----- On Nov 17, 2016, at 1:51 AM, Lai Jiangshan jiangshan...@gmail.com wrote:
> On Fri, Nov 4, 2016 at 12:29 AM, Mathieu Desnoyers
> <mathieu.desnoy...@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.desnoy...@efficios.com>
>> CC: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
>> CC: Josh Triplett <j...@joshtriplett.org>
>> CC: Steven Rostedt <rost...@goodmis.org>
>> CC: Lai Jiangshan <jiangshan...@gmail.com>
>> CC: <sta...@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 <jiangshan...@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 ?
Thanks,
Mathieu
>
> thanks,
> Lai
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
