On Mon, Mar 23, 2026 at 01:53:17PM -0700, Andrew Morton wrote: >On Mon, 9 Mar 2026 10:07:09 +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. >> >> ... >> >> 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.
Yes, a v8 is on the way. >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! That's a fair point. I should have included numbers from the start. 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. The larger the system, the more costly redundant broadcast IPIs become. Thanks, Lance
