----- On Nov 17, 2016, at 1:51 AM, Lai Jiangshan [email protected] wrote:
> On Fri, Nov 4, 2016 at 12:29 AM, Mathieu Desnoyers > <[email protected]> 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 <[email protected]> >> CC: "Paul E. McKenney" <[email protected]> >> CC: Josh Triplett <[email protected]> >> CC: Steven Rostedt <[email protected]> >> CC: Lai Jiangshan <[email protected]> >> CC: <[email protected]> [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 <[email protected]> > > 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 ? Thanks, Mathieu > > thanks, > Lai -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
