----- On Jul 27, 2017, at 4:06 PM, Paul E. McKenney [email protected] wrote:
> 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! Thanks! More below, > > 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. > I would prefer: * @MEMBARRIER_CMD_PRIVATE_EXPEDITED: * Execute a memory barrier on each running * thread belonging to the same process as the current * thread. Upon return from system call, the * caller thread is ensured that all its running * threads siblings have passed through a state * where all memory accesses to user-space * addresses match program order between entry * to and return from the system call * (non-running threads are de facto in such a * state). This only covers threads from the * same processes as the caller thread. This * command returns 0. mainly because this is a uapi/ header, installed into distribution headers. So I want to remove any implementation reference from that text. >> * >> * 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. I'm planning to do exactly this for v2. > >> + */ >> +#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. Sure, I'll use this in my v2. Thanks! Mathieu > > 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 -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
