linux-kernel - Re: [PATCH 4/7] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT

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Message-ID: <87o73t9jw4.fsf@oracle.com>
Date: Wed, 09 Oct 2024 12:05:47 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: 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, frederic@...nel.org,
        efault@....de
Subject: Re: [PATCH 4/7] rcu: handle quiescent states for PREEMPT_RCU=n,
 PREEMPT_COUNT=y



Ankur Arora <ankur.a.arora@...cle.com> writes:

> With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
> states for read-side critical sections via rcu_all_qs().
> One reason why this was needed, was lacking preempt-count, the tick
> handler has no way of knowing whether it is executing in a read-side
> critical section or not.
>
> With PREEMPT_LAZY=y, there can be configurations with PREEMPT_COUNT=y,
> PREEMPT_RCU=n, where cond_resched() is a stub that does not provide
> quiescent states via rcu_all_qs().
>
> So, use the availability of preempt_count() to report quiescent states
> in rcu_flavor_sched_clock_irq().

A note about the inverse of this case, where we might have long running
loops which only temporarily enable preemption and thus would be
unlikely to align themselves with the tick: in prior discussions [1]
Paul had pointed the need for providing for forcing a context switch
in such a scenario.

I had a patch which did that, but I think it is unnecessary since this
clause in rcu_sched_clock_irq() should already handle it.

   void rcu_sched_clock_irq(int user) {
        ...
        /* The load-acquire pairs with the store-release setting to true. */
        if (smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {
                /* Idle and userspace execution already are quiescent states. */
                if (!rcu_is_cpu_rrupt_from_idle() && !user) {
                        set_tsk_need_resched(current);
                        set_preempt_need_resched();
                }
                __this_cpu_write(rcu_data.rcu_urgent_qs, false);
        }

Paul?

--
ankur