Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

Dave Hansen
2022-04-11T06:41:54Z

On 4/9/22 10:52, Kirill A. Shutemov wrote:
> On Fri, Apr 08, 2022 at 12:11:58PM -0700, Dave Hansen wrote:
>> On 4/5/22 16:43, Kirill A. Shutemov wrote:
>>> Kernel only needs to accept memory once after boot, so during the boot
>>> and warm up phase there will be a lot of memory acceptance. After things
>>> are settled down the only price of the feature if couple of checks for
>>> PageUnaccepted() in allocate and free paths. The check refers a hot
>>> variable (that also encodes PageBuddy()), so it is cheap and not visible
>>> on profiles.
>>
>> Let's also not sugar-coat this. Page acceptance is hideously slow.
>> It's agonizingly slow. To boot, it's done holding a global spinlock
>> with interrupts disabled (see patch 6/8). At the very, very least, each
>> acceptance operation involves a couple of what are effectively ring
>> transitions, a 2MB memset(), and a bunch of cache flushing.
>>
>> The system is going to be downright unusable during this time, right?
...
>> Do we need anything more discrete to tell users when acceptance is over?
>
> I can imagine setups that where acceptance is never over. A VM running
> a workload with fixed dataset can have planty of memory unaccepted.
>
> I don't think "make it over" should be the goal.

I agree, there will be users that don't care when acceptance is over.
But, I'm also sure that there are users that will care deeply.

>> For instance, maybe they run something and it goes really slow, they
>> watch "accept_memory" until it stops. They rejoice at their good
>> fortune! Then, memory allocation starts falling over to a new node and
>> the agony beings anew.
>>
>> I can think of dealing with this in two ways:
>>
>> cat /sys/.../unaccepted_pages_left
>>
>> which just walks the bitmap and counts the amount of pages remaining. or
>> something like:
>>
>> echo 1 > /sys/devices/system/node/node0/make_the_pain_stop
>>
>> Which will, well, make the pain stop on node0.
>
> Sure we can add handles. But API is hard. Maybe we should wait and see
> what is actually needed. (Yes, I'm lazy.:)

Let's just call out the possible (probable?) need for new ABI here.
Maybe it will cue folks who care to speak up.