Re: [RFC PATCH bpf-next 0/9] mm, bpf: Add BPF into /proc/meminfo

[Vlastimil Babka <vbabka@suse.cz>]

On 12/13/22 15:56, Hyeonggon Yoo wrote:
> On Tue, Dec 13, 2022 at 07:52:42PM +0800, Yafang Shao wrote:
>> On Tue, Dec 13, 2022 at 1:54 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>> >
>> > On 12/12/22 01:37, Yafang Shao wrote:
>> > > Currently there's no way to get BPF memory usage, while we can only
>> > > estimate the usage by bpftool or memcg, both of which are not reliable.
>> > >
>> > > - bpftool
>> > >   `bpftool {map,prog} show` can show us the memlock of each map and
>> > >   prog, but the memlock is vary from the real memory size. The memlock
>> > >   of a bpf object is approximately
>> > >   `round_up(key_size + value_size, 8) * max_entries`,
>> > >   so 1) it can't apply to the non-preallocated bpf map which may
>> > >   increase or decrease the real memory size dynamically. 2) the element
>> > >   size of some bpf map is not `key_size + value_size`, for example the
>> > >   element size of htab is
>> > >   `sizeof(struct htab_elem) + round_up(key_size, 8) + round_up(value_size, 8)`
>> > >   That said the differece between these two values may be very great if
>> > >   the key_size and value_size is small. For example in my verifaction,
>> > >   the size of memlock and real memory of a preallocated hash map are,
>> > >
>> > >   $ grep BPF /proc/meminfo
>> > >   BPF:             1026048 B <<< the size of preallocated memalloc pool
>> > >
>> > >   (create hash map)
>> > >
>> > >   $ bpftool map show
>> > >   3: hash  name count_map  flags 0x0
>> > >           key 4B  value 4B  max_entries 1048576  memlock 8388608B
>> > >
>> > >   $ grep BPF /proc/meminfo
>> > >   BPF:            84919344 B
>> > >
>> > >   So the real memory size is $((84919344 - 1026048)) which is 83893296
>> > >   bytes while the memlock is only 8388608 bytes.
>> > >
>> > > - memcg
>> > >   With memcg we only know that the BPF memory usage is less than
>> > >   memory.usage_in_bytes (or memory.current in v2). Furthermore, we only
>> > >   know that the BPF memory usage is less than $MemTotal if the BPF
>> > >   object is charged into root memcg :)
>> > >
>> > > So we need a way to get the BPF memory usage especially there will be
>> > > more and more bpf programs running on the production environment. The
>> > > memory usage of BPF memory is not trivial, which deserves a new item in
>> > > /proc/meminfo.
>> > >
>> > > This patchset introduce a solution to calculate the BPF memory usage.
>> > > This solution is similar to how memory is charged into memcg, so it is
>> > > easy to understand. It counts three types of memory usage -
>> > >  - page
>> > >    via kmalloc, vmalloc, kmem_cache_alloc or alloc pages directly and
>> > >    their families.
>> > >    When a page is allocated, we will count its size and mark the head
>> > >    page, and then check the head page at page freeing.
>> > >  - slab
>> > >    via kmalloc, kmem_cache_alloc and their families.
>> > >    When a slab object is allocated, we will mark this object in this
>> > >    slab and check it at slab object freeing. That said we need extra memory
>> > >    to store the information of each object in a slab.
>> > >  - percpu
>> > >    via alloc_percpu and its family.
>> > >    When a percpu area is allocated, we will mark this area in this
>> > >    percpu chunk and check it at percpu area freeing. That said we need
>> > >    extra memory to store the information of each area in a percpu chunk.
>> > >
>> > > So we only need to annotate the allcation to add the BPF memory size,
>> > > and the sub of the BPF memory size will be handled automatically at
>> > > freeing. We can annotate it in irq, softirq or process context. To avoid
>> > > counting the nested allcations, for example the percpu backing allocator,
>> > > we reuse the __GFP_ACCOUNT to filter them out. __GFP_ACCOUNT also make
>> > > the count consistent with memcg accounting.
>> >
>> > So you can't easily annotate the freeing places as well, to avoid the whole
>> > tracking infrastructure?
>> 
>> The trouble is kfree_rcu().  for example,
>>     old_item = active_vm_item_set(ACTIVE_VM_BPF);
>>     kfree_rcu();
>>     active_vm_item_set(old_item);
>> If we want to pass the ACTIVE_VM_BPF into the deferred rcu context, we
>> will change lots of code in the RCU subsystem. I'm not sure if it is
>> worth it.
> 
> (+Cc rcu folks)
> 
> IMO adding new kfree_rcu() varient for BPF that accounts BPF memory
> usage would be much less churn :)

Alternatively, just account the bpf memory as freed already when calling
kfree_rcu()? I think the amount of memory "in flight" to be freed by rcu is
a separate issue (if it's actually an issue) and not something each
kfree_rcu() user should think about separately?

>> 
>> >  I thought there was a patchset for a whole
>> > bfp-specific memory allocator, where accounting would be implemented
>> > naturally, I would imagine.
>> >
>> 
>> I posted a patchset[1] which annotates both allocating and freeing
>> several months ago.
>> But unfortunately after more investigation and verification I found
>> the deferred freeing context is a problem, which can't be resolved
>> easily.
>> That's why I finally decided to annotate allocating only.
>> 
>> [1]. https://lore.kernel.org/linux-mm/20220921170002.29557-1-laoar.shao@gmail.com/
>> 
>> > > To store the information of a slab or a page, we need to create a new
>> > > member in struct page, but we can do it in page extension which can
>> > > avoid changing the size of struct page. So a new page extension
>> > > active_vm is introduced. Each page and each slab which is allocated as
>> > > BPF memory will have a struct active_vm. The reason it is named as
>> > > active_vm is that we can extend it to other areas easily, for example in
>> > > the future we may use it to count other memory usage.
>> > >
>> > > The new page extension active_vm can be disabled via CONFIG_ACTIVE_VM at
>> > > compile time or kernel parameter `active_vm=` at runtime.
>> >
>> > The issue with page_ext is the extra memory usage, so it was rather intended
>> > for debugging features that can be always compiled in, but only enabled at
>> > runtime when debugging is needed. The overhead is only paid when enabled.
>> > That's at least the case of page_owner and page_table_check. The 32bit
>> > page_idle is rather an oddity that could have instead stayed 64-bit only.
>> >
>> 
>> Right, it seems currently page_ext is for debugging purposes only.
>> 
>> > But this is proposing a page_ext functionality supposed to be enabled at all
>> > times in production, with the goal of improved accounting. Not an on-demand
>> > debugging. I'm afraid the costs will outweight the benefits.
>> >
>> 
>> The memory overhead of this new page extension is (8/4096), which is
>> 0.2% of total memory. Not too big to be acceptable.
> 
> It's generally unacceptable to increase sizeof(struct page)
> (nor enabling page_ext by default, and that's the why page_ext is for
> debugging purposes only)
> 
>> If the user really
>> thinks this overhead is not accepted, he can set "active_vm=off" to
>> disable it.
> 
> I'd say many people won't welcome adding 0.2% of total memory by default
> to get BPF memory usage. 

Agreed.

>> To reduce the memory overhead further, I have a bold idea.
>> Actually we don't need to allocate such a page extension for every
>> page,  while we only need to allocate it if the user needs to access
>> it. That said it seems that we can allocate some kind of page
>> extensions dynamically rather than preallocate at booting, but I
>> haven't investigated it deeply to check if it can work. What do you
>> think?

There's lots of benefits (simplicity) of page_ext being allocated as it is
today. What you're suggesting will be better solved (in few years :) by
Matthew's bold ideas about shrinking the current struct page and allocating
usecase-specific descriptors.

>> > Just a quick thought, in case the bpf accounting really can't be handled
>> > without marking pages and slab objects - since memcg already has hooks there
>> > without need of page_ext, couldn't it be done by extending the memcg infra
>> > instead?
>> >
>> 
>> We need to make sure the accounting of BPF memory usage is still
>> workable even without memcg, see also the previous discussion[2].
>> 
>> [2]. https://lore.kernel.org/linux-mm/Yy53cgcwx+hTll4R@slm.duckdns.org/
>