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@xxxxxxxxx> > > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx> > > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx> > > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx> > > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > > Cc: "Zhang, Qiang1" <qiang1.zhang@xxxxxxxxx> > > To: rcu@xxxxxxxxxxxxxxx > > --- > > 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 > >