Date:   Thu, 27 Jul 2017 13:26:29 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH] membarrier: expedited private command

On Thu, Jul 27, 2017 at 08:18:47PM +0000, Mathieu Desnoyers wrote:
> 
> ----- On Jul 27, 2017, at 4:06 PM, Paul E. McKenney paulmck@...ux.vnet.ibm.com 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,

At at least one had it pass their initial testing.

> > Please see other feedback inline below.
> > 
> >> [ Compile-tested only! ]
> >> 
> >> CC: Peter Zijlstra <peterz@...radead.org>
> >> CC: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> >> CC: Boqun Feng <boqun.feng@...il.com>
> >> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> >> ---
> >>  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.

Fine by me, as long as there is documentation.  ;-)

> >>   *
> >>   * 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.

Fair enough.

> >> + */
> >> +#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.

Sounds good!

							Thanx, Paul

> 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
> 

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