Messages in this thread | | | Subject | Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command | From | Avi Kivity <> | Date | Thu, 5 Oct 2017 15:22:40 +0300 |
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On 10/05/2017 07:23 AM, Nicholas Piggin wrote: > On Wed, 4 Oct 2017 14:37:53 -0700 > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > >> 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.
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