Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it

[Gilad Ben-Yossef]

On Thu, Mar 22, 2012 at 7:18 PM, Chris Metcalf <cmetcalf@xxxxxxxxxx> wrote:
> On 3/22/2012 3:38 AM, Gilad Ben-Yossef wrote:
>> On Wed, Mar 21, 2012 at 4:54 PM, Christoph Lameter <cl@xxxxxxxxx> wrote:
>>> On Wed, 21 Mar 2012, Frederic Weisbecker wrote:
>>>
>>>> If RCU is waiting for the current CPU to complete a grace
>>>> period, don't turn off the tick. Unlike dynctik-idle, we
>>>> are not necessarily going to enter into rcu extended quiescent
>>>> state, so we may need to keep the tick to note current CPU's
>>>> quiescent states.
>>> Is there any way for userspace to know that the tick is not off yet due to
>>> this? It would make sense for us to have busy loop in user space that
>>> waits until the OS has completed all processing if that avoids future
>>> latencies for the application.
>>>
>> I previously suggested having the user register to receive a signal
>> when the tick
>> is turned off. Since the tick is always turned off the user task is
>> the current task
>> by design, *I think* you can simply mark the signal pending when you
>> turn the tick off.
>>
>> The user would register a signal handler to set a flag when it is
>> called and then busy
>> loop waiting for a flag to clear.
>
> This sounds plausible, but the kernel would have to know that the tick not
> only was stopped currently, but also would still be stopped when the signal
> handler's sigreturn syscall was performed.

Well, I'd say send a signal when the tick is turned off and another
signal when it's
turned on again.


> The problem we've seen is that
> it's sometimes somewhat nondeterministic when the kernel might decide it
> needed some more ticking, once you let kernel code start to run.  For
> example, for RCU ops the kernel can choose to ignore the nohz cpuset cores
> when they're running userspace code only, but as soon as they get back into
> the kernel for any reason, you may need to schedule a grace period, and so
> just returning from the "you have no more ticks!" signal handler ends up
> causing ticks to be scheduled.

There is no real difference from the user stand point between the
return signal sys call
doing something that causes the tick to be turned on and an IPI or
timer that turns on
the tick a nano second after the signal return system call returned.

The return signal syscall setting the tick on is just a private,
though annoying, case of the
tick getting turned on by something.

> The approach we took for the Tilera dataplane mode was to have a syscall
> that would hold the task in the kernel until any ticks were done, and only
> then return to userspace.  (This is the same set_dataplane() syscall that
> also offers some flags to control and debug the dataplane stuff in general;
> in fact the "hold in kernel" support is a mode we set for all syscalls, to
> keep things deterministic.)  This way the "busy loop" is done in the
> kernel, but in fact we explicitly go into idle until the next tick, so it's
> lower-power.
>

Yes, I saw that. My gripe with it is that puts the policy of what to do
while we wait for the tick to go away in the kernel. I usually hate the
kernel to take decisions on what to do. I want it to give mechanisms
and let the programmer set the policy.- e.g. have a led blink while
you're waiting for the
and the tick to go away so that the poor end user will know we are
still waiting for
the starts to align just right...

I'm not sure that is so big a deal, but that is why I thought of a
signal handler.

> An alternative approach, not so good for power but at least avoiding the
> "use the kernel to avoid the kernel" aspect of signals, would be to
> register a location in userspace that the kernel would write to when it
> disabled the tick, and userspace could then just spin reading memory.
>

That's cool for letting you know when the tick goes away but not for alarming
you when it suddenly came back... :-)

Gilad

> --
> Chris Metcalf, Tilera Corp.
> http://www.tilera.com
>



-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@xxxxxxxxxxxxx
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
http://benyossef.com

"If you take a class in large-scale robotics, can you end up in a
situation where the homework eats your dog?"
 -- Jean-Baptiste Queru
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