----- On Sep 18, 2017, at 2:46 PM, Alan Stern [email protected] wrote:
> On Mon, 18 Sep 2017, Mathieu Desnoyers wrote: > >> 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. >> >> [ Runtime testing on the PowerPC architecture would be welcome. ] >> >> Signed-off-by: Mathieu Desnoyers <[email protected]> > >> +++ b/arch/powerpc/include/asm/membarrier.h >> @@ -0,0 +1,37 @@ >> +#ifndef _ASM_POWERPC_MEMBARRIER_H >> +#define _ASM_POWERPC_MEMBARRIER_H >> + >> +static inline void membarrier_arch_sched_in(struct task_struct *prev, >> + struct task_struct *next) >> +{ >> + /* >> + * Only need the full barrier when switching between processes. >> + */ >> + if (likely(!test_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED) >> + || prev->mm == next->mm)) I think I should use: test_ti_thread_flag(next, TIF_MEMBARRIER_PRIVATE_EXPEDITED) instead of test_thread_flag above. >> + return; >> + >> + /* >> + * The membarrier system call requires a full memory barrier >> + * after storing to rq->curr, before going back to user-space. >> + */ >> + smp_mb(); >> +} >> +static inline void membarrier_arch_fork(struct task_struct *t, >> + unsigned long clone_flags) >> +{ >> + /* >> + * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread >> + * fork is protected by siglock. membarrier_arch_fork is called >> + * with siglock held. >> + */ >> + if (t->mm->membarrier_private_expedited) >> + set_ti_thread_flag(t, TIF_MEMBARRIER_PRIVATE_EXPEDITED); >> +} > > Why have two separate bitflags for the same thing? Can't you just use > the mm->membarrier_private_expedited flag everywhere and forget about > TIF_MEMBARRIER_PRIVATE_EXPEDITED? The goal here is to save chasing through the task->mm-> pointer in sched_in(), and directly access the process registration state through a thread flag. Thanks, Mathieu > > Alan Stern -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
