Re: [RFC PATCH v2] membarrier: expedited private command

[Paul E. McKenney]

On Thu, Jul 27, 2017 at 05:13:14PM -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 the
> thread calling sys_membarrier.
> 
> Scheduler-wise, it requires that we add a memory barrier after context
> switching between processes (which have different mm). Interestingly,
> there is already a memory barrier in mmdrop(), so we only need to add
> a barrier when switching from a kernel thread to a userspace thread.
> We also don't need to add the barrier when switching to a kernel thread,
> because it has no userspace memory mapping, which makes ordering of
> user-space memory accesses pretty much useless.
> 
> * Benchmark
> 
> A stress-test benchmark of sched pipe shows that it does not add
> significant overhead to the scheduler switching between processes:
> 
> 100 runs of:
> 
> taskset 01 ./perf bench sched pipe
> 
> Running 'sched/pipe' benchmark:
> Executed 1000000 pipe operations between two processes
> 
> Hardware: CPU: Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz
> 
> A) With 4.13.0-rc2+
>    at commit a97fb594bc7d ("virtio-net: fix module unloading")
> 
> avg.:     2.923 usecs/op
> std.dev:  0.057 usecs/op
> 
> B) With this commit:
> 
> avg.:     2.916 usecs/op
> std.dev:  0.043 usecs/op
> 
> Changes since v1:
> - move membarrier code under kernel/sched/ because it uses the
>   scheduler runqueue,
> - only add the barrier when we switch from a kernel thread. The case
>   where we switch from a user-space thread is already handled by
>   the atomic_dec_and_test() in mmdrop().
> - add a comment to mmdrop() documenting the requirement on the implicit
>   memory barrier.
> 
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> CC: Boqun Feng <boqun.feng@xxxxxxxxx>
> CC: Andrew Hunter <ahh@xxxxxxxxxx>
> CC: Maged Michael <maged.michael@xxxxxxxxx>
> CC: gromer@xxxxxxxxxx
> CC: Avi Kivity <avi@xxxxxxxxxxxx>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>

Looks much better, thank you!

I have queued this in place of my earlier patch for the moment.  If there
are no objections, I will push this into the upcoming v4.14 merge window.
If someone else wants to push it into v4.14, I am of course fine with
that, and am happy to give it a reviewed-by along the way.  But if there
are objections to your patch (suitably modified based on additional
review and testing, of course) going into v4.14, I can always fall back
to pushing my earlier simpler but less housebroken patch.  ;-)

							Thanx, Paul

> ---
>  MAINTAINERS                     |  2 +-
>  include/linux/sched/mm.h        |  5 +++
>  include/uapi/linux/membarrier.h | 23 +++++++++++--
>  kernel/Makefile                 |  1 -
>  kernel/sched/Makefile           |  1 +
>  kernel/sched/core.c             | 27 ++++++++++++++++
>  kernel/{ => sched}/membarrier.c | 72 ++++++++++++++++++++++++++++++++++++++++-
>  7 files changed, 126 insertions(+), 5 deletions(-)
>  rename kernel/{ => sched}/membarrier.c (59%)
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index f66488dfdbc9..3b035584272f 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -8621,7 +8621,7 @@ M:	Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
>  M:	"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
>  L:	linux-kernel@xxxxxxxxxxxxxxx
>  S:	Supported
> -F:	kernel/membarrier.c
> +F:	kernel/sched/membarrier.c
>  F:	include/uapi/linux/membarrier.h
> 
>  MEMORY MANAGEMENT
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 2b24a6974847..5c5384d9ae0f 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
>  extern void __mmdrop(struct mm_struct *);
>  static inline void mmdrop(struct mm_struct *mm)
>  {
> +	/*
> +	 * Implicit full memory barrier provided by
> +	 * atomic_dec_and_test() is required by membarrier. See comments
> +	 * around membarrier_expedited_mb_after_set_current().
> +	 */
>  	if (unlikely(atomic_dec_and_test(&mm->mm_count)))
>  		__mmdrop(mm);
>  }
> diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
> index e0b108bd2624..6d47b3249d8a 100644
> --- a/include/uapi/linux/membarrier.h
> +++ b/include/uapi/linux/membarrier.h
> @@ -40,14 +40,33 @@
>   *                          (non-running threads are de facto in such a
>   *                          state). This covers threads from all processes
>   *                          running on the system. This command returns 0.
> + * @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. The "expedited" commands
> + *                          complete faster than the non-expedited ones,
> + *                          they never block, but have the downside of
> + *                          causing extra overhead.
>   *
>   * 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/Makefile b/kernel/Makefile
> index 4cb8e8b23c6e..9c323a6daa46 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -108,7 +108,6 @@ obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
>  obj-$(CONFIG_JUMP_LABEL) += jump_label.o
>  obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
>  obj-$(CONFIG_TORTURE_TEST) += torture.o
> -obj-$(CONFIG_MEMBARRIER) += membarrier.o
> 
>  obj-$(CONFIG_HAS_IOMEM) += memremap.o
> 
> diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
> index 53f0164ed362..78f54932ea1d 100644
> --- a/kernel/sched/Makefile
> +++ b/kernel/sched/Makefile
> @@ -25,3 +25,4 @@ obj-$(CONFIG_SCHED_DEBUG) += debug.o
>  obj-$(CONFIG_CGROUP_CPUACCT) += cpuacct.o
>  obj-$(CONFIG_CPU_FREQ) += cpufreq.o
>  obj-$(CONFIG_CPU_FREQ_GOV_SCHEDUTIL) += cpufreq_schedutil.o
> +obj-$(CONFIG_MEMBARRIER) += membarrier.o
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 17c667b427b4..01e3b881ab3a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2724,6 +2724,32 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
>  		put_user(task_pid_vnr(current), current->set_child_tid);
>  }
> 
> +static void membarrier_expedited_mb_after_set_current(struct mm_struct *mm,
> +		struct mm_struct *oldmm)
> +{
> +	if (!IS_ENABLED(CONFIG_MEMBARRIER))
> +		return;
> +	/*
> +	 * __schedule()->
> +	 *   finish_task_switch()->
> +	 *    if (mm)
> +	 *      mmdrop(mm) ->
> +	 *        atomic_dec_and_test()
> +	 * takes care of issuing a memory barrier when oldmm is
> +	 * non-NULL. We also don't need the barrier when switching to a
> +	 * kernel thread, nor when we switch between threads belonging
> +	 * to the same process.
> +	 */
> +	if (likely(oldmm || !mm || mm == oldmm))
> +		return;
> +	/*
> +	 * 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 +2763,7 @@ 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
> diff --git a/kernel/membarrier.c b/kernel/sched/membarrier.c
> similarity index 59%
> rename from kernel/membarrier.c
> rename to kernel/sched/membarrier.c
> index 9f9284f37f8d..f80828b0b607 100644
> --- a/kernel/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -17,12 +17,79 @@
>  #include <linux/syscalls.h>
>  #include <linux/membarrier.h>
>  #include <linux/tick.h>
> +#include <linux/cpumask.h>
> +
> +#include "sched.h"	/* for cpu_rq(). */
> 
>  /*
>   * 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. */
> +}
> +
> +static void membarrier_private_expedited(void)
> +{
> +	int cpu, this_cpu;
> +	bool fallback = false;
> +	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. */
> +		fallback = true;
> +	}
> +
> +	/*
> +	 * Skipping the current CPU is OK even through we can be
> +	 * migrated at any point. The current CPU, at the point where we
> +	 * read raw_smp_processor_id(), is ensured to be in program
> +	 * order with respect to the caller thread. Therefore, we can
> +	 * skip this CPU from the iteration.
> +	 */
> +	this_cpu = raw_smp_processor_id();
> +	cpus_read_lock();
> +	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) {
> +			if (!fallback)
> +				__cpumask_set_cpu(cpu, tmpmask);
> +			else
> +				smp_call_function_single(cpu, ipi_mb, NULL, 1);
> +		}
> +		rcu_read_unlock();
> +	}
> +	cpus_read_unlock();
> +	if (!fallback) {
> +		smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
> +		free_cpumask_var(tmpmask);
> +	}
> +
> +	/*
> +	 * 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 +131,9 @@ 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;
>  	}
> -- 
> 2.11.0
>