On Wed, 4 Oct 2017 14:37:53 -0700 "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
> From: Mathieu Desnoyers <mathieu.desnoy...@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. > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h > index 3a19c253bdb1..4af1b719c65f 100644 > --- a/include/linux/sched/mm.h > +++ b/include/linux/sched/mm.h > @@ -205,4 +205,54 @@ static inline void memalloc_noreclaim_restore(unsigned > int flags) > current->flags = (current->flags & ~PF_MEMALLOC) | flags; > } > > +#ifdef CONFIG_MEMBARRIER > + > +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS > +#include <asm/membarrier.h> > +#else > +static inline void membarrier_arch_switch_mm(struct mm_struct *prev, > + struct mm_struct *next, struct task_struct *tsk) > +{ > +} This is no longer required in architecture independent code, is it?