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

[Boqun Feng]

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 ;-)

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

[1]: https://lore.kernel.org/lkml/20250414060055.341516-1-boqun.feng@xxxxxxxxx/

Regards,
Boqun

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