linux-kernel - Re: [PATCH 1/1] Reduce synchronize

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Date:   Mon, 27 Mar 2023 19:47:48 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Zhang, Qiang1" <qiang1.zhang@...el.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        RCU <rcu@...r.kernel.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH 1/1] Reduce synchronize_rcu() waiting time

On Mon, Mar 27, 2023 at 11:21:23AM +0000, Zhang, Qiang1 wrote:
> > > From: Uladzislau Rezki (Sony) <urezki@...il.com>
> > > Sent: Tuesday, March 21, 2023 6:28 PM
> > > [...]
> > > Subject: [PATCH 1/1] Reduce synchronize_rcu() waiting time
> > > 
> > > A call to a synchronize_rcu() can be expensive from time point of view.
> > > Different workloads can be affected by this especially the ones which use this
> > > API in its time critical sections.
> > > 
> > 
> > This is interesting and meaningful research. ;-)
> > 
> > > For example in case of NOCB scenario the wakeme_after_rcu() callback
> > > invocation depends on where in a nocb-list it is located. Below is an example
> > > when it was the last out of ~3600 callbacks:
> >
> 
> 
> 
> Can it be implemented separately as follows?  it seems that the code is simpler
> (only personal opinion)  😊.
> 
> But I didn't test whether this reduce synchronize_rcu() waiting time
> 
> +static void rcu_poll_wait_gp(struct rcu_tasks *rtp)
> +{
> +       unsigned long gp_snap;
> +
> +       gp_snap = start_poll_synchronize_rcu();
> +       while (!poll_state_synchronize_rcu(gp_snap))
> +               schedule_timeout_idle(1);
> +}
> +
> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func);
> +DEFINE_RCU_TASKS(rcu_poll, rcu_poll_wait_gp, call_rcu_poll,
> +                 "RCU Poll");
> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func)
> +{
> +       call_rcu_tasks_generic(rhp, func, &rcu_poll);
> +}
> +EXPORT_SYMBOL_GPL(call_rcu_poll);
> +
> +void synchronize_rcu_poll(void)
> +{
> +       synchronize_rcu_tasks_generic(&rcu_poll);
> +}
> +EXPORT_SYMBOL_GPL(synchronize_rcu_poll);
> +
> +static int __init rcu_spawn_poll_kthread(void)
> +{
> +       cblist_init_generic(&rcu_poll);
> +       rcu_poll.gp_sleep = HZ / 10;
> +       rcu_spawn_tasks_kthread_generic(&rcu_poll);
> +       return 0;
> +}
> 
Uh.. I am working on v2 of original patch where i need to add a Kconfig
parameter. You are inventing a new API :)

--
Uladzislau Rezki