Re: [GIT PULL] Re: REGRESSION: Performance regressions fromswitching anon_vma->lock to mutex

[Paul E. McKenney]

On Thu, Jun 16, 2011 at 09:03:35AM +0200, Ingo Molnar wrote:
> 
> * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > 
> > 
> > Ingo Molnar <mingo@xxxxxxx> wrote:
> > >
> > > I have this fix queued up currently:
> > >
> > >  09223371deac: rcu: Use softirq to address performance regression
> > 
> > I really don't think that is even close to enough.
> 
> Yeah.
> 
> > It still does all the callbacks in the threads, and according to 
> > Peter, about half the rcu time in the threads remained..
> 
> You are right - things that are a few percent on a 24 core machine 
> will definitely go exponentially worse on larger boxen. We'll get rid 
> of the kthreads entirely.

I did indeed at one time have access to larger test systems than I
do now, and I clearly need to fix that.  :-/

> The funny thing about this workload is that context-switches are 
> really a fastpath here and we are using anonymous IRQ-triggered 
> softirqs embedded in random task contexts as a workaround for that.

The other thing that the IRQ-triggered softirqs do is to get the callbacks
invoked in cases where a CPU-bound user thread is never context switching.
Of course, one alternative might be to set_need_resched() to force entry
into the scheduler as needed.

> [ I think we'll have to revisit this issue and do it properly:
>   quiescent state is mostly defined by context-switches here, so we
>   could do the RCU callbacks from the task that turns a CPU
>   quiescent, right in the scheduler context-switch path - perhaps
>   with an option for SCHED_FIFO tasks to *not* do GC.

I considered this approach for TINY_RCU, but dropped it in favor of
reducing the interlocking between the scheduler and RCU callbacks.
Might be worth revisiting, though.  If SCHED_FIFO task omit RCU callback
invocation, then there will need to be some override for CPUs with lots
of SCHED_FIFO load, probably similar to RCU's current blimit stuff.

>   That could possibly be more cache-efficient than softirq execution,
>   as we'll process a still-hot pool of callbacks instead of doing
>   them only once per timer tick. It will also make the RCU GC
>   behavior HZ independent. ]

Well, the callbacks will normally be cache-cold in any case due to the
grace-period delay, but on the other hand, both tick-independence and
the ability to shield a given CPU from RCU callback execution might be
quite useful.  The tick currently does the following for RCU:

1.	Informs RCU of user-mode execution (rcu_sched and rcu_bh
	quiescent state).

2.	Informs RCU of non-dyntick idle mode (again, rcu_sched and
	rcu_bh quiescent state).

3.	Kicks the current CPU's RCU core processing as needed in
	response to actions from other CPUs.

Frederic's work avoiding ticks in long-running user-mode tasks
might take care of #1, and it should be possible to make use of
the current dyntick-idle APIs to deal with #2.  Replacing #3
efficiently will take some thought.

> In any case the proxy kthread model clearly sucked, no argument about 
> that.

Indeed, I lost track of the global nature of real-time scheduling.  :-(

Whatever does the boosting will need to have process context and
can be subject to delays, so that pretty much needs to be a kthread.
But it will context-switch quite rarely, so should not be a problem.

							Thanx, Paul
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