Re: [Xenomai-help] Mode switch when using RT heap on ARM

[Wolfgang Grandegger <wg@domain.hid>]

Gilles Chanteperdrix wrote:
> Wolfgang Grandegger wrote:
>> Gilles Chanteperdrix wrote:
>>> Wolfgang Grandegger wrote:
>>>> Hello,
>>>>
>>>> I realized that accessing memory allocated with rt_heap_alloc() causes
>>>> mode switches on ARM i.mx31. The attached patch provides a demo program
>>>> to demonstrate the problem, which actually does *not* show up on my
>>>> PowerPC TQM5200 board.
>>> Index: xenomai-2.4.x/examples/native/rtheap.c
>>> ===================================================================
>>> --- /dev/null
>>> +++ xenomai-2.4.x/examples/native/rtheap.c
>>> @@ -0,0 +1,164 @@
>>> +#include <stdio.h>
>>> +#include <signal.h>
>>> +#include <unistd.h>
>>> +#include <sys/mman.h>
>>> +
>>> +#include <native/task.h>
>>> +#include <native/timer.h>
>>> +#include <native/heap.h>
>>> +
>>> +#define USE_HEAP
>>> +//#define USE_SIGXCPU
>>> +
>>> +#define HEAP_SIZE (1024*1024)
>>> +#define HEAP_MODE 0		/* Local heap. */
>>> +
>>> +RT_HEAP heap_desc;
>>> +RT_TASK demo_task;
>>> +
>>> +
>>> +static int block_sizes[] = {16, 20, 90, 150, 310, 800, 1000};
>>> +
>>> +/* NOTE: error handling omitted. */
>>> +
>>> +void demo(void *arg)
>>> +{
>>> +	RTIME now, previous;
>>> +	void *block;
>>> +	int sizes = sizeof(block_sizes) / sizeof(int);
>>> +	int count = 0;
>>> +	int size, err;
>>> +	int *ptr;
>>> +
>>> +#ifdef USE_SIGXCPU
>>> +	/* Ask Xenomai to warn us upon switches to secondary mode. */
>>> +	rt_task_set_mode(0, T_WARNSW, NULL);
>>> +#endif
>>> +
>>> +	/*
>>> +	 * Arguments: &task (NULL=self),
>>> +	 *            start time,
>>> +	 *            period (here: 1 s)
>>> +	 */
>>> +	rt_task_set_periodic(NULL, TM_NOW, 10000000);
>>> +	previous = rt_timer_read();
>>> +
>>> +	while (1) {
>>> +		rt_task_wait_period(NULL);
>>> +		now = rt_timer_read();
>>> +
>>> +		size = block_sizes[count % sizes];
>>> +#ifdef USE_HEAP
>>> +		/*
>>> +		 * Request a 16-bytes block, asking for a non-blocking call
>>> +		 * since only Xenomai tasks may block.
>>> +		 */
>>> +		err = rt_heap_alloc(&heap_desc, size, TM_NONBLOCK, &block);
>>> +		if (err) {
>>> +			printf("rt_heap_alloc() failed with %d\n", err);
>>> +			break;
>>> +		}
>>> +
>>> +		ptr = (int *)block;
>>> +		*ptr = 0xdeadbeef;
>>> +		if (*ptr != 0xdeadbeef) {
>>> +			printf("Write/read test to heap failed\n");
>>> +			break;
>>> +		}
>>> +
>>> +		ptr = (int *)(block + size - sizeof(int));
>>>
>>> This is not guaranteed to be aligned on an int. Could you try:
>>>
>>> 		ptr = (int *)(((unsigned long) block + size - sizeof(int)) & ~3);
>> With aligned accesses rtheap reports just3 mode switches in both modes,
>> 0 and H_NONCACHED. But already one mode switch is too much!
> 
> I hope we get several mode switches because the allocator returns
> different pages. To workaround this, we will have to cause a fault on
> each page.  We can do this either in kernel or user-space.

Doing it in kernel-space, e.g. in xnheap_init_mapped(), does not help.
Anyhow, I think the memory pages get touched in kernel space already here:

http://www.rts.uni-hannover.de/xenomai/lxr/source/ksrc/nucleus/heap.c#096

But the mode switches disappear when I touch the allocated memory (read
and write back) in __map_heap_memory() write after the __real_mmap(:

http://www.rts.uni-hannover.de/xenomai/lxr/source/src/skins/native/heap.c#031

Wolfgang.