intel x86 server, the CAL interrupt count in
> /proc/interrupts dropped from 646,316 to 785 when collapsing a 20 GiB
> range with this series applied.

Well that's nice.

Which other architectures could utilize this?

> David Hildenbrand did the initial implementation. I built on his work and
> relied on off-list discussions to push it further - thanks a lot David!
>
> ...
>
>  arch/x86/include/asm/tlb.h      | 18 +++++++++++++++++-
>  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, 67 insertions(+), 1 deletion(-)

Can the x86 maintainers please review these changes?