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

[Ingo Molnar]

* Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:

> 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.

We'll have a scheduler tick in any case, which will act as a 
worst-case RCU tick.

My main point is that we need to check whether this solution improves 
performance over the current softirq code. I think there's a real 
chance that it improves things like VFS workloads, because it 
provides (much!) lower grace period latencies hence provides 
fundamentally better cache locality.

If a workload pays the cost of frequent scheduling then it might as 
well use a beneficial side-effect of that scheduling: high-freq grace 
periods ...

If it improves performance we can figure out all the loose ends. If 
it doesnt then the loose ends are not worth worrying about.

Thanks,

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/