Messages in this thread | | | Date | Tue, 27 Apr 2010 16:41:39 -0700 | From | Saravana Kannan <> | Subject | Re: CPUfreq - udelay() interaction issues |
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Hi Mathieu,
Thanks for taking the time to provide your input. More responses below.
Mathieu Desnoyers wrote: > * 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.
This will take us back to the scalability problem because we now have to go through every process running on a CPU to update their udelay loop counters whenever the CPU freq changes.
>> 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.
If it's a !PREEMPT kernel, we don't have to worry about the CPUfreq changing on us. CPU freq is changed in a deferrable work queue context.
>> 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.
As mentioned earlier, cpufreq change can't happen when udelay is running in !PREEMPT kernel (which is where in_atomic() won't work). Btw, I actually wasn't referring to the real in_atomic() macro (I remembered it having limitations). But now that you mentioned the limitation, it might not be a problem after all.
>> 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...
Hmm... set affinity does seem more complicated than what I expected.
>> Does my better explained solution look palatable? > > Nope, not on a multiprocessor system.
Yes, set affinity seems to be a problem.
Didn't get to work on this for the past few days. Let me think more about this before I get back. In the mean time, if you can come up with a relatively simple solution without scalability issues, I would be glad to drop my existing solution.
Thanks again for the input.
-Saravana
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