Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

[Paul E. McKenney]

On Tue, Jul 25, 2017 at 12:27:01PM +0800, Boqun Feng wrote:
> On Mon, Jul 24, 2017 at 02:58:16PM -0700, Paul E. McKenney wrote:
> > The sys_membarrier() system call has proven too slow for some use
> > cases, which has prompted users to instead rely on TLB shootdown.
> > Although TLB shootdown is much faster, it has the slight disadvantage
> > of not working at all on arm and arm64.  This commit therefore adds
> > an expedited option to the sys_membarrier() system call.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > ---
> >  include/uapi/linux/membarrier.h | 11 +++++++++++
> >  kernel/membarrier.c             |  7 ++++++-
> >  2 files changed, 17 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
> > index e0b108bd2624..ba36d8a6be61 100644
> > --- a/include/uapi/linux/membarrier.h
> > +++ b/include/uapi/linux/membarrier.h
> > @@ -40,6 +40,16 @@
> >   *                          (non-running threads are de facto in such a
> >   *                          state). This covers threads from all processes
> >   *                          running on the system. This command returns 0.
> > + * @MEMBARRIER_CMD_SHARED_EXPEDITED:  Execute a memory barrier on all
> > + *			    running threads, but in an expedited fashion.
> > + *                          Upon return from system call, the caller thread
> > + *                          is ensured that all running threads have passed
> > + *                          through a state where all memory accesses to
> > + *                          user-space addresses match program order between
> > + *                          entry to and return from the system call
> > + *                          (non-running threads are de facto in such a
> > + *                          state). This covers threads from all processes
> > + *                          running on the system. This command returns 0.
> >   *
> >   * Command to be passed to the membarrier system call. The commands need to
> >   * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to
> > @@ -48,6 +58,7 @@
> >  enum membarrier_cmd {
> >  	MEMBARRIER_CMD_QUERY = 0,
> >  	MEMBARRIER_CMD_SHARED = (1 << 0),
> > +	MEMBARRIER_CMD_SHARED_EXPEDITED = (2 << 0),
> 
> Should this better be "(1 << 1)" ;-)

Same value, but yes, much more aligned with the intent.  Good catch,
thank you, fixed!

							Thanx, Paul

> Regards,
> Boqun
> 
> >  };
> >  
> >  #endif /* _UAPI_LINUX_MEMBARRIER_H */
> > diff --git a/kernel/membarrier.c b/kernel/membarrier.c
> > index 9f9284f37f8d..b749c39bb219 100644
> > --- a/kernel/membarrier.c
> > +++ b/kernel/membarrier.c
> > @@ -22,7 +22,8 @@
> >   * Bitmask made from a "or" of all commands within enum membarrier_cmd,
> >   * except MEMBARRIER_CMD_QUERY.
> >   */
> > -#define MEMBARRIER_CMD_BITMASK	(MEMBARRIER_CMD_SHARED)
> > +#define MEMBARRIER_CMD_BITMASK	(MEMBARRIER_CMD_SHARED |		\
> > +				 MEMBARRIER_CMD_SHARED_EXPEDITED)
> >  
> >  /**
> >   * sys_membarrier - issue memory barriers on a set of threads
> > @@ -64,6 +65,10 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> >  		if (num_online_cpus() > 1)
> >  			synchronize_sched();
> >  		return 0;
> > +	case MEMBARRIER_CMD_SHARED_EXPEDITED:
> > +		if (num_online_cpus() > 1)
> > +			synchronize_sched_expedited();
> > +		return 0;
> >  	default:
> >  		return -EINVAL;
> >  	}
> > -- 
> > 2.5.2
> >