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 <paul...@linux.vnet.ibm.com>
> > ---
> > 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
> >
