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