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Subject: Re: [PATCH v3] lib: stackdepot: Add support to configure
 STACK_HASH_SIZE
From:   Vijayanand Jitta <vjitta@codeaurora.org>
To:     Alexander Potapenko <glider@google.com>
Cc:     Minchan Kim <minchan@kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        dan.j.williams@intel.com, broonie@kernel.org,
        Masami Hiramatsu <mhiramat@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Konovalov <andreyknvl@google.com>, qcai@redhat.com,
        ylal@codeaurora.org, vinmenon@codeaurora.org,
        kasan-dev <kasan-dev@googlegroups.com>
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Message-ID: <6110a26b-dc87-b6f9-e679-aa60917403de@codeaurora.org>
Date:   Mon, 21 Dec 2020 16:44:36 +0530
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On 12/18/2020 2:10 PM, Vijayanand Jitta wrote:
> 
> 
> On 12/17/2020 4:24 PM, Alexander Potapenko wrote:
>>>> Can you provide an example of a use case in which the user wants to
>>>> use the stack depot of a smaller size without disabling it completely,
>>>> and that size cannot be configured statically?
>>>> As far as I understand, for the page owner example you gave it's
>>>> sufficient to provide a switch that can disable the stack depot if
>>>> page_owner=off.
>>>>
>>> There are two use cases here,
>>>
>>> 1. We don't want to consume memory when page_owner=off ,boolean flag
>>> would work here.
>>>
>>> 2. We would want to enable page_owner on low ram devices but we don't
>>> want stack depot to consume 8 MB of memory, so for this case we would
>>> need a configurable stack_hash_size so that we can still use page_owner
>>> with lower memory consumption.
>>>
>>> So, a configurable stack_hash_size would work for both these use cases,
>>> we can set it to '0' for first case and set the required size for the
>>> second case.
>>
>> Will a combined solution with a boolean boot-time flag and a static
>> CONFIG_STACKDEPOT_HASH_SIZE work for these cases?
>> I suppose low-memory devices have a separate kernel config anyway?
>>
> 
> Yes, the combined solution will also work but i think having a single
> run time config is simpler instead of having two things to configure.
> 

To add to it we started of with a CONFIG first, after the comments from
Minchan (https://lkml.org/lkml/2020/11/3/2121) we decided to switch to
run time param.

Quoting Minchan's comments below:

"
1. When we don't use page_owner, we don't want to waste any memory for
stackdepot hash array.
2. When we use page_owner, we want to have reasonable stackdeport hash array

With this configuration, it couldn't meet since we always need to
reserve a reasonable size for the array.
Can't we make the hash size as a kernel parameter?
With it, we could use it like this.

1. page_owner=off, stackdepot_stack_hash=0 -> no more wasted memory
when we don't use page_owner
2. page_owner=on, stackdepot_stack_hash=8M -> reasonable hash size
when we use page_owner.
"

Thanks,
Vijay
>> My concern is that exposing yet another knob to users won't really
>> solve their problems, because the hash size alone doesn't give enough
>> control over stackdepot memory footprint (we also have stack_slabs,
>> which may get way bigger than 8Mb).
>>
> 
> True, stack_slabs can consume more memory but they consume most only
> when stack depot is used as they are allocated in stack_depot_save path.
> when stack depot is not used they consume 8192 * sizeof(void) bytes at
> max. So nothing much we can do here since static allocation is not much
> and memory consumption depends up on stack depot usage, unlike
> stack_hash_table where 8mb is preallocated.
> 

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