linux-kernel - Re: [PATCH] srcu: Add detection of boot CPU srcu_data structure's->srcu

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Message-ID: <Y4RyK4D25IFwWK3P@piliu.users.ipa.redhat.com>
Date:   Mon, 28 Nov 2022 16:32:43 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Zqiang <qiang1.zhang@...el.com>, paulmck@...nel.org,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] srcu: Add detection of boot CPU srcu_data
 structure's->srcu_cblist

On Thu, Nov 17, 2022 at 03:20:25PM +0100, Frederic Weisbecker wrote:
> On Thu, Nov 17, 2022 at 07:20:50PM +0800, Zqiang wrote:
> > Before SRCU_SIZE_WAIT_CALL srcu_size_state is reached, the srcu
> > callback only insert to boot-CPU srcu_data structure's->srcu_cblist
> > and other CPU srcu_data structure's->srcu_cblist is always empty. so
> > before SRCU_SIZE_WAIT_CALL is reached, need to correctly check boot-CPU
> > pend cbs in srcu_might_be_idle().
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> > ---
> >  kernel/rcu/srcutree.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 6af031200580..6d9af9901765 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -1098,8 +1098,11 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
> >  	unsigned long tlast;
> >  
> >  	check_init_srcu_struct(ssp);
> > -	/* If the local srcu_data structure has callbacks, not idle.  */
> > -	sdp = raw_cpu_ptr(ssp->sda);
> > +	/* If the boot CPU or local srcu_data structure has callbacks, not idle.  */
> > +	if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_CALL)
> > +		sdp = per_cpu_ptr(ssp->sda, get_boot_cpu_id());
> > +	else
> > +		sdp = raw_cpu_ptr(ssp->sda);
> 
> While at it if someone is interested in documenting/commenting on the meaning of
> all these SRCU_SIZE_* things, it would be much appreciated!
> 

Due to a conflict understanding to the code, once hesitate to jump in.
But anyway, just bold to move ahead.

I have send a series "[PATCH 0/3] srcu: shrink the num of
srcu_size_state", which opens the discussion. (Have Cced you and Zqiang)

Please have a look.

Thanks,

	Pingfan

> Thanks.
> 
> >  	spin_lock_irqsave_rcu_node(sdp, flags);
> >  	if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
> >  		spin_unlock_irqrestore_rcu_node(sdp, flags);
> > -- 
> > 2.25.1
> >