On Thu, Dec 01, 2022 at 09:53:36PM +0800, Pingfan Liu wrote:
> 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.
>
Sorry to update late, I was interrupted by something else.
I used two machine to test
-1. on 144 cpus hpe-dl560gen10
modprobe rcutorture torture_type=srcud fwd_progress=144 fwd_progress_holdoff=1 shutdown_secs=36000 stat_interval=60 verbose=1
# no patch
[37587.632155] srcud: End-test grace-period state: g28287244 f0x0 total-gps=28287244
#my patch
[36443.468017] srcud: End-test grace-period state: g29026056 f0x0 total-gps=29026056
-2. on 256 cpus amd-milan
modprobe rcutorture torture_type=srcud fwd_progress=256 fwd_progress_holdoff=1 shutdown_secs=36000 stat_interval=60 verbose=1
# no patch
[36093.605732] srcud: End-test grace-period state: g10850284 f0x0 total-gps=10850284
#my patch
[36093.856713] srcud: End-test grace-period state: g10672632 f0x0 total-gps=10672632
The first test shows that it has about 2.6% improvement, while the
second test shows it has no significant effect.
I wonder if any test case can be in flavour of my patch. Any suggestion?
Thanks,
Pingfan
> 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
> > >
