Re: [RFC PATCH v4 3/4] hazptr: Implement Hazard Pointers

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

On 2026-01-08 11:34, Frederic Weisbecker wrote:
> Le Fri, Dec 19, 2025 at 09:22:19AM -0500, Mathieu Desnoyers a écrit :
>> On 2025-12-18 19:43, Boqun Feng wrote:
>>> On Thu, Dec 18, 2025 at 12:35:18PM -0500, Mathieu Desnoyers wrote:
>>> [...]
>>>>> Could you utilize this[1] to see a
>>>>> comparison of the reader-side performance against RCU/SRCU?
>>>>
>>>> Good point ! Let's see.
>>>>
>>>> On a AMD 2x EPYC 9654 96-Core Processor with 192 cores,
>>>> hyperthreading disabled,
>>>> CONFIG_PREEMPT=y,
>>>> CONFIG_PREEMPT_RCU=y,
>>>> CONFIG_PREEMPT_HAZPTR=y.
>>>>
>>>> scale_type                 ns
>>>> -----------------------
>>>> hazptr-smp-mb             13.1   <- this implementation
>>>> hazptr-barrier            11.5   <- replace smp_mb() on acquire with barrier(), requires IPIs on synchronize.
>>>> hazptr-smp-mb-hlist       12.7   <- replace per-task hp context and per-cpu overflow lists by hlist.
>>>> rcu                       17.0
>>>
>>> Hmm.. now looking back, how is it possible that hazptr is faster than
>>> RCU on the reader-side? Because a grace period was happening and
>>> triggered rcu_read_unlock_special()? This is actualy more interesting.
>> So I could be entirely misreading the code, but, we have:
>>
>> rcu_flavor_sched_clock_irq():
>> [...]
>>          /* If GP is oldish, ask for help from rcu_read_unlock_special(). */
>>          if (rcu_preempt_depth() > 0 &&
>>              __this_cpu_read(rcu_data.core_needs_qs) &&
>>              __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
>>              !t->rcu_read_unlock_special.b.need_qs &&
>>              time_after(jiffies, rcu_state.gp_start + HZ))
>>                  t->rcu_read_unlock_special.b.need_qs = true;
>>
>> which means we set need_qs = true as a result from observing
>> cpu_no_qs.b.norm == true.
>>
>> This is sufficient to trigger calls (plural) to rcu_read_unlock_special()
>> from __rcu_read_unlock.
>>
>> But then if we look at rcu_preempt_deferred_qs_irqrestore()
>> which we would expect to clear the rcu_read_unlock_special.b.need_qs
>> state, we have this:
>>
>>          special = t->rcu_read_unlock_special;
>>          if (!special.s && !rdp->cpu_no_qs.b.exp) {
>>                  local_irq_restore(flags);
>>                  return;
>>          }
>>          t->rcu_read_unlock_special.s = 0;
>>
>> which skips over clearing the state unless there is an expedited
>> grace period required.
>>
>> So unless I'm missing something, we should _also_ clear that state
>> when it's invoked after rcu_flavor_sched_clock_irq, so the next
>> __rcu_read_unlock won't all call into rcu_read_unlock_special().
>>
>> I'm adding a big warning about sleep deprivation and possibly
>> misunderstanding the whole thing. What am I missing ?
> 
> As far as I can tell, this skips clearing the state if the state is
> already cleared. Or am I even more sleep deprived than you? :o)

No, you are right. The (!x && !y) pattern confused me, but the
code is correct. Good thing I've put a warning about sleep
deprivation. ;-)

Sorry for the noise.

Thanks,

Mathieu

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