On 10/05/2017 07:23 AM, Nicholas Piggin wrote:
On Wed, 4 Oct 2017 14:37:53 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
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.
