Session: Developer's Diary: It's About Time ELCE 2011

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Session Details

ELCE 2011, Prague
Wolfram Sang
| HD | SD thanks to | Free Electrons


A Talk about timeouts and delays.




Transcribed by
Peter Huewe
Verified by
1 -

Introduction of and by Wolfram Sang

My talks which I usually entitle “Developers Diary” I want to share experiences I made in my everyday work I want to [show you] problems I encountered, things I've found out [and] try to give best practices. As Linux in general is a very huge project and I pretty much want [...]

A lot of people think “oh my problem [fits here] as well” - so if you have something to add, feel free to do so. I hope at the end of the talk we're all a bit enlightened than before.

This time I want to talk about dealing with time in device drivers and I chose to talk about two groups:

The first I will be talking about [are] delays, when you deal with hardware and sometime you activate something and have to wait for a certain amount of time until something happens.

And the second will be timeouts, you do something and if it not does something in a certain time you'll have to deal with it.

Both are not very complicated matters [on their own] but let's have a look:

- The basics of delays:

There are two kinds, usually, One [called] busy waiting: the cpu burns cycles while waiting and the other one is blocking: you say “okay I’m going to wait, let me go to sleep, and you’ll come back later so the cpu can do something in between”.

[Poll:] Who knew this before? Okay that's what I kinda expected, so I'm not telling you rocket science. I know that.

[Poll:]Who is writing device drivers for the linux kernel here? [few hands rise] Great! That's the audience I was hoping for. So let's get a bit interactive - [Q] What kind of delay functions do you know [responses by the audience] - schedule Does it delay? - udelay - mdelay - sleep – which ones - usleep -msleep we're getting into discussion already -usleep_range -ndelay ? - nanosleep? - msleep_interruptible Not bad – was it all? [Guy in the audience joking] a for cycle loop Is there an sdelay`?

So let’start with the solution:

- ndelay
- udelay
- mdelay
luckily no sdelay
no usleep
- usleep_range  which you can give a time frame
- msleep
- msleep_interruptible
- ssleep

But it's already interesting to see that here, as you just recognized here are quite a bunch of people doing kernel code and there seems to be some agreements and disagreements of what is available.

And now think about implicit things which might happen in the back[ground] [...]

There can be things happening you're not expecting or whatever so it's really pays off to have a look at this once in a while so you get the foundations right so you're really knowing what you're doing when you're using such functions.

Let’s have a short look at the delay headers. [opens include/linux/delay.h]

I’ll call this talk a little bit ARM specific - I hope nobody will be angry about being ARM specific, but it basically applies to all platform - because one of the thing that’s interesting is that the delay functions for ARM are architecture specific.

So this is a general Linux header file – and you can see here in those lines that [...] for mdelay there is a fallback so if the architecture does not define it basically falls back to udelay.

There’s also an ndelay, the general fall back if the architecture does not define one is based on udelay – so that udelay seems to be a pretty important function in all that.

We see msleep, msleep_interruptible, usleep_range and ssleep, they are all defined here, so they are independent of the architecture. If we look at the architecture specific implementation of udelay, which we just found out is a very core function, you see that in the arm case it’s not even a function but the first […] encounter with a define, which tracks if it’s a constant value or not.

That’s the places where you have to look if you want to dive in deeper.

So how I got interested in the topic was we had a customer problem and he said “yeah the system is working fine but when we press the touch the system becomes unusable and further analysis have shown that 25% of the cpu was busy doing something”.

I first expected an interrupt storm or whatever but that couldn’t be found out so I started looking at code, especially at the touchscreen driver and this was the thing I found. [slide shows the function in drivers/input/touchscreen/wm9712.c]

This is a delay inside the driver, which takes a value from the [lookup]table and [the] array offset is given to the function, which is (as you can see on the next slide) more or less a module parameter. It says “okay we need that to get the correct value. The delay can be set by setting the delay to n where n is the array position of the delay in the array table below.” This is not very transparent to the user; it is said “long delays for 1ms are supported for completeness but are not recommended” – that’s a very useful comment but it’s totally invisible for the user using the module.

That’s what happened to our customer – I think it was also caused little bit broken hardware – he didn’t get the touch[screen] working so here’s the “mighty table” which is feeding the udelay. The standard is this one [indicating 200us], so it will in general burn like 200 us, our customer ended up being here [indicating 3ms] – so on a regular basis for every poll this routine was nearly burning 3ms – and this was indeed the cause for the total cpu loss, so it just lost all the cpu power doing nothing. Sadly this is still in the kernel – because once the problem was found out, the customer said “yeah we can fix that”, and took the hardware away from me, so I think they did some nasty stuff to get it working – so this is still present in the kernel, sadly.

So this gave me the idea – udelay is a pretty dangerous function.

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