Re: CPUfreq - udelay() interaction issues

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Saravana Kannan (skannan@codeaurora.org) wrote:
[...]
>
> Seems a bit more complicated than what I had in mind. This is touching  
> the scheduler I think we can get away without having to. Also, there is  
> no simple implementation for the "slowpath" that can guarantee the delay  
> without starting over the loop and hoping not to get interrupted or just  
> giving up and doing a massively inaccurate delay (like msleep, etc).

Not necessarily. Another way to do it: we could keep the udelay loop counter in
the task struct. When ondemand changes frequency, and upon migration, this
counter would be adapted to the current cpu frequency.

>
> I was thinking of something along the lines of this:
>
> udelay()
> {
>   if (!is_atomic())

see hardirq.h:

/*
 * Are we running in atomic context?  WARNING: this macro cannot
 * always detect atomic context; in particular, it cannot know about
 * held spinlocks in non-preemptible kernels.  Thus it should not be
 * used in the general case to determine whether sleeping is possible.
 * Do not use in_atomic() in driver code.
 */
#define in_atomic()     ((preempt_count() & ~PREEMPT_ACTIVE) != PREEMPT_INATOMIC_BASE)

Sorry, your scheme is broken on !PREEMPT kernels.

> 	down_read(&freq_sem);
>   /* else
> 	do nothing since cpufreq can't interrupt you.
>   */

This comment seems broken. in_atomic() can return true because preemption is
disabled, thus letting cpufreq interrupts coming in.

>
>   call usual code since cpufreq is not going to preempt you.
>
>   if (!is_atomic())
> 	up_read(&freq_sem);
> }
>
> __cpufreq_driver_target(...)
> {
>   down_write(&freq_sem);
>   cpufreq_driver->target(...);
>   up_write(&freq_sem);
> }
>
> In the implementation of the cpufreq driver, they just need to make sure  
> they always increase the LPJ _before_ increasing the freq and decrease  
> the LPJ _after_ decreasing the freq. This is make sure that when an  
> interrupt handler preempts the cpufreq driver code (since atomic  
> contexts aren't looking at the r/w semaphore) the LPJ value will be good  
> enough to satisfy the _at least_ guarantee of udelay().
>
> For the CPU switching issue, I think the solution I proposed is quite  
> simple and should work.

You mean this ?

>>>> udelay(us)
>>>> {
>>>>    set cpu affinity to current CPU;
>>>>    Do the usual udelay code;
>>>>    restore cpu affinity status;
>>>> }

Things like lock scalability and performance degradations comes to my mind. We
can expect some drivers to make very heavy use of udelay(). This should not
bring a 4096-core box to its knees. sched_setaffinity() is very far from being
lightweight, as it locks cpu hotplug (that's a global mutex protecting a
refcount), allocates memory, manipulates cpumasks, etc...

>
> Does my better explained solution look palatable?

Nope, not on a multiprocessor system.

Thanks,

Mathieu



-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com