sleep

[Tobin C. Harding]

On Tue, Nov 14, 2017 at 02:52:06PM -0200, Bruno E. O. Meneguele wrote:
> On 26-10, Tobin C. Harding wrote:
> > Is there any 'non-process' context apart from interrupt context? I had a
> > re-read of sleep sections in ldd3 and in Robert Love's book but am still
> > not totally clear on this.
> > 
> > The reason for the question is understanding when we cannot sleep.
> 
> Well, AFAIK you can simplify things to interrupt vs process context, but
> you might be aware that inside interrupt context there are others (NMI,
> HWIRQ, SOFTIRQ, ..), infos that you can find in Robert's book. What
> confused me for sometime was the 'atomic' vs 'interrupt' naming, but
> after reading Robert's book it cames to the fact that they're the same.
> 
> The general rule is that in any interrupt context you can't sleep, and
> why is that? Because interrupts aren't processes, there isn't an actual
> process that the scheduler can put to sleep and wake it up later. It's
> basically a design decision. Kernel-RT patches provide the feature of
> sleepable interrupts, since the goal there is the latency and not
> performance. That said, in short, the idea in Linux Kernel (I'm not an
> expert, far from that actually, so anyone can correct me) is to make
> interrupts atomic in behalf of simplicity and performance.
> 
> But hey, be careful, there are that talk about top vs bottom halves
> mechanisms (to defer work) right? Top are always in interrupt context,
> but bottom ones can run in process context. The most basic ones are
> Softirqs, Tasklets and Workqueues, the former is the most low-level
> available and I won't talk about it, but basicaly softirqs and tasklets
> run in interrupt context, thus can't sleep, while workqueues is handled
> in process context, thus can sleep normally (it can be rescheduled).
> 
> But as I said, I'm far from expert in the subject. What I've said here
> is just what I know about :)
> 
> Hope it helps you.

thanks for your response,
Tobin.
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