linux-kernel - RE: [PATCH] rcu: Only boost rcu reader tasks with lower priority than boost kthreads

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Message-ID: <PH0PR11MB5880569E024586505CD18630DA0A9@PH0PR11MB5880.namprd11.prod.outlook.com>
Date:   Wed, 9 Mar 2022 03:04:19 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     Uladzislau Rezki <urezki@...il.com>
CC:     "paulmck@...nel.org" <paulmck@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        "frederic@...nel.org" <frederic@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>
Subject: RE: [PATCH] rcu: Only boost rcu reader tasks with lower priority than
 boost kthreads


> On Mon, Mar 07, 2022 at 02:03:17AM +0000, Zhang, Qiang1 wrote:
> > On 3/4/2022 2:56 PM, Zqiang wrote:
> > > When RCU_BOOST is enabled, the boost kthreads will boosting 
> > > readers who are blocking a given grace period, if the current 
> > > reader tasks have a higher priority than boost kthreads(the boost 
> > > kthreads priority not always 1, if the kthread_prio is set), 
> > > boosting is useless, skip current task and select next task to 
> > > boosting, reduce the time for a given grace period.
> > > 
> > > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> 
> Adding to CC to get more eyes on this.  I am not necessarily opposed 
> to it, but I don't do that much RT work myself these days.
> 
> 							Thanx, Paul
> 
> > > ---
> > >   kernel/rcu/tree_plugin.h | 10 +++++++++-
> > >   1 file changed, 9 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h 
> > > index c3d212bc5338..d35b6da66bbd 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -12,6 +12,7 @@
> > >    */
> > >   
> > >   #include "../locking/rtmutex_common.h"
> > > +#include <linux/sched/deadline.h>
> > >   
> > >   static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
> > >   {
> > > @@ -1065,13 +1066,20 @@ static int rcu_boost(struct rcu_node *rnp)
> > >   	 * section.
> > >   	 */
> > >   	t = container_of(tb, struct task_struct, rcu_node_entry);
> > > +	if (!rnp->exp_tasks && (dl_task(t) || t->prio <= current->prio)) {
> > > +		tb = rcu_next_node_entry(t, rnp);
> > > +		WRITE_ONCE(rnp->boost_tasks, tb);
> > > +		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> > > +		goto end;
> > > +	}
> > > +
>>>>Why do you bypass the expedited grace period and boost any tasks anyway?
>>>>Same way the expedited gp can be blocked by higher prior tasks SCHED_DEADLINE or SCHED_FIFO.

Thanks advice, it really ignores expedited grace period, may be
should generate 'boost_exp_tasks' pointer to point 'rnp->exp_tasks', do the same thing
as normal grace period.

Thanks
Zqiang

>>>>
>>>>Thanks!
>>>>
>>>>--
>>>>Vlad Rezki