linux-kernel - Re: [PATCH v5 1/1] Create debugfs file with virtio balloon usage information

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Date:   Mon, 25 Jul 2022 13:36:55 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alexander Atanasov <alexander.atanasov@...tuozzo.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     kernel@...nvz.org, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/1] Create debugfs file with virtio balloon usage
 information

On 25.07.22 13:27, Alexander Atanasov wrote:
> Hi,
> 
> On 18/07/2022 14:35, David Hildenbrand wrote:
>> On 14.07.22 15:20, Alexander Atanasov wrote:
>>> Allow the guest to know how much it is ballooned by the host.
>>> It is useful when debugging out of memory conditions.
>>>
>>> When host gets back memory from the guest it is accounted
>>> as used memory in the guest but the guest have no way to know
>>> how much it is actually ballooned.
>>>
>>> Signed-off-by: Alexander Atanasov <alexander.atanasov@...tuozzo.com>
>>> ---
>>>   drivers/virtio/virtio_balloon.c     | 79 +++++++++++++++++++++++++++++
>>>   include/uapi/linux/virtio_balloon.h |  1 +
>>>   2 files changed, 80 insertions(+)
>>>
>>> V2:
>>>   - fixed coding style
>>>   - removed pretty print
>>> V3:
>>>   - removed dublicate of features
>>>   - comment about balooned_pages more clear
>>>   - convert host pages to balloon pages
>>> V4:
>>>   - added a define for BALLOON_PAGE_SIZE to make things clear
>>> V5:
>>>   - Made the calculatons work properly for both ways of memory accounting
>>>     with or without deflate_on_oom
>>>   - dropped comment
>>>
> [....]
>>> +	u32 num_pages, total_pages, current_pages;
>>> +	struct sysinfo i;
>>> +
>>> +	si_meminfo(&i);
>>> +
>>> +	seq_printf(f, "%-22s: %d\n", "page_size", VIRTIO_BALLOON_PAGE_SIZE);
>> Why? Either export all in ordinary page size or in kB. No need to
>> over-complicate the interface with a different page size that users
>> don't actually care about.
>>
>> I'd just stick to /proc/meminfo and use kB.
> 
> The point is to expose some internal balloon data. Balloon works with 
> pages not with kB  and users can easily calculate kB.

Pages translate to KB. kB are easy to consume by humans instead of pages
with variable apge sizes

> 
>>> +
>>> +	seq_printf(f, "%-22s: %u\n", "ballooned_pages", num_pages);
>>> +
>>> +	if (virtio_has_feature(b->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) {
>>> +		total_pages = guest_to_balloon_pages(i.totalram);
>>> +		current_pages = guest_to_balloon_pages(i.totalram) - num_pages;
>>> +	} else {
>>> +		total_pages = guest_to_balloon_pages(i.totalram) +  num_pages;
>>> +		current_pages = guest_to_balloon_pages(i.totalram);
>>> +	}
>>> +
>>> +	/* Total Memory for the guest from host */
>>> +	seq_printf(f, "%-22s: %u\n", "total_pages", total_pages);
>>> +
>>> +	/* Current memory for the guest */
>>> +	seq_printf(f, "%-22s: %u\n", "current_pages", current_pages);
>> The think I detest about that interface (total/current) is that it's
>> simply not correct -- because i.totalram for example doesn't include
>> things like (similar to MemTotal in /proc/meminfo)
>>
>> a) crashkernel
>> b) early boot allocations (e.g., memmap)
>> c) any kind of possible memory (un)hotplug in the future
>>
>> I'd really suggest to just KIS and not mess with i.totalram.
>>
>> Exposing how much memory is inflated and some way to identify how that
>> memory is accounted in /proc/meminfo should be good enough.
>>
>> E.g., the output here could simply be
>>
>> "Inflated: 1024 kB"
>> "MemTotalReduced: 1024 kB"
>>
>> That would even allow in the future for flexibility when it comes to how
>> much/what was actually subtracted from MemTotal.
> 
> 
> The point of the debug interface is to expose some of the balloon driver 
> internals to the guest.
> 
> The users of this are user space processes that try to work according to 
> the memory pressure and nested virtualization.
> 
> I haven't seen one userspace software that expects total ram to change, 
> it should be constant with one exception hotplug. But if you do  hotplug 
> RAM you know that and you can restart what you need to restart.
> 
> So instead of messing with totalram with adding or removing pages /it 
> would even be optimization since now it is done page by page/ i suggest 
> to just account the inflated memory as used as in the deflate_on_oom 
> case now. It is confusing and inconsistent as it is now. How to  explain 
> to a vps user why his RAM is constantly changing?
> 
> And the file can go just to
> 
> inflated: <pages>
> 
> ballon_page_size:  4096
> 
> or
> 
> inflated: kB
> 
> I prefer pages because on theory guest and host can different size and 
> if at some point guest starts to make requests to the host for pages it 
> must somehow know what the host/balloon page is. Since you have all the 
> information at one place it is easy to calculate kB. But you can not 
> calculate pages from only kB.

The host can only inflate in guest-page base sizes. How that translates
to host-page base sizes is absolutely irrelevant.

Who should care about pages? Absolutely irrelevant.

Guest pages: kB / getpagesize()
Logical balloon pages: kB / 4k
Host pages: ???

> 
> Later on when hotplug comes it can add it's own data to the file so it 
> can be used to see the amount that is added to the total ram.
> 
> It can add
> 
> hotplugged: <pages>
> 
> 
> What do you think?

Let's keep this interface simple, please.

-- 
Thanks,

David / dhildenb