I actually managed to modify entry.S to call C function to set up large
stack lazily and it works. I will be sending patch soon.
On Friday, February 2, 2018 at 4:43:07 PM UTC-5, Waldek Kozaczuk wrote:
>
> I am trying to implement the idea that Nadav proposed to save memory when
> implementing callstack switch during syscall. Here is excerpt from our
> conversation:
>
> "
>
>>
>> I was also wondering that existing implementation of syscall stack makes
>> every app thread allocate it upon creation eagerly whether syscall are
>> going to be made or not. For example java apps (which I believe use libc
>> functions) would be penalized by memory usage. I was wondering how
>> difficult it would be to make allocation of syscall stack lazily only when
>> first syscall is made.
>>
>
> The problem is that we won't have a stack to do this allocation work on.
>
> One thing we could do have for all threads a tiny syscall stack (doesn't
> even need to be a whole page, can be smaller!), and the first thing a
> syscall does is
> to check if this thread has this tiny stack, and if it does it allocates a
> different one. The stack needs to be just big enough for malloc() to run.
> We could
> even avoid calling malloc() directly and just send a message to a
> different thread to do this for us, but I think this will be an overkill.
> "
>
> So I am trying to avoid messing with assembly as much as possible. Is it
> possible to implement it almost all in C by calling a function from
> linux.cc/syscall_wrapper() that would lazily allocate larger stack?
> Obviously we would still need to change RSP register to new value and then
> restore it back with could be done with asm("...") instruction. Is it
> possible?
>
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