On Thu, Jul 27, 2017 at 02:59:43PM -0400, Mathieu Desnoyers wrote: > Implement MEMBARRIER_CMD_PRIVATE_EXPEDITED with IPIs using cpumask built > from all runqueues for which current thread's mm is the same as our own. > > Scheduler-wise, it requires that we add a memory barrier after context > switching between processes (which have different mm). > > It would be interesting to benchmark the overhead of this added barrier > on the performance of context switching between processes. If the > preexisting overhead of switching between mm is high enough, the > overhead of adding this extra barrier may be insignificant.
I sent this along to the people asking for faster sys_membarrier(), CCing you, thank you! Please see other feedback inline below. > [ Compile-tested only! ] > > CC: Peter Zijlstra <[email protected]> > CC: Paul E. McKenney <[email protected]> > CC: Boqun Feng <[email protected]> > Signed-off-by: Mathieu Desnoyers <[email protected]> > --- > include/uapi/linux/membarrier.h | 8 +++-- > kernel/membarrier.c | 76 > ++++++++++++++++++++++++++++++++++++++++- > kernel/sched/core.c | 21 ++++++++++++ > 3 files changed, 102 insertions(+), 3 deletions(-) > > diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h > index e0b108bd2624..6a33c5852f6b 100644 > --- a/include/uapi/linux/membarrier.h > +++ b/include/uapi/linux/membarrier.h > @@ -40,14 +40,18 @@ > * (non-running threads are de facto in such a > * state). This covers threads from all processes > * running on the system. This command returns 0. > + * TODO: documentation. How about something like this? * @MEMBARRIER_CMD_PRIVATE_EXPEDITED: IPI each CPU running a thread belonging * to the same process as the current thread, causing * the recipient CPU to execute a full memory-barrier * instruction. The same-process determination is * made using the task_struct ->mm field: If some * other CPU's currently running task has the same * value in its ->mm field as the requesting thread, * that CPU is IPIed. > * > * Command to be passed to the membarrier system call. The commands need to > * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to > * the value 0. > */ > enum membarrier_cmd { > - MEMBARRIER_CMD_QUERY = 0, > - MEMBARRIER_CMD_SHARED = (1 << 0), > + MEMBARRIER_CMD_QUERY = 0, > + MEMBARRIER_CMD_SHARED = (1 << 0), > + /* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */ > + /* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */ > + MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3), > }; > > #endif /* _UAPI_LINUX_MEMBARRIER_H */ > diff --git a/kernel/membarrier.c b/kernel/membarrier.c > index 9f9284f37f8d..8c6c0f96f617 100644 > --- a/kernel/membarrier.c > +++ b/kernel/membarrier.c > @@ -19,10 +19,81 @@ > #include <linux/tick.h> > > /* > + * XXX For cpu_rq(). Should we rather move > + * membarrier_private_expedited() to sched/core.c or create > + * sched/membarrier.c ? Any reason not to move the entirety of membarrier.c there? Assuming that Peter is OK with this, of course. > + */ > +#include "sched/sched.h" > + > +/* > * Bitmask made from a "or" of all commands within enum membarrier_cmd, > * except MEMBARRIER_CMD_QUERY. > */ > -#define MEMBARRIER_CMD_BITMASK (MEMBARRIER_CMD_SHARED) > +#define MEMBARRIER_CMD_BITMASK \ > + (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED) > + > +static void ipi_mb(void *info) > +{ > + smp_mb(); /* IPIs should be serializing but paranoid. */ I am good with paranoia! ;-) > +} > + > +static void membarrier_private_expedited_ipi_each(void) > +{ > + int cpu; > + > + for_each_online_cpu(cpu) { > + struct task_struct *p; > + > + rcu_read_lock(); > + p = task_rcu_dereference(&cpu_rq(cpu)->curr); > + if (p && p->mm == current->mm) > + smp_call_function_single(cpu, ipi_mb, NULL, 1); > + rcu_read_unlock(); > + } > +} > + > +static void membarrier_private_expedited(void) > +{ > + int cpu, this_cpu; > + cpumask_var_t tmpmask; > + > + if (num_online_cpus() == 1) > + return; > + > + /* > + * Matches memory barriers around rq->curr modification in > + * scheduler. > + */ > + smp_mb(); /* system call entry is not a mb. */ > + > + if (!alloc_cpumask_var(&tmpmask, GFP_NOWAIT)) { > + /* Fallback for OOM. */ > + membarrier_private_expedited_ipi_each(); > + goto end; > + } > + > + this_cpu = raw_smp_processor_id(); > + for_each_online_cpu(cpu) { > + struct task_struct *p; > + > + if (cpu == this_cpu) > + continue; > + rcu_read_lock(); > + p = task_rcu_dereference(&cpu_rq(cpu)->curr); > + if (p && p->mm == current->mm) > + __cpumask_set_cpu(cpu, tmpmask); > + rcu_read_unlock(); > + } > + smp_call_function_many(tmpmask, ipi_mb, NULL, 1); > + free_cpumask_var(tmpmask); > +end: > + /* > + * Memory barrier on the caller thread _after_ we finished > + * waiting for the last IPI. Matches memory barriers around > + * rq->curr modification in scheduler. > + */ > + smp_mb(); /* exit from system call is not a mb */ > +} > > /** > * sys_membarrier - issue memory barriers on a set of threads > @@ -64,6 +135,9 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags) > if (num_online_cpus() > 1) > synchronize_sched(); > return 0; > + case MEMBARRIER_CMD_PRIVATE_EXPEDITED: > + membarrier_private_expedited(); > + return 0; > default: > return -EINVAL; > } > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 17c667b427b4..f171d2aaaf82 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2724,6 +2724,26 @@ asmlinkage __visible void schedule_tail(struct > task_struct *prev) > put_user(task_pid_vnr(current), current->set_child_tid); > } > > +#ifdef CONFIG_MEMBARRIER > +static void membarrier_expedited_mb_after_set_current(struct mm_struct *mm, > + struct mm_struct *oldmm) > +{ > + if (likely(mm == oldmm)) > + return; /* Thread context switch, same mm. */ > + /* > + * When switching between processes, membarrier expedited > + * private requires a memory barrier after we set the current > + * task. > + */ > + smp_mb(); > +} > +#else /* #ifdef CONFIG_MEMBARRIER */ > +static void membarrier_expedited_mb_after_set_current(struct mm_struct *mm, > + struct mm_struct *oldmm) > +{ > +} > +#endif /* #else #ifdef CONFIG_MEMBARRIER */ Why not something like this? Shorter, easier to read, and should generate the same code. static void membarrier_expedited_mb_after_set_current(struct mm_struct *mm, struct mm_struct *oldmm) { if (!IS_ENABLED(CONFIG_MEMBARRIER)) return; if (likely(mm == oldmm)) return; /* Thread context switch, same mm. */ /* * When switching between processes, membarrier expedited * private requires a memory barrier after we set the current * task. */ smp_mb(); } > + > /* > * context_switch - switch to the new MM and the new thread's register state. > */ > @@ -2737,6 +2757,7 @@ context_switch(struct rq *rq, struct task_struct *prev, > > mm = next->mm; > oldmm = prev->active_mm; > + membarrier_expedited_mb_after_set_current(mm, oldmm); > /* > * For paravirt, this is coupled with an exit in switch_to to > * combine the page table reload and the switch backend into > -- > 2.11.0 >
