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Re: Scheduler startup question


Hi Michael,

I might remember wrong but I think in case of SMP target, the lock is not taken in Vector.S but directly after entering interrupt_end. Of course this is spinlock based so it might delay posting/scheduling of the DSR.

Christophe

On 3/2/2014 9:19 PM, Michael Jones wrote:
Jurgen,

I think I fully understand how the scheduler locking works during interrupt now. Vectors.S takes the lock, and interrupt_end clears it. However, the normal technique of incrementing the lock count does not work with SMP. The problem is that another CPU may have the lock. Incrementing anyway leads to assertions. Attempting to take the lock with the spinlock can lead to deadlocks or an unresponsive network application.

So I changed things so that in Vectors.S, during an interrupt, an attempt at locking is made. This means trying to take a spinlock that might fail. If the lock is taken, interrupt_end is called. If the lock fails, interrupt_end is not called.

This means that a DSR may not be posted on that interrupt. This can cause some latency based on the real time clock interrupt rate, or time until a thread switch. However, it is stable and assertion free. Also, a HAL could implement a timeout on the try spinlock which might reduce latency.

To support the try and testing if the lock was taken, I had to add some functions to the kernel. The following wiki page has been updated to reflect the kernel changes.

https://sourceforge.net/p/ecosfreescale/wiki/SMP%20Kernel/

Anyone with SMP knowledge might want to take a look. There may be better solutions to some of these problems. But at least for now, the IMX6 SMP HAL seems stable and I can run IO intensive Lua scripts over telnet reliably, even when the client aborts.

The client abort means telnet has to kill a thread. This was quite a challenge. Telnet is creating a separate heap for Lua so it can kill the thread and reclaim memory. The remaining problem is closing file handles. I still get some assertions when a handle is sometimes killed by a thread that does not own it. I don't think that can be solved without adding some new functions dedicated to clean up of file handles by an outside thread.

Mike



On Feb 26, 2014, at 11:40 PM, Lambrecht Jürgen <J.Lambrecht@TELEVIC.com> wrote:

As far as I know the scheduler is started after cyg_user_start(), used by your application to initialize everything.  Do you use cyg_user_start?


Verzonden vanaf Samsung Mobile



-------- Oorspronkelijk bericht --------
Van: Michael Jones <mjones@linear.com>
Datum:
Aan: ecos discuss <ecos-discuss@sourceware.org>
Onderwerp: [ECOS] Scheduler startup question


I have a question about proper scheduler locking startup behavior.

The context is I am cleaning up my iMX6 HAL and attempting to make things work without a couple of kernel hacks I added to make it work.

The question has to do with sched_lock. By default this has a value of 1, so during startup the scheduler is locked.

When there is an interrupt, sched_lock is incremented in Vectors.S, and decremented in interrupt_end.

However, I am getting an assert in sync.h which is part of the BSD stack. The assert is because it expects the lock to be zero.

The question is, during the startup process, how does the lock get set to zero after initialization? Is it supposed to stay 1 while hardware is initialized and through all the constructors, etc? Is it cleared by the scheduler somehow? Is the HAL supposed to zero it at some point during startup?

My HAL is part of the ARM hal, so if this is device specific, it is the ARM HAL I am working with.

Mike
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