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

[Frederic Weisbecker]

On Thu, Jun 16, 2011 at 10:25:50PM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > > 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.
> 
> Yeah - but this workload didnt have that.
> 
> > Of course, one alternative might be to set_need_resched() to force 
> > entry into the scheduler as needed.
> 
> No need for that: we can just do the callback not in softirq but in 
> regular syscall context in that case, in the return-to-userspace 
> notifier. (see TIF_USER_RETURN_NOTIFY and the USER_RETURN_NOTIFIER 
> facility)
> 
> Abusing a facility like setting need_resched artificially will 
> generally cause trouble.

If the task enqueued callbacks in the kernel, thus started a new grace period,
it might return to userspace before every CPUs have completed that grace period,
and you need that full completion to happen before invoking the callbacks.

I think you need to keep the tick in such case because you can't count on
the other CPUs to handle that completion as they may be all idle.

So when you resume to userspace and you started a GP, either you find another
CPU to handle the GP completion and callbacks executions, or you keep the tick
until you are done.
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