On Wed, Nov 30, 2022 at 12:39:53PM -0500, Joel Fernandes wrote:
>
>
> > On Nov 30, 2022, at 11:55 AM, Boqun Feng <boqun.feng@xxxxxxxxx> 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.
> >
> > Do you have any number of the potential performance improvement?
>
>
> Agreed. Another question - have you verified the workqueue behavior after this change (say using tracing), regardless of what the workqueue documentation says?
>
I will try to answer this question after figuring out a test case.
Thanks,
Pingfan
> Thanks,
>
> - Joel
>
>
>
> >
> > 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
> >>
