Re: [PATCH v1] kernel/trace:check the val against the available mem

[Joel Fernandes]

Hi Steven,

On Fri, Mar 30, 2018 at 2:42 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Fri, 30 Mar 2018 17:30:31 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
>> I'll take a look at si_mem_available() that Joel suggested and see if
>> we can make that work.
>
> Wow, this appears to work great! Joel and Zhaoyang, can you test this?
>
> -- Steve
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index a2fd3893cc02..32a803626ee2 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1164,6 +1164,11 @@ static int __rb_allocate_pages(long nr_pages, struct list_head *pages, int cpu)
>         struct buffer_page *bpage, *tmp;
>         long i;
>
> +       /* Check if the available memory is there first */
> +       i = si_mem_available();
> +       if (i < nr_pages)

Does it make sense to add a small margin here so that after ftrace
finishes allocating, we still have some memory left for the system?
But then then we have to define a magic number :-|

> +               return -ENOMEM;
> +

I tested in Qemu with 1GB memory, I am always able to get it to fail
allocation even without this patch without causing an OOM. Maybe I am
not running enough allocations in parallel or something :)

The patch you shared using si_mem_available is working since I'm able
to allocate till the end without a page allocation failure:

bash-4.3# echo 237800 > /d/tracing/buffer_size_kb
bash: echo: write error: Cannot allocate memory
bash-4.3# echo 237700 > /d/tracing/buffer_size_kb
bash-4.3# free -m
             total         used         free       shared      buffers
Mem:           985          977            7           10            0
-/+ buffers:                977            7
Swap:            0            0            0
bash-4.3#

I think this patch is still good to have, since IMO we should not go
and get page allocation failure (even if its a non-OOM) and subsequent
stack dump from mm's allocator, if we can avoid it.

Tested-by: Joel Fernandes <joelaf@xxxxxxxxxx>

thanks,

- Joel