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

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

On 2026/2/5 23:05, David Hildenbrand (Arm) wrote:
> On 2/5/26 16:01, Lance Yang wrote:
>>
>>
>> On 2026/2/5 21:25, David Hildenbrand (Arm) wrote:
>>> On 2/2/26 16:52, Lance Yang wrote:
>>>>
>>>>
>>>>
>>>> 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.
> 
> GUP-fast is 3%, right? Any way we can reduce that to 1% and call it 
> noise? :)

Yes, GUP-fast is ~3%. I'll keep trying to do that, but first getting
the low-hanging fruit done :)