On Tue, 24 Mar 2026 16:52:36 +0800 Lance Yang <[email protected]> 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?
