[PATCH v2] nptl: Add backoff mechanism to spinlock loop

[Noah Goldstein]

On Wed, Mar 30, 2022 at 6:54 AM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
>
>
> On 28/03/2022 05:47, Wangyang Guo via Libc-alpha wrote:
> > When mutiple threads waiting for lock at the same time, once lock owner
> > releases the lock, waiters will see lock available and all try to lock,
> > which may cause an expensive CAS storm.
> >
> > Binary exponential backoff with random jitter is introduced. As try-lock
> > attempt increases, there is more likely that a larger number threads
> > compete for adaptive mutex lock, so increase wait time in exponential.
> > A random jitter is also added to avoid synchronous try-lock from other
> > threads.
> >
> > v2: Remove read-check before try-lock for performance.
> >
> > Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
> > ---
> >  nptl/pthread_mutex_lock.c | 25 ++++++++++++++++---------
> >  1 file changed, 16 insertions(+), 9 deletions(-)
> >
> > diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
> > index d2e652d151..7e75ec1cba 100644
> > --- a/nptl/pthread_mutex_lock.c
> > +++ b/nptl/pthread_mutex_lock.c
> > @@ -26,6 +26,7 @@
> >  #include <futex-internal.h>
> >  #include <stap-probe.h>
> >  #include <shlib-compat.h>
> > +#include <random-bits.h>
> >
> >  /* Some of the following definitions differ when pthread_mutex_cond_lock.c
> >     includes this file.  */
> > @@ -64,11 +65,6 @@ lll_mutex_lock_optimized (pthread_mutex_t *mutex)
> >  # define PTHREAD_MUTEX_VERSIONS 1
> >  #endif
> >
> > -#ifndef LLL_MUTEX_READ_LOCK
> > -# define LLL_MUTEX_READ_LOCK(mutex) \
> > -  atomic_load_relaxed (&(mutex)->__data.__lock)
> > -#endif
> > -
> >  static int __pthread_mutex_lock_full (pthread_mutex_t *mutex)
> >       __attribute_noinline__;
> >
> > @@ -138,17 +134,28 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
> >         int cnt = 0;
> >         int max_cnt = MIN (max_adaptive_count (),
> >                            mutex->__data.__spins * 2 + 10);
> > +       int spin_count, exp_backoff = 1;
> > +       unsigned int jitter = random_bits ();
>
> This will issue a syscall for architectures that do not have clock_gettime
> on vDSO, which is a performance regression.  You will need to move the
> jitter setup to be arch-specific, where the generic interface setting
> no random jitter.

What would be the best init jitter for arch w/ only syscall timers? TID? Or
something else?
>
> >         do
> >           {
> > -           if (cnt++ >= max_cnt)
> > +           /* In each loop, spin count is exponential backoff plus
> > +              random jitter, random range is [0, exp_backoff-1].  */
> > +           spin_count = exp_backoff + (jitter & (exp_backoff - 1));
> > +           cnt += spin_count;
> > +           if (cnt >= max_cnt)
> >               {
> > +               /* If cnt exceeds max spin count, just go to wait
> > +                  queue.  */
> >                 LLL_MUTEX_LOCK (mutex);
> >                 break;
> >               }
> > -           atomic_spin_nop ();
> > +           do
> > +               atomic_spin_nop ();
> > +           while (--spin_count > 0);
> > +           /* Binary exponential backoff, prepare for next loop.  */
> > +           exp_backoff <<= 1;
> >           }
> > -       while (LLL_MUTEX_READ_LOCK (mutex) != 0
> > -              || LLL_MUTEX_TRYLOCK (mutex) != 0);
> > +       while (LLL_MUTEX_TRYLOCK (mutex) != 0);
> >
> >         mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
> >       }