Re: [PATCH] Fix: disable sys_membarrier when nohz_full is enabled

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Nov 17, 2016 at 11:46:34AM +0000, Mathieu Desnoyers wrote:
> ----- On Nov 17, 2016, at 1:51 AM, Lai Jiangshan jiangshanlai@gmail.com wrote:
> 
> > On Fri, Nov 4, 2016 at 12:29 AM, Mathieu Desnoyers
> > <mathieu.desnoyers@efficios.com> wrote:
> >> Userspace applications should be allowed to expect the membarrier system
> >> call with MEMBARRIER_CMD_SHARED command to issue memory barriers on
> >> nohz_full CPUs, but synchronize_sched() does not take those into
> >> account.
> >>
> >> Given that we do not want unrelated processes to be able to affect
> >> real-time sensitive nohz_full CPUs, simply return ENOSYS when membarrier
> >> is invoked on a kernel with enabled nohz_full CPUs.
> >>
> >> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> >> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> >> CC: Josh Triplett <josh@joshtriplett.org>
> >> CC: Steven Rostedt <rostedt@goodmis.org>
> >> CC: Lai Jiangshan <jiangshanlai@gmail.com>
> >> CC: <stable@vger.kernel.org>    [3.10+]
> >> ---
> >>  kernel/membarrier.c | 4 ++++
> >>  1 file changed, 4 insertions(+)
> >>
> >> diff --git a/kernel/membarrier.c b/kernel/membarrier.c
> >> index 536c727..9f9284f 100644
> >> --- a/kernel/membarrier.c
> >> +++ b/kernel/membarrier.c
> >> @@ -16,6 +16,7 @@
> >>
> >>  #include <linux/syscalls.h>
> >>  #include <linux/membarrier.h>
> >> +#include <linux/tick.h>
> >>
> >>  /*
> >>   * Bitmask made from a "or" of all commands within enum membarrier_cmd,
> >> @@ -51,6 +52,9 @@
> >>   */
> >>  SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> >>  {
> >> +       /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
> >> +       if (tick_nohz_full_enabled())
> >> +               return -ENOSYS;
> > 
> > I guess this code needs to be moved down into the branch of
> > "case MEMBARRIER_CMD_SHARED" to match its comment.
> 
> No, that would be unexpected from user-space. Either a system
> call is implemented or not, not "implemented for some parameters".
> 
> We also want MEMBARRIER_CMD_QUERY to return -ENOSYS in this case,
> and all other parameter values to also return -ENOSYS (rather than
> -EINVAL).
> 
> If a system call that returns successfully on CMD_QUERY or EINVAL,
> user-space may assume it will not have to handle ENOSYS in the
> next calls.
> 
> 
> > 
> > Acked-by: Lai Jiangshan <jiangshanlai@gmail.com>
> > 
> > But I'm afraid, in the future, tick_nohz_full will become a default y
> > feature. thus it makes sys_membarrier() always disabled. we might
> > need a new MEMBARRIER_CMD_XXX to handle it?
> 
> This may require that we send an IPI to nohz_full CPUs, which will
> disturb them real-time wise. Any better ideas ?

Restrict the IPIs to CPUs running the process executing the
sys_membarrier() system call.  This would mean that CPUs only
are interrupted by their own application's request.

							Thanx, Paul