On Wed, 2013-06-26 at 15:40 -0700, Davidlohr Bueso wrote: > On Wed, 2013-06-26 at 15:28 -0700, Tim Chen wrote: > > We want to add optimistic spinning to rwsems because we've noticed that > > the writer rwsem does not perform as well as mutexes. Tim noticed that > > for exim (mail server) workloads, when reverting commit 4fc3f1d6 and > > Davidlohr > > noticed it when converting the i_mmap_mutex to a rwsem in some aim7 > > workloads. We've noticed that the biggest difference, in a nutshell, is > > when we fail to acquire a mutex in the fastpath, optimistic spinning > > comes in to play and we can avoid a large amount of unnecessary sleeping > > and wait queue overhead. > > > > For rwsems on the other hand, upon entering the writer slowpath in > > rwsem_down_write_failed(), we just acquire the ->wait_lock, add > > ourselves to the wait_queue and blocking until we get the lock. > > > > Reviewed-by: Peter Zijlstra <[email protected]> > > Reviewed-by: Peter Hurley <[email protected]> > > Signed-off-by: Tim Chen <[email protected]> > > Signed-off-by: Davidlohr Bueso <[email protected]> > > --- > > include/linux/rwsem.h | 3 + > > init/Kconfig | 9 +++ > > kernel/rwsem.c | 29 +++++++++- > > lib/rwsem.c | 150 > > +++++++++++++++++++++++++++++++++++++++++++++---- > > 4 files changed, 179 insertions(+), 12 deletions(-) > > > > diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h > > index 0616ffe..0c5933b 100644 > > --- a/include/linux/rwsem.h > > +++ b/include/linux/rwsem.h > > @@ -29,6 +29,9 @@ struct rw_semaphore { > > #ifdef CONFIG_DEBUG_LOCK_ALLOC > > struct lockdep_map dep_map; > > #endif > > +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER > > + struct task_struct *owner; > > +#endif > > }; > > > > extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore > > *sem); > > diff --git a/init/Kconfig b/init/Kconfig > > index 9d3a788..1c582d1 100644 > > --- a/init/Kconfig > > +++ b/init/Kconfig > > @@ -1595,6 +1595,15 @@ config TRACEPOINTS > > > > source "arch/Kconfig" > > > > +config RWSEM_SPIN_ON_WRITE_OWNER > > + bool "Optimistic spin write acquisition for writer owned rw-sem" > > + default n > > + depends on SMP > > + help > > + Allows a writer to perform optimistic spinning if another writer own > > + the read write semaphore. This gives a greater chance for writer to > > + acquire a semaphore before blocking it and putting it to sleep. > > + > > Quoting from kernel/mutex.c: > > "The rationale is that if the lock owner is running, it is likely to > release the lock soon." > > It would be good to add that to the Kconfig.
Sounds good. Tim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
