Re: [RFC PATCH] membarrier: expedited private command

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 27, 2017, at 4:06 PM, Paul E. McKenney paulmck@linux.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,

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


>>   *
>>   * 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