----- On Oct 5, 2017, at 8:22 AM, Avi Kivity avi@xxxxxxxxxxxx wrote: > 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. Thanks Avi for your input on the userspace API. Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
