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

[Mathieu Desnoyers]

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!

Thanks,

Mathieu

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