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

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

On 2026/2/5 21:25, David Hildenbrand (Arm) wrote:
> On 2/2/26 16:52, Lance Yang wrote:
>>
>>
>> On 2026/2/2 23:09, Peter Zijlstra wrote:
>>> On Mon, Feb 02, 2026 at 10:37:39PM +0800, Lance Yang wrote:
>>>>
>>>>
>>>>
>>>> PT_RECLAIM=y does have IPI for unshare/collapse — those paths call
>>>> tlb_flush_unshared_tables() (for hugetlb unshare) and 
>>>> collapse_huge_page()
>>>> (in khugepaged collapse), which already send IPIs today (broadcast 
>>>> to all
>>>> CPUs via tlb_remove_table_sync_one()).
>>>>
>>>> What PT_RECLAIM=y doesn't need IPI for is table freeing (
>>>> __tlb_remove_table_one() uses call_rcu() instead). But table 
>>>> modification
>>>> (unshare, collapse) still needs IPI to synchronize with lockless 
>>>> walkers,
>>>> regardless of PT_RECLAIM.
>>>>
>>>> So PT_RECLAIM=y is not broken; it already has IPI where needed. This 
>>>> series
>>>> just makes those IPIs targeted instead of broadcast. Does that clarify?
>>>
>>> Oh bah, reading is hard. I had missed they had more table_sync_one() 
>>> calls,
>>> rather than remove_table_one().
>>>
>>> So you *can* replace table_sync_one() with rcu_sync(), that will provide
>>> the same guarantees. Its just a 'little' bit slower on the update side,
>>> but does not incur the read side cost.
>>
>> Yep, we could replace the IPI with synchronize_rcu() on the sync side:
>>
>> - Currently: TLB flush → send IPI → wait for walkers to finish
>> - With synchronize_rcu(): TLB flush → synchronize_rcu() -> waits for 
>> grace period
>>
>> Lockless walkers (e.g. GUP-fast) use local_irq_disable(); 
>> synchronize_rcu() also
>> waits for regions with preemption/interrupts disabled, so it should 
>> work, IIUC.
>>
>> And then, the trade-off would be:
>> - Read side: zero cost (no per-CPU tracking)
>> - Write side: wait for RCU grace period (potentially slower)
>>
>> For collapse/unshare, that write-side latency might be acceptable :)
>>
>> @David, what do you think?
> 
> Given that we just fixed the write-side latency from breaking Oracle's 
> databases completely, we have to be a bit careful here :)

Yep, agreed.

> 
> The thing is: on many x86 configs we don't need *any* TLB flushed or RCU 
> syncs.

Right. Looks like that is low-hanging fruit. I'll send that out 
separately :)

> 
> So "how much slower" are we talking about, especially on bigger/loaded 
> systems?

Unfortunately the numbers are pretry bad. On an x86-64 64-core system
under high load, each synchronize_rcu() is about *22.9* ms on average ...

So for now, neither approach looks good: tracking on the read side adss
cost to GUP-fast, and syncing on the write side e.g. synchronize_rcu()
is too slow on large systems.