Re: [PATCH] locking/rwsem: Teach contention tracing about optimistic spinning

[Namhyung Kim]

Hi Davidlohr,

On Wed, Apr 27, 2022 at 9:04 AM Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:
>
> Similar to the mutex counterpart, we can further distinguish the
> types of contention. Similarly this patch also introduces potentially
> various _begin() tracepoints with a single respective _end().

Thanks for doing this.  I was thinking about it as well.

>
> - The original _begin() for down_write() is moved further up,
> right after we know optimistic spinning is bust.
>
> - For the scenario that spins after setting the hand-off bit and
> failing to grab the lock the code is change to duplicate the
> rwsem_try_write_lock() upon a OWNER_NULL, which minimizes the
> amounts of _begin() in the wait-loop - but also gets rid of a
> goto label and the logic is pretty encapsulated in the branch.
> In such cases the common case will be to acquire the lock,
> but if it is stolen in that window, this path won't see the
> signal_pending() now in the iteration and block. This should
> be benign as the check will come in after waking if the lock
> can still not be taken.
>
> Signed-off-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> ---
> rtmutexes' top waiter will also do optimitic spinning, but
> I don't think it is worth adding tracepoints here as it
> is quite minimal, unlike the osq stuff.
>
>  include/trace/events/lock.h |  4 +++-
>  kernel/locking/rwsem.c      | 29 +++++++++++++++++++++--------
>  2 files changed, 24 insertions(+), 9 deletions(-)
>
> diff --git a/include/trace/events/lock.h b/include/trace/events/lock.h
> index 9ebd081e057e..0f68a3e69a9f 100644
> --- a/include/trace/events/lock.h
> +++ b/include/trace/events/lock.h
> @@ -15,6 +15,7 @@
>  #define LCB_F_RT       (1U << 3)
>  #define LCB_F_PERCPU   (1U << 4)
>  #define LCB_F_MUTEX    (1U << 5)
> +#define LCB_F_RWSEM    (1U << 6)
>
>
>  #ifdef CONFIG_LOCKDEP
> @@ -115,7 +116,8 @@ TRACE_EVENT(contention_begin,
>                                 { LCB_F_WRITE,          "WRITE" },
>                                 { LCB_F_RT,             "RT" },
>                                 { LCB_F_PERCPU,         "PERCPU" },
> -                               { LCB_F_MUTEX,          "MUTEX" }
> +                               { LCB_F_MUTEX,          "MUTEX" },
> +                               { LCB_F_RWSEM,          "RWSEM" }
>                           ))
>  );

Well I'm ok with this but it'd be better if we can do this
without adding a new flag.  Originally a mutex can be
identified with 0, and a rwsem with either of READ or WRITE.

The MUTEX flag was added to note optimistic spins
on mutex and now we need something similar for
rwsem.  Then can we change the MUTEX to OPTIMISTIC
if it's not too late?

>
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 9d1db4a54d34..c38f990cacea 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -1057,7 +1057,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
>         if (!wake_q_empty(&wake_q))
>                 wake_up_q(&wake_q);
>
> -       trace_contention_begin(sem, LCB_F_READ);
> +       trace_contention_begin(sem, LCB_F_RWSEM | LCB_F_READ);
>
>         /* wait to be given the lock */
>         for (;;) {
> @@ -1101,9 +1101,14 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>         DEFINE_WAKE_Q(wake_q);
>
>         /* do optimistic spinning and steal lock if possible */
> -       if (rwsem_can_spin_on_owner(sem) && rwsem_optimistic_spin(sem)) {
> -               /* rwsem_optimistic_spin() implies ACQUIRE on success */
> -               return sem;
> +       if (rwsem_can_spin_on_owner(sem)) {
> +               trace_contention_begin(sem,
> +                                      LCB_F_RWSEM | LCB_F_WRITE | LCB_F_SPIN);
> +               if (rwsem_optimistic_spin(sem)) {
> +                       /* rwsem_optimistic_spin() implies ACQUIRE on success */
> +                       trace_contention_end(sem, 0);
> +                       return sem;
> +               }
>         }
>
>         /*
> @@ -1115,6 +1120,8 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>         waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
>         waiter.handoff_set = false;
>
> +       trace_contention_begin(sem, LCB_F_RWSEM | LCB_F_WRITE);
> +
>         raw_spin_lock_irq(&sem->wait_lock);
>         rwsem_add_waiter(sem, &waiter);
>
> @@ -1137,7 +1144,6 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>
>         /* wait until we successfully acquire the lock */
>         set_current_state(state);
> -       trace_contention_begin(sem, LCB_F_WRITE);
>
>         for (;;) {
>                 if (rwsem_try_write_lock(sem, &waiter)) {
> @@ -1161,18 +1167,25 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>                 if (waiter.handoff_set) {
>                         enum owner_state owner_state;
>
> +                       trace_contention_begin(sem, LCB_F_RWSEM |
> +                                              LCB_F_WRITE | LCB_F_SPIN);
>                         preempt_disable();
>                         owner_state = rwsem_spin_on_owner(sem);
>                         preempt_enable();
>
> -                       if (owner_state == OWNER_NULL)
> -                               goto trylock_again;
> +                       if (owner_state == OWNER_NULL) {
> +                               raw_spin_lock_irq(&sem->wait_lock);
> +                               if (rwsem_try_write_lock(sem, &waiter))
> +                                       break;
> +                               raw_spin_unlock_irq(&sem->wait_lock);
> +                       }
> +
> +                       trace_contention_begin(sem, LCB_F_RWSEM | LCB_F_WRITE);

I'm afraid that it'd generate many contention_begin
trace events for a single lock acquisition.

Thanks,
Namhyung

>                 }
>
>                 schedule();
>                 lockevent_inc(rwsem_sleep_writer);
>                 set_current_state(state);
> -trylock_again:
>                 raw_spin_lock_irq(&sem->wait_lock);
>         }
>         __set_current_state(TASK_RUNNING);
> --
> 2.36.0
>