Re: [RFC PATCH v4 4/4] hazptr: Migrate per-CPU slots to backup slot on context switch

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-02-23 11:54, Boqun Feng wrote:
> Hi Mathieu,
> 
> On Thu, Dec 18, 2025 at 07:21:28PM -0500, Mathieu Desnoyers wrote:
>> On 2025-12-18 17:16, Boqun Feng wrote:
>> [...]
>>>
>>> I would suggest we make CONFIG_PREEMPT_HAZPTR always enabled hence no
>>> need for a config, do we have the measurement of the additional cost?
>>
>> Removing the PREEMPT_HAZPTR brings read-side performance
>> from 13.1 down to 5.1 ns. So there is a surprising amount
>> of work that goes into list_add/list_del.
>>
>> I just noticed that I was running a kernel with CONFIG_LIST_HARDENED=y.
>> Reruning refscale for PREEMPT_HAZPTR=y with list hardening disabled
>> goes from 13.1 ns to 12.4ns, so not a huge win.
>>
>> I did not notice much difference in terms of scheduler
>> performance with a quick hackbench run, but I cannot
>> claim it is an extensive benchmark in any way.
>>
>>>
>>> I think you need to add interrupt disabling for chain/unchain because of
>>> the potential readers in interrupt and then you can avoid the preempt
>>> disabling in hazptr_release() I think. Let's aim for supporting readers
>>> in interrupt handler, because at least lockdep needs that.
>>
>> OK, I'll look into it!
>>
> 
> Gentle ping on this. I want to make some forward progress on this ;-)

Thanks for the ping, I was focused on other things. I do have an updated
branch to post.

> 
> I suggest we make PREEMPT_HAZPTR enabled by default and support readers
> in interrupt handler.

Yes, that's what I have in my updated branch.

> The rest missing part is an async thread, we could
> utilize some code in my previous patchset [1]. Let me know whether you
> think you have the cycle for this, otherwise I could add this into your
> series ;-) Thanks!

If that's OK with you, I could post my updated hazptr series for
feedback, and leave the async thread adaptation to you.

Thanks,

Mathieu

> 
> [1]: https://lore.kernel.org/lkml/20250414060055.341516-1-boqun.feng@gmail.com/
> 
> Regards,
> Boqun
> 
>> Thanks,
>>
>> Mathieu
>>
>> -- 
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> https://www.efficios.com


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com