linux-kernel - Re: [PATCH v2 3/6] rcu: limit PREEMPT

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Message-ID: <874j3qfyca.fsf@oracle.com>
Date: Thu, 28 Nov 2024 20:39:01 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        peterz@...radead.org, tglx@...utronix.de, paulmck@...nel.org,
        mingo@...nel.org, bigeasy@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com, efault@....de, sshegde@...ux.ibm.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v2 3/6] rcu: limit PREEMPT_RCU configurations


Frederic Weisbecker <frederic@...nel.org> writes:

> Le Tue, Nov 26, 2024 at 10:19:05PM -0800, Ankur Arora a écrit :
>>
>> Ankur Arora <ankur.a.arora@...cle.com> writes:
>>
>> > Frederic Weisbecker <frederic@...nel.org> writes:
>> >
>> >> Le Mon, Nov 25, 2024 at 01:40:39PM -0800, Ankur Arora a écrit :
>> >>>
>> >>> Frederic Weisbecker <frederic@...nel.org> writes:
>> >>>
>> >>> > Le Wed, Nov 06, 2024 at 12:17:55PM -0800, Ankur Arora a écrit :
>> >>> >> PREEMPT_LAZY can be enabled stand-alone or alongside PREEMPT_DYNAMIC
>> >>> >> which allows for dynamic switching of preemption models.
>> >>> >>
>> >>> >> The choice of PREEMPT_RCU or not, however, is fixed at compile time.
>> >>> >>
>> >>> >> Given that PREEMPT_RCU makes some trade-offs to optimize for latency
>> >>> >> as opposed to throughput, configurations with limited preemption
>> >>> >> might prefer the stronger forward-progress guarantees of PREEMPT_RCU=n.
>> >>> >>
>> >>> >> Accordingly, explicitly limit PREEMPT_RCU=y to the latency oriented
>> >>> >> preemption models: PREEMPT, PREEMPT_RT, and the runtime configurable
>> >>> >> model PREEMPT_DYNAMIC.
>> >>> >>
>> >>> >> This means the throughput oriented models, PREEMPT_NONE,
>> >>> >> PREEMPT_VOLUNTARY and PREEMPT_LAZY will run with PREEMPT_RCU=n.
>> >>> >>
>> >>> >> Cc: Paul E. McKenney <paulmck@...nel.org>
>> >>> >> Cc: Peter Zijlstra <peterz@...radead.org>
>> >>> >> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> >>> >> ---
>> >>> >>  kernel/rcu/Kconfig | 2 +-
>> >>> >>  1 file changed, 1 insertion(+), 1 deletion(-)
>> >>> >>
>> >>> >> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
>> >>> >> index 5a7ff5e1cdcb..9d52f87fac27 100644
>> >>> >> --- a/kernel/rcu/Kconfig
>> >>> >> +++ b/kernel/rcu/Kconfig
>> >>> >> @@ -18,7 +18,7 @@ config TREE_RCU
>> >>> >>
>> >>> >>  config PREEMPT_RCU
>> >>> >>  	bool
>> >>> >> -	default y if PREEMPTION
>> >>> >> +	default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
>> >>> >>  	select TREE_RCU
>> >>> >>  	help
>> >>> >>  	  This option selects the RCU implementation that is
>> >>> >
>> >>> > Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
>> >>> >
>> >>> > But looking at !CONFIG_PREEMPT_RCU code on tree_plugin.h, I see
>> >>> > some issues now that the code can be preemptible. Well I think
>> >>> > it has always been preemptible but PREEMPTION && !PREEMPT_RCU
>> >>> > has seldom been exerciced (or was it even possible?).
>> >>> >
>> >>> > For example rcu_read_unlock_strict() can be called with preemption
>> >>> > enabled so we need the following otherwise the rdp is unstable, the
>> >>> > norm value becomes racy (though automagically fixed in rcu_report_qs_rdp())
>> >>> > and rcu_report_qs_rdp() might warn.
>> >>> >
>> >>> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>> >>> > index 58d84c59f3dd..368f00267d4e 100644
>> >>> > --- a/include/linux/rcupdate.h
>> >>> > +++ b/include/linux/rcupdate.h
>> >>> > @@ -95,9 +95,9 @@ static inline void __rcu_read_lock(void)
>> >>> >
>> >>> >  static inline void __rcu_read_unlock(void)
>> >>> >  {
>> >>> > -	preempt_enable();
>> >>> >  	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
>> >>> >  		rcu_read_unlock_strict();
>> >>> > +	preempt_enable();
>> >>> >  }
>> >>> >
>> >>> >  static inline int rcu_preempt_depth(void)
>> >>>
>> >>> Based on the discussion on the thread, how about keeping this and
>> >>> changing the preempt_count check in rcu_read_unlock_strict() instead?
>> >>>
>> >>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> >>> index 1c7cbd145d5e..8fc67639d3a7 100644
>> >>> --- a/kernel/rcu/tree_plugin.h
>> >>> +++ b/kernel/rcu/tree_plugin.h
>> >>> @@ -831,8 +831,15 @@ dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
>> >>>  void rcu_read_unlock_strict(void)
>> >>>  {
>> >>>         struct rcu_data *rdp;
>> >>> +       int pc = ((preempt_count() & PREEMPT_MASK) >> PREEMPT_SHIFT);
>> >>
>> >> This should be in_atomic_preempt_off(), otherwise softirqs and IRQs are
>> >> spuriously accounted as quiescent states.
>> >
>> > Not sure I got that. Won't ((preempt_count() & PREEMPT_MASK) >> PREEMPT_SHIFT)
>> > give us task only preempt count?
>>
>> Oh wait. I see your point now. My check is too narrow.
>>
>> Great. This'll work:
>>
>> -       if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
>> +       if (irqs_disabled() || in_atomic_preempt_off()|| !rcu_state.gp_kthread)
>>
>> Thanks
>
> Do you plan to integrate this in a further version of your set? Or should I send
> a patch?

Sure. I can add it to v3. Okay, if I add your suggested-by?

--
ankur