On Fri, Feb 19, 2016 at 6:55 PM, haris iqbal <haris.p...@gmail.com> wrote:
> On Fri, Feb 19, 2016 at 6:00 PM, Dmitry V. Levin <l...@altlinux.org> wrote:
>> Hi,
>>
>> On Fri, Feb 19, 2016 at 05:47:40PM +0530, haris iqbal wrote:
>>> Hello.
>>>
>>> I was going through the code of strace.c, when I found this line in
>>> init() function.
>>>
>>> tcbtabsize = argc; /* Surely enough for all -p args. */
>>
>>> That set me thinking is it really is what I think it is. Can it really
>>> be that strace will allocate that many tcbtab pointers and tcp
>>> structures as many arguments I give to the program.
>>
>> The fun of the situation is that after commit v4.6-258-ge8172b7
>> this is no longer enough for all -p args.
>>
>>> So I set out to find out whether there is a way to make strace take a
>>> lot of memory, without actually using it.
>>>
>>> Firstly I found out that if there is a way to give large number of
>>> arguments and still make strace work as it should. It turns out that
>>> there is. One can give as many -i (or some other such arguments) as
>>> arguments and the strace would work properly.
>>>
>>> Then, I wanted to make sure whether the code is actually allocating
>>> that many resources as as there are arguments. So I went into the
>>> cleanup() function that is called through error_msg_and_die(). And
>>> there I added debugging code to see how many of them were actually
>>> allocated. It turns out that they were allocated.
>>> I added a print statement in the for loop of cleanup(). It ran for as
>>> many number of times as there were arguments. Although the usefull one
>>> was the first one only, which had the pid of the process being traced
>>> (strace was run with -p <pid> option).
>>>
>>> So, summing it all up. I ran a script which gave strace a -p option
>>> and a process pid to trace. Along with that some 600000 -i options. It
>>> ran successfully with a memory consumption of around 100mb. Thats a
>>> lot for a small system with limited memory.
>>
>> OK, so what would you suggest to change in initial tcb table allocation?
>
> There are still many portions of code which I have not understood
> properly, but I am to make a suggestion with whatever knowledge I have
> of the code. I would say to postpone the allocation till alloctcb() is
> called. But I can see many places where alloctcb() is called is by
> checking whether tcp->pid is present or not. And for that check, one
> would need an allocated structure with a 0 value.
>
> Are we allocating tcb table for multiple -p options only or does it
> have some other purpose?
>
Ok. I have come up with a separate memory model for tcbtab. In this
model, we will use a linked list instead of a global array of pointers
tcbtab.
The structure
struct s_tcbtab
{
struct tcb* data;
struct s_tcbtab* next;
}
And a global head of the linked list.
struct s_tcbtab* head_tcbtab = NULL;
Now, no need for the following code in init()
/* Allocate the initial tcbtab. */
tcbtabsize = argc; /* Surely enough for all -p args. */
tcbtab = xcalloc(tcbtabsize, sizeof(tcbtab[0]));
tcp = xcalloc(tcbtabsize, sizeof(*tcp));
for (tcbi = 0; tcbi < tcbtabsize; tcbi++)
tcbtab[tcbi] = tcp++;
tcbtabsize will be 0.
The changes will be made in the alloctab(). The code that will be added is
if(head_tcbtab == NULL)
{
// for the first allocation
head_tcbtab = malloc(sizeof(s_tcbtab));
head_tcbtab->next = NULL
tcp = head_tcbtab -> data = malloc(sizeof(struct tcb));
tcbtabsize++;
}
else
{
// further adding tabs
temp_tcb = head_tcbtab;
while(temp_tcb->next != NULL)
temp_tcb = temp_tcb->next;
temp_tcb->next = malloc(sizeof(s_tcbtab));
temp_tcb = temp_tcb->next;
temp_tcb->next = NULL;
tcp = temp_tcb->data = malloc(sizeof(struct tcb));
tcbtabsize++;
}
after this, the code can continue with the normal
memset(tcp, 0, sizeof(*tcp));
tcp->pid = pid;
#if SUPPORTED_PERSONALITIES > 1
tcp->currpers = current_personality;
.
.
return tcp;
Additional code has to be changed at places where there is a for loop
iteration like in startup_attach()
we have to change this
for (tcbi = 0; tcbi < tcbtabsize; tcbi++)
{
tcp = tcbtab[tcbi];
..
to
tcbi = head_tcbtab;
while(tcbi != NULL)
{
tcp = tcbi->data;
..
>>
>>
>> --
>> ldv
>>
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>
>
>
> --
>
> With regards,
>
> Md Haris Iqbal,
> Placement Coordinator, MTech IT
> NITK Surathkal,
> Contact: +91 8861996962
--
With regards,
Md Haris Iqbal,
Placement Coordinator, MTech IT
NITK Surathkal,
Contact: +91 8861996962
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