Re: [PATCH] srcu: switch work func to allow concurrent gp

[Pingfan Liu <kernelfans@gmail.com>]

On Wed, Nov 30, 2022 at 08:53:43AM -0800, Boqun Feng wrote:
> On Wed, Nov 30, 2022 at 04:39:02PM +0800, Pingfan Liu wrote:
> > ssp->srcu_cb_mutex is introduced to allow the other srcu state machine
> > to advance as soon as possible. But according to the implement of
> > workqueue, the same work_struct is serialized and can not run
> > concurrently in fact.
> > 
> > Quoting from Documentation/core-api/workqueue.rst
> > "
> > Non-reentrance Conditions
> > =========================
> > 
> > Workqueue guarantees that a work item cannot be re-entrant if the following
> > conditions hold after a work item gets queued:
> > 
> >         1. The work function hasn't been changed.
> >         2. No one queues the work item to another workqueue.
> >         3. The work item hasn't been reinitiated.
> > "
> > 
> > To allow the concurrence to some extent, it can be achieved by changing
> > the work function to break the conditions. As a result, when
> > srcu_gp_end() releases srcu_gp_mutex, a new state machine can begin.
> > 
> > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > Cc: "Paul E. McKenney" <paulmck@kernel.org>
> > Cc: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Josh Triplett <josh@joshtriplett.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: "Zhang, Qiang1" <qiang1.zhang@intel.com>
> > To: rcu@vger.kernel.org
> > ---
> >  kernel/rcu/srcutree.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 1c304fec89c0..56dd9bb2c8b8 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -75,6 +75,7 @@ static bool __read_mostly srcu_init_done;
> >  static void srcu_invoke_callbacks(struct work_struct *work);
> >  static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay);
> >  static void process_srcu(struct work_struct *work);
> > +static void process_srcu_wrap(struct work_struct *work);
> >  static void srcu_delay_timer(struct timer_list *t);
> >  
> >  /* Wrappers for lock acquisition and release, see raw_spin_lock_rcu_node(). */
> > @@ -763,6 +764,11 @@ static void srcu_gp_end(struct srcu_struct *ssp)
> >  		cbdelay = 0;
> >  
> >  	WRITE_ONCE(ssp->srcu_last_gp_end, ktime_get_mono_fast_ns());
> > +	/* Change work func so work can be concurrent */
> > +	if (ssp->work.work.func == process_srcu_wrap)
> > +		ssp->work.work.func = process_srcu;
> > +	else
> > +		ssp->work.work.func = process_srcu_wrap;
> 
> This looks really hacky ;-) It would be good that workqueue has an API
> to allow "resetting" a work.
> 

Indeed, it is hacky and it can be done by using alternative work_struct
so that from the workqueue API, it is intact.

> Do you have any number of the potential performance improvement?
> 

I will try to bring out one. But the result may heavily depend on the
test case.


Thanks,

	Pingfan

> Regards,
> Boqun
> 
> >  	rcu_seq_end(&ssp->srcu_gp_seq);
> >  	gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
> >  	if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, gpseq))
> > @@ -1637,6 +1643,19 @@ static void process_srcu(struct work_struct *work)
> >  	srcu_reschedule(ssp, curdelay);
> >  }
> >  
> > +/*
> > + * The ssp->work is expected to be concurrent to some extent, but the current
> > + * workqueue does not support the concurrence on the same work. (Refer to the
> > + * section "Non-reentrance Conditions" in the file workqueue.rst)
> > + * Resolving it by changing the work func.
> > + *
> > + * Prevent compilering from optimizing out it.
> > + */
> > +static __used void process_srcu_wrap(struct work_struct *work)
> > +{
> > +	process_srcu(work);
> > +}
> > +
> >  void srcutorture_get_gp_data(enum rcutorture_type test_type,
> >  			     struct srcu_struct *ssp, int *flags,
> >  			     unsigned long *gp_seq)
> > -- 
> > 2.31.1
> >