Re: [PATCH v3] lib: stackdepot: Add support to configure STACK_HASH_SIZE

[Alexander Potapenko]

On Fri, Dec 11, 2020 at 1:45 PM Vijayanand Jitta <vjitta@xxxxxxxxxxxxxx> wrote:
>
>
>
> On 12/11/2020 2:06 PM, Alexander Potapenko wrote:
> > On Thu, Dec 10, 2020 at 6:01 AM <vjitta@xxxxxxxxxxxxxx> wrote:
> >>
> >> From: Yogesh Lal <ylal@xxxxxxxxxxxxxx>
> >>
> >> Add a kernel parameter stack_hash_order to configure STACK_HASH_SIZE.
> >>
> >> Aim is to have configurable value for STACK_HASH_SIZE, so that one
> >> can configure it depending on usecase there by reducing the static
> >> memory overhead.
> >>
> >> One example is of Page Owner, default value of STACK_HASH_SIZE lead
> >> stack depot to consume 8MB of static memory. Making it configurable
> >> and use lower value helps to enable features like CONFIG_PAGE_OWNER
> >> without any significant overhead.
> >
> > Can we go with a static CONFIG_ parameter instead?
> > Guess most users won't bother changing the default anyway, and for
> > CONFIG_PAGE_OWNER users changing the size at boot time is not strictly
> > needed.
> >
> Thanks for review.
>
> One advantage of having run time parameter is we can simply set it to a
> lower value at runtime if page_owner=off thereby reducing the memory
> usage or use default value if we want to use page owner so, we have some
> some flexibility here. This is not possible with static parameter as we
> have to have some predefined value.

If we are talking about a configuration in which page_owner is the
only stackdepot user in the system, then for page_owner=off it
probably makes more sense to disable stackdepot completely instead of
setting it to a lower value. This is a lot easier to do in terms of
correctness.
But if there are other users (e.g. KASAN), their stackdepot usage may
actually dominate that of page_owner.

> >> -static struct stack_record *stack_table[STACK_HASH_SIZE] = {
> >> -       [0 ...  STACK_HASH_SIZE - 1] = NULL
> >> +static unsigned int stack_hash_order = 20;
> >
> > Please initialize with MAX_STACK_HASH_ORDER instead.
> >
>
> Sure, will update this.
>


> >> +
> >> +static int __init init_stackdepot(void)
> >> +{
> >> +       size_t size = (STACK_HASH_SIZE * sizeof(struct stack_record *));
> >> +
> >> +       stack_table = vmalloc(size);
> >> +       memcpy(stack_table, stack_table_def, size);
> >
> > Looks like you are assuming stack_table_def already contains some data
> > by this point.
> > But if STACK_HASH_SIZE shrinks this memcpy() above will just copy some
> > part of the table, whereas the rest will be lost.
> > We'll need to:
> > - either explicitly decide we can afford losing this data (no idea how
> > bad this can potentially be),
> > - or disallow storing anything prior to full stackdepot initialization
> > (then we don't need stack_table_def),
> > - or carefully move all entries to the first part of the table.
> >
> > Alex
> >
>
> The hash for stack_table_def is computed using the run time parameter
> stack_hash_order, though stack_table_def is a bigger array it will only
> use the entries that are with in the run time configured STACK_HASH_SIZE
> range. so, there will be no data loss during copy.

Do we expect any data to be stored into stack_table_def before
setup_stack_hash_order() is called?
If the answer is no, then we could probably drop stack_table_def and
allocate the table right in setup_stack_hash_order()?

> Thanks,
> Vijay
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
> member of Code Aurora Forum, hosted by The Linux Foundation



-- 
Alexander Potapenko
Software Engineer

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