Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Steven Rostedt (rostedt@goodmis.org) wrote:
> On Sat, 2010-01-09 at 16:03 -0800, Paul E. McKenney wrote:
> > On Sat, Jan 09, 2010 at 06:16:40PM -0500, Steven Rostedt wrote:
> > > On Sat, 2010-01-09 at 18:05 -0500, Steven Rostedt wrote:
> > > 
> > > > Then we should have O(tasks) for spinlocks taken, and 
> > > > O(min(tasks, CPUS)) for IPIs.
> > > 
> > > And for nr tasks >> CPUS, this may help too:
> > > 
> > > > cpumask = 0;
> > > > foreach task {
> > > 
> > > 	if (cpumask == online_cpus)
> > > 		break;
> > > 
> > > > 	spin_lock(task_rq(task)->rq->lock);
> > > > 	if (task_rq(task)->curr == task)
> > > > 		cpu_set(task_cpu(task), cpumask);
> > > > 	spin_unlock(task_rq(task)->rq->lock);
> > > > }
> > > > send_ipi(cpumask);
> > 
> > Good point, erring on the side of sending too many IPIs is safe.  One
> > might even be able to just send the full set if enough of the CPUs were
> > running the current process and none of the remainder were running
> > real-time threads.  And yes, it would then be necessary to throttle
> > calls to sys_membarrier().
> > 
> 
> If you need to throttle calls to sys_membarrier(), than why bother
> optimizing it? Again, this is like calling synchronize_sched() in the
> kernel, which is a very heavy operation, and should only be called by
> those that are not performance critical.
> 
> Why are we struggling so much with optimizing the slow path?
> 
> Here's how I take it. This method is much better that sending signals to
> all threads. The advantage the sys_membarrier gives us, is also a way to
> keep user rcu_read_locks barrier free, which means that rcu_read_locks
> are quick and scale well.
> 
> So what if we have a linear decrease in performance with the number of
> threads on the write side?

Hrm, looking at arch/x86/include/asm/mmu_context.h

switch_mm(), which is basically called each time the scheduler needs to
change the current task, does a

cpumask_clear_cpu(cpu, mm_cpumask(prev));

and

cpumask_set_cpu(cpu, mm_cpumask(next));

which precise goal is to stop the flush ipis for the previous mm. The
100$ question is : why do we have to confirm that the thread is indeed
on the runqueue (taking locks and everything) when we could simply just
bluntly use the mm_cpumask for our own IPIs ?

cpumask_clear_cpu and cpumask_set_cpu translate into clear_bit/set_bit.
cpumask_next does a find_next_bit on the cpumask.

clear_bit/set_bit are atomic and not reordered on x86. PowerPC also uses
ll/sc loops in bitops.h, so I think it should be pretty safe to assume
that mm_cpumask is, by design, made to be used as cpumask to send a
broadcast IPI to all CPUs which run threads belonging to a given
process.

So, how about just using mm_cpumask(current) for the broadcast ? Then we
don't even need to allocate our own cpumask neither.

Or am I missing something ? I just sounds too simple.

Thanks,

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
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