Messages in this thread | | | Date | Thu, 8 May 2014 16:12:14 -0400 (EDT) | From | Nicolas Pitre <> | Subject | Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems |
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On Thu, 8 May 2014, Russell King - ARM Linux wrote:
> Anything which is expecting precise timings from udelay() is broken. > Firstly, udelay() does _not_ guarantee to give you a delay of at least > the requested period - it tries to give an _approximate_ delay. > > The first thing to realise is that loops_per_jiffy is calibrated with > interrupts _on_, which means that the calculated loops_per_jiffy is > the number of iterations in a jiffy _minus_ the time it takes for the > timer interrupt to be processed. This means loops_per_jiffy will > always be smaller than the number of loops that would be executed > within the same period. > > This leads to udelay() always producing slightly shorter than > requested delays - this is quite measurable.
OK, this is certainly bad. Hopefully it won't be that far off like it would when the CPU is in the middle of a clock freq transition.
> It gets worse when you consider the utter mess that the L2 cache code > is in - where on many platforms we calibrate udelay() with the cache > off, which results in loops_per_jiffy being smaller than it would > otherwise be (meaning shorter delays.) > > So, that's two reasons there why loops_per_jiffy will be smaller than > it should be at boot, and therefore udelay() will be inaccurate. > > Another reason udelay() can be unaccurate is if interrupts are on, and > you have USB present. USB interrupt processing can take on the order > of 10s of milliseconds even on 800MHz or faster ARM CPUs. If you > receive one of those mid-udelay(), your CPU will be occupied elsewhere. > > Another reason is preempt. If the kernel can preempt during udelay(), > your delay will also be much longer than you requested. No, disabling > preemption in udelay() is not on, that would increase preemption > latency. > > So, the only /real/ solution if you want proper delays is for udelay() > to use a timer or counter, and this is should always the preferred > method where it's available. Quite rightly, we're not hacking udelay() > stuff to work around not having that, or if someone configures it out.
What about using a default based on ktime_get(), or even sched_clock(), when SMP and cpufreq are configured in?
Nicolas
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