Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table

[Lance Yang <lance.yang@linux.dev>]

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On Mon, 2 Feb 2026 10:54:14 +0100, Peter Zijlstra wrote:
> On Mon, Feb 02, 2026 at 03:45:54PM +0800, Lance Yang wrote:
> > When freeing or unsharing page tables we send an IPI to synchronize with
> > concurrent lockless page table walkers (e.g. GUP-fast). Today we broadcast
> > that IPI to all CPUs, which is costly on large machines and hurts RT
> > workloads[1].
> > 
> > This series makes those IPIs targeted. We track which CPUs are currently
> > doing a lockless page table walk for a given mm (per-CPU
> > active_lockless_pt_walk_mm). When we need to sync, we only IPI those CPUs.
> > GUP-fast and perf_get_page_size() set/clear the tracker around their walk;
> > tlb_remove_table_sync_mm() uses it and replaces the previous broadcast in
> > the free/unshare paths.
> 
> I'm confused. This only happens when !PT_RECLAIM, because if PT_RECLAIM
> __tlb_remove_table_one() actually uses RCU.
> 
> So why are you making things more expensive for no reason?

You're right that when CONFIG_PT_RECLAIM is set, __tlb_remove_table_one()
uses call_rcu() and we never call any sync there — this series doesn't
touch that path.

In the !PT_RECLAIM table-free path (same __tlb_remove_table_one() branch
that calls tlb_remove_table_sync_mm(tlb->mm) before __tlb_remove_table),
we're not adding any new sync; we're replacing the existing broadcast IPI
(tlb_remove_table_sync_one()) with targeted IPIs (tlb_remove_table_sync_mm()).

One thing I just realized: when CONFIG_MMU_GATHER_RCU_TABLE_FREE is not
set, the sync path isn't used at all (tlb_remove_table_sync_one() and
friends aren't even compiled), so we don't need the tracker in that config.

Thanks for raising this!
Lance