Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

----- On Oct 5, 2017, at 8:22 AM, Avi Kivity avi@scylladb.com wrote:

On 10/05/2017 07:23 AM, Nicholas Piggin wrote:

quoted

On Wed,  4 Oct 2017 14:37:53 -0700
"Paul E. McKenney" [off-list ref] wrote:

quoted

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Provide a new command allowing processes to register their intent to use
the private expedited command.

This allows PowerPC to skip the full memory barrier in switch_mm(), and
only issue the barrier when scheduling into a task belonging to a
process that has registered to use expedited private.

Processes are now required to register before using
MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.

Changes since v1:
- Use test_ti_thread_flag(next, ...) instead of test_thread_flag() in
   powerpc membarrier_arch_sched_in(), given that we want to specifically
   check the next thread state.
- Add missing ARCH_HAS_MEMBARRIER_HOOKS in Kconfig.
- Use task_thread_info() to pass thread_info from task to
   *_ti_thread_flag().

Changes since v2:
- Move membarrier_arch_sched_in() call to finish_task_switch().
- Check for NULL t->mm in membarrier_arch_fork().
- Use membarrier_sched_in() in generic code, which invokes the
   arch-specific membarrier_arch_sched_in(). This fixes allnoconfig
   build on PowerPC.
- Move asm/membarrier.h include under CONFIG_MEMBARRIER, fixing
   allnoconfig build on PowerPC.
- Build and runtime tested on PowerPC.

Changes since v3:
- Simply rely on copy_mm() to copy the membarrier_private_expedited mm
   field on fork.
- powerpc: test thread flag instead of reading
   membarrier_private_expedited in membarrier_arch_fork().
- powerpc: skip memory barrier in membarrier_arch_sched_in() if coming
   from kernel thread, since mmdrop() implies a full barrier.
- Set membarrier_private_expedited to 1 only after arch registration
   code, thus eliminating a race where concurrent commands could succeed
   when they should fail if issued concurrently with process
   registration.
- Use READ_ONCE() for membarrier_private_expedited field access in
   membarrier_private_expedited. Matches WRITE_ONCE() performed in
   process registration.

Changes since v4:
- Move powerpc hook from sched_in() to switch_mm(), based on feedback
   from Nicholas Piggin.

For now, the powerpc approach is okay by me. I plan to test
others (e.g., taking runqueue locks) on larger systems, but that can
be sent as an incremental patch at a later time.

The main thing I would like is for people to review the userspace API.

As a future satisfied user of the expedited private membarrier syscall,
I am happy with the change.

Thanks Avi for your input on the userspace API.

Mathieu


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