Message-Id: <20180814170618.GA24813@linux.vnet.ibm.com>
Date:   Tue, 14 Aug 2018 10:06:18 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     joel@...lfernandes.org, mathieu.desnoyers@...icios.com,
        peterz@...radead.org, tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] Make call_srcu() available during very early boot

On Tue, Aug 14, 2018 at 12:49:45PM -0400, Steven Rostedt wrote:
> On Tue, 14 Aug 2018 09:24:48 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > Event tracing is moving to SRCU in order to take advantage of the fact
> > that SRCU may be safely used from idle and even offline CPUs.  However,
> > event tracing can invoke call_srcu() very early in the boot process,
> > even before workqueue_init_early() is invoked (let alone rcu_init()).
> > Therefore, call_srcu()'s attempts to queue work fail miserably.
> > 
> > This commit therefore detects this situation, and refrains from attempting
> > to queue work before rcu_init() time, but does everything else that it
> > would have done, and in addition, adds the srcu_struct to a global list.
> > The rcu_init() function now invokes a new srcu_init() function, which
> > is empty if CONFIG_SRCU=n.  Otherwise, srcu_init() queues work for
> > each srcu_struct on the list.  This all happens early enough in boot
> > that there is but a single CPU with interrupts disabled, which allows
> > synchronization to be dispensed with.
> > 
> > Of course, the queued work won't actually be invoked until after
> > workqueue_init() is invoked, which happens shortly after the scheduler
> > is up and running.  This means that although call_srcu() may be invoked
> > any time after per-CPU variables have been set up, there is still a very
> > narrow window when synchronize_srcu() won't work, and this window
> > extends from the time that the scheduler starts until the time that
> > workqueue_init() returns.  This can be fixed in a manner similar to
> > the fix for synchronize_rcu_expedited() and friends, but until someone
> > actually needs to use synchronize_srcu() during this window, this fix
> > is added churn for no benefit.
> > 
> > Finally, note that Tree SRCU's new srcu_init() function invokes
> > queue_work() rather than the queue_delayed_work() function that is invoked
> > post-boot.  The reason is that queue_delayed_work() will (as you would
> > expect) post a timer, and timers have not yet been initialized.  So use
> > of queue_delayed_work() avoids the complaints about use of uninitialized
> 
> You mean "So use of queue_work() avoids .." ?

Indeed I do!  Fixed.

> > spinlocks that would otherwise result.  Besides, delay is in any case
> > provide by the aforementioned fact that the queued work won't actually
> > be invoked until after the scheduler is up and running.
> > 
> > Requested-by: Steven Rostedt <rostedt@...dmis.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index f41d2fb09f87..2b5c0822e683 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -36,6 +36,7 @@ struct srcu_struct {
> >  	struct rcu_head *srcu_cb_head;	/* Pending callbacks: Head. */
> >  	struct rcu_head **srcu_cb_tail;	/* Pending callbacks: Tail. */
> >  	struct work_struct srcu_work;	/* For driving grace periods. */
> > +	struct list_head srcu_boot_entry; /* Early-boot callbacks. */
> 
> I really don't like increasing the size of a structure for a field that
> is hardly ever used.
> 
> Is there a way we could make a union, or reuse one of the other fields,
> as we know that synchronize_srcu() can't be used yet (and if it is,
> either warn, or just make it a nop). And when we call srcu_init() and
> remove the srcu_struct from the list, we can then initialize whatever
> we used as the temporary boot up list field.

I will take a look.  If nothing else, I could union it with the
struct work_struct, since it cannot be used that early anyway.  ;-)

Or I could just use the work_struct that is already inside the struct
work_struct.  Tejun, would you be OK with that?

For whatever it is worth, synchronize_srcu() is perfectly legal way
early in boot, at least as early as call_srcu().  The reason is that
until the scheduler starts, synchronize_srcu() is a no-op.

> srcu_init is called when we are still running only one CPU correct?

Yes, single CPU interrupts disabled.

> >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >  	struct lockdep_map dep_map;
> >  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> > @@ -48,6 +49,7 @@ void srcu_drive_gp(struct work_struct *wp);
> >  	.srcu_wq = __SWAIT_QUEUE_HEAD_INITIALIZER(name.srcu_wq),	\
> >  	.srcu_cb_tail = &name.srcu_cb_head,				\
> >  	.srcu_work = __WORK_INITIALIZER(name.srcu_work, srcu_drive_gp),	\
> > +	.srcu_boot_entry = LIST_HEAD_INIT(name.srcu_boot_entry),	\
> >  	__SRCU_DEP_MAP_INIT(name)					\
> >  }
> >  
> > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> > index 745d4ca4dd50..86ad97111315 100644
> > --- a/include/linux/srcutree.h
> > +++ b/include/linux/srcutree.h
> > @@ -94,6 +94,7 @@ struct srcu_struct {
> >  						/*  callback for the barrier */
> >  						/*  operation. */
> >  	struct delayed_work work;
> > +	struct list_head srcu_boot_entry;	/* Early-boot callbacks. */
> >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >  	struct lockdep_map dep_map;
> >  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> > @@ -105,12 +106,13 @@ struct srcu_struct {
> >  #define SRCU_STATE_SCAN2	2
> >  
> >  #define __SRCU_STRUCT_INIT(name, pcpu_name)				\
> > -	{								\
> > -		.sda = &pcpu_name,					\
> > -		.lock = __SPIN_LOCK_UNLOCKED(name.lock),		\
> > -		.srcu_gp_seq_needed = 0 - 1,				\
> > -		__SRCU_DEP_MAP_INIT(name)				\
> > -	}
> > +{									\
> > +	.sda = &pcpu_name,						\
> > +	.lock = __SPIN_LOCK_UNLOCKED(name.lock),			\
> > +	.srcu_gp_seq_needed = 0 - 1,					\
> 
> Interesting initialization of -1. This was there before, but still
> interesting none the less.

If I recall correctly, this subterfuge suppresses compiler complaints
about initializing an unsigned long with a negative number.  :-/

							Thanx, Paul

> > +	.srcu_boot_entry = LIST_HEAD_INIT(name.srcu_boot_entry),	\
> > +	__SRCU_DEP_MAP_INIT(name)					\
> > +}
> >  
> >
> 
> -- Steve
> 

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