Re: [RFC PATCH] membarrier: expedited private command

[Paul E. McKenney]

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 <peterz@xxxxxxxxxxxxx>
> CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> CC: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> ---
>  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
>