Messages in this thread | | | Subject | Re: [PATCH v3] lib: stackdepot: Add support to configure STACK_HASH_SIZE | From | Vijayanand Jitta <> | Date | Wed, 16 Dec 2020 18:36:37 +0530 |
| |
On 12/16/2020 1:56 PM, Alexander Potapenko wrote: > On Wed, Dec 16, 2020 at 4:43 AM Vijayanand Jitta <vjitta@codeaurora.org> wrote: >> >> >> >> On 12/14/2020 4:02 PM, Vijayanand Jitta wrote: >>> >>> >>> On 12/14/2020 3:04 PM, Alexander Potapenko wrote: >>>> On Mon, Dec 14, 2020 at 5:02 AM Vijayanand Jitta <vjitta@codeaurora.org> wrote: >>>>> >>>>> >>>>> >>>>> On 12/11/2020 6:55 PM, Alexander Potapenko wrote: >>>>>> On Fri, Dec 11, 2020 at 1:45 PM Vijayanand Jitta <vjitta@codeaurora.org> wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>> On 12/11/2020 2:06 PM, Alexander Potapenko wrote: >>>>>>>> On Thu, Dec 10, 2020 at 6:01 AM <vjitta@codeaurora.org> wrote: >>>>>>>>> >>>>>>>>> From: Yogesh Lal <ylal@codeaurora.org> >>>>>>>>> >>>>>>>>> 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()? >>>>>> >>>>> >>>>> Yes, we do see an allocation from stack depot even before init is called >>>>> from kasan, thats the reason for having stack_table_def. >>>>> This is the issue reported due to that on v2, so i added stack_table_def. >>>>> https://lkml.org/lkml/2020/12/3/839 >>>> >>>> But at that point STACK_HASH_SIZE is still equal to 1L << >>>> MAX_STACK_HASH_ORDER, isn't it? >>>> Then we still need to take care of the records that fit into the >>>> bigger array, but not the smaller one. >>>> >>> >>> At this point early_param is already called which sets stack_hash_order. >>> So, STACK_HASH_SIZE will be set to this updated value and not >>> MAX_STACK_HASH_SIZE.So, no additional entires in the bigger array. >>> >>> Thanks, >>> Vijay >>> >> >> Let me know if there are any other concerns. > > I still think there are implicit assumptions that should at least be > written down in the comments.
Sure, will add the comments.
> As far as I understand the code, here is what happens here: > > 0. No stacks are recorded. > 1. early_param is called to set stack_hash_order to a value > potentially smaller than MAX_STACK_HASH_SIZE. > 2. KASAN (or other users) records some stacks into stack_table_def > (capped at new STACK_HASH_SIZE) > 3. init_stackdepot() allocates a new stack_table and copies the > contents of stack_table_def into it > 4. Further stacks are recorded into the new stack_table > > If this is how the things work, I agree we don't need to account for > the part of stack_table_def past STACK_HASH_SIZE. > Not allocating stack_table when setting stack_hash_order is probably > also justified, as we don't have SLAB or vmalloc at that point. >
That's Right.
> But I am still curious if a runtime parameter that disables the > stackdepot completely will solve your problem. > Allocating a small amount of memory when you actually don't want to > allocate any sounds suboptimal. >
I think disabling stack depot completely should be fine, we can make STACK_HASH_SIZE as runtime parameter instead of stack_hash_order and set stack_hash_size to 0 to disable stack depot completely.
Minchan, This should be fine right ? Do you see any issue with disabling stack depot completely ?
Thanks, Vijay
>> Thanks, >> Vijay >> >>>>> Thanks, >>>>> Vijay >>>>> >>>>>>> Thanks, >>>>>>> Vijay >>>>>>> >>>>>>> -- >>>>>>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a >>>>>>> member of Code Aurora Forum, hosted by The Linux Foundation >>>>>> >>>>>> >>>>>> >>>>> >>>>> -- >>>>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a >>>>> member of Code Aurora Forum, hosted by The Linux Foundation >>>> >>>> >>>> >>> >> >> -- >> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a >> member of Code Aurora Forum, hosted by The Linux Foundation > > >
-- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
| |