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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 24, 2017, at 5:58 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com 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.

Is this now possible because the synchronize_sched_expedited()
implementation does not require to send IPIs to all CPUS ? I
suspect that using tree srcu now solves this somehow, but can
you tell us a bit more about why it is now OK to expose this
to user-space ?

The commit message here does not explain why it is OK real-time
wise to expose this feature as a system call.

Thanks,

Mathieu


> 
> Signed-off-by: Paul E. McKenney <paulmck@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),
> };
> 
> #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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com