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Date: Wed, 4 Jan 2023 21:11:46 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Zqiang <qiang1.zhang@...el.com>
Cc: frederic@...nel.org, quic_neeraju@...cinc.com,
joel@...lfernandes.org, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rcu: Rework tick dependency setting into
rcu_exp_handler()
On Thu, Jan 05, 2023 at 11:40:00AM +0800, Zqiang wrote:
> Currently, when first find out the expedited grace period is not end
> and timeout occurred, we set tick dependency for CPUs that have not
> yet reported the quiescent state in the rcu_node structure's->expmask
> but need to eliminate races between set and clear tick dependency, setting
> CPU tick dependency need to hold rcu_node structure's->lock.
>
> This commit move tick dependency setting into rcu_exp_handler(), set tick
> dependency when fail to report a quiescent state and clear tick dependency
> in rcu_report_exp_rdp(). [from Frederic Weisbecker suggestion]
>
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
First, a big "thank you" to you an Frederic for investigating this
approach!
So which is better, this patch or the one that I already have queued?
The advantage of the patch of yours that I already have queued is that
CPUs that respond in some other way within a millisecond do not get hit
with an additional scheduling-clock interrupt.
On the other hand, if the CPU goes through a quiescent state before the
next scheduling-clock interrupt arrives, rcu_report_exp_cpu_mult() will
shut down the tick before it happens. Plus if the CPU waits a full tick
before reaching a quiescent state, then the tick_dep_set_cpu() called from
synchronize_rcu_expedited_wait() is going to send along an IPI anyway.
Except that invoking tick_dep_set_cpu() on the CPU itself will also
do an IPI from tick_dep_set_cpu() because of IRQ_WORK_INIT_HARD(), right?
Which means that the patch below gets us an extra self-IPI, right?
Or am I misreading the code?
In addition, doesn't tick_dep_clear_cpu() just clear a bit? Won't that
mean that the next scheduling-clock interrupt will happen, just that the
one after that won't? (Give or take kernel-to-user or kernel-to-idle
transitions that might happen in the meantime.)
Am I missing something in my attempts to model the costs?
Thanx, Paul
> ---
> kernel/rcu/tree_exp.h | 43 ++++++++++++++++++++-----------------------
> 1 file changed, 20 insertions(+), 23 deletions(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 7cc4856da081..f1e947675727 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -586,39 +586,16 @@ static bool synchronize_rcu_expedited_wait_once(long tlimit)
> static void synchronize_rcu_expedited_wait(void)
> {
> int cpu;
> - unsigned long j;
> unsigned long jiffies_stall;
> unsigned long jiffies_start;
> unsigned long mask;
> int ndetected;
> - struct rcu_data *rdp;
> struct rcu_node *rnp;
> struct rcu_node *rnp_root = rcu_get_root();
> - unsigned long flags;
>
> trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
> jiffies_stall = rcu_exp_jiffies_till_stall_check();
> jiffies_start = jiffies;
> - if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
> - if (synchronize_rcu_expedited_wait_once(1))
> - return;
> - rcu_for_each_leaf_node(rnp) {
> - raw_spin_lock_irqsave_rcu_node(rnp, flags);
> - mask = READ_ONCE(rnp->expmask);
> - for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
> - rdp = per_cpu_ptr(&rcu_data, cpu);
> - if (rdp->rcu_forced_tick_exp)
> - continue;
> - rdp->rcu_forced_tick_exp = true;
> - if (cpu_online(cpu))
> - tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
> - }
> - raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> - }
> - j = READ_ONCE(jiffies_till_first_fqs);
> - if (synchronize_rcu_expedited_wait_once(j + HZ))
> - return;
> - }
>
> for (;;) {
> if (synchronize_rcu_expedited_wait_once(jiffies_stall))
> @@ -729,6 +706,24 @@ static void rcu_exp_sel_wait_wake(unsigned long s)
>
> #ifdef CONFIG_PREEMPT_RCU
>
> +static void rcu_exp_set_tickdep(struct rcu_data *rdp)
> +{
> + int cpu = rdp->cpu;
> +
> + if (tick_nohz_full_cpu(cpu) && rcu_inkernel_boot_has_ended() &&
> + cpu_online(cpu)) {
> + /*
> + * The rcu_exp_handler() can be invoked from stop machine,
> + * at this time CPU has disabled all interrupts and offline,
> + * in which case, we don't need requeue IPI or irq work.
> + * which is not a problem since rcu_report_dead() does the
> + * QS report.
> + */
> + rdp->rcu_forced_tick_exp = true;
> + tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
> + }
> +}
> +
> /*
> * Remote handler for smp_call_function_single(). If there is an
> * RCU read-side critical section in effect, request that the
> @@ -757,6 +752,7 @@ static void rcu_exp_handler(void *unused)
> WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
> set_tsk_need_resched(t);
> set_preempt_need_resched();
> + rcu_exp_set_tickdep(rdp);
> }
> return;
> }
> @@ -778,6 +774,7 @@ static void rcu_exp_handler(void *unused)
> if (rnp->expmask & rdp->grpmask) {
> WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
> t->rcu_read_unlock_special.b.exp_hint = true;
> + rcu_exp_set_tickdep(rdp);
> }
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> return;
> --
> 2.25.1
>
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