Re: [PATCH 5/7] ramoops: Split ftrace buffer space into per-CPU zones

[Joel Fernandes]

Hi Kees,

On Mon, Oct 10, 2016 at 4:59 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Sun, Oct 9, 2016 at 10:15 AM, Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
>> On Fri, Oct 7, 2016 at 10:28 PM, Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
>>> If FTRACE_PER_CPU flag is passed to ramoops pdata, split the space into
>>> multiple zones depending on the number of CPUs.
>>>
>>> This speeds up the performance of function tracing by about 280% in my tests as
>>> we avoid the locking. The trade off being lesser space available per CPU.  Let
>>> the ramoops user decide which option they want based on pdata flag.
>>>
>>> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
>>> ---
>>>  fs/pstore/ram.c            | 70 +++++++++++++++++++++++++++++++++++-----------
>>>  include/linux/pstore_ram.h |  3 ++
>>>  2 files changed, 56 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
>> [..]
>>> @@ -391,14 +418,21 @@ static void ramoops_free_przs(struct ramoops_context *cxt)
>>>  {
>>>         int i;
>>>
>>> -       if (!cxt->przs)
>>> -               return;
>>> +       /* Free dump PRZs */
>>> +       if (cxt->przs) {
>>> +               for (i = 0; i < cxt->max_dump_cnt; i++)
>>> +                       persistent_ram_free(cxt->przs[i]);
>>>
>>> -       for (i = 0; i < cxt->max_dump_cnt; i++)
>>> -               persistent_ram_free(cxt->przs[i]);
>>> +               kfree(cxt->przs);
>>> +               cxt->max_dump_cnt = 0;
>>> +       }
>>>
>>> -       kfree(cxt->przs);
>>> -       cxt->max_dump_cnt = 0;
>>> +       /* Free ftrace PRZs */
>>> +       if (cxt->fprzs) {
>>> +               for (i = 0; i < nr_cpu_ids; i++)
>>> +                       persistent_ram_free(cxt->przs[i]);
>>
>> I am supposed to free fprzs[i] here, instead of przs[i]. Also need to
>> check flag and free correct number of zones. Will fix for v2, sorry
>> about that.
>
> I think the cpu id needs to be bounds-checked against the size of the
> allocation. In theory, CPU hot-plug could grow the number of CPUs
> after pstore is initialized.

nr_cpu_ids is fixed to the number of possible CPUs regardless of if
hotplug is being used or not. I did a hotplug test as well to confirm
this. So if I boot on 4 CPU machine with only 2 CPUs online, then
nr_cpu_ids is 4.

Thanks,
Joel