On Tue, Nov 14, 2023 at 07:39:40PM +0100, David Hildenbrand wrote:
> On 14.11.23 19:02, Sumanth Korikkar wrote:
> > Implement MEM_PHYS_ONLINE and MEM_PHYS_OFFLINE memory notifiers on s390
> >
...
> > arch/s390/mm/init.c | 16 +++++++++++++++-
> > drivers/s390/char/sclp_cmd.c | 33 ++++++++++++++++++++++++++++++---
> > 2 files changed, 45 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
> > index 8d9a60ccb777..db505ed590b2 100644
> > --- a/arch/s390/mm/init.c
> > +++ b/arch/s390/mm/init.c
> > @@ -288,6 +288,12 @@ int arch_add_memory(int nid, u64 start, u64 size,
> > rc = vmem_add_mapping(start, size);
> > if (rc)
> > return rc;
> > + /*
> > + * If MHP_MEMMAP_ON_MEMORY is enabled, perform __add_pages() during memory
> > + * onlining phase
> > + */
> > + if (params->altmap)
> > + return 0;
>
>
> So we'd have added memory blocks without a memmap? Sorry, but this seems to
> further hack into the s390x direction.
This new approach has the advantage that we do not need to allocate any
additional memory during online phase, neither for direct mapping page
tables nor struct pages, so that memory hotplug can never fail.
The old approach (without altmap) is already a hack, because we add
the memmap / struct pages, but for memory that is not really accessible.
And with all the disadvantage of pre-allocating struct pages from system
memory.
The new approach allows to better integrate s390 to the existing
interface, and also make use of altmap support, which would eliminate
the major disadvantage of the old behaviour. So from s390 perspective,
this new mechanism would be preferred, provided that there is no
functional issue with the "added memory blocks without a memmap"
approach.
Do you see any functional issues, e.g. conflict with common
code?
>
> Maybe s390x should just provide a dedicate interface to add these memory
> blocks instead of adding them during boot and then relying on the old way of
> using online/offline set them online/offline.
Existing behavior:
The current 'lsmem -a' command displays both online and standby memory.
interface changes:
If a new interface is introduced and standby memory is no longer listed,
the following consequences might occur:
1. Running 'lsmem -a' would only show online memory, potentially leading
to user complaints.
2. standby memory addition would need:
* echo "standby memory addr" > /sys/devices/system/memory/probe
As far as I understand, this interface is already deprecated.
3. To remove standby memory, a new interface probe_remove is needed
* echo "standby memory addr" > /sys/devices/system/memory/probe_remove
4. Users may express a need to identify standby memory addresses,
resulting in the creation of another interface to list these standby
memory ranges.
Hence, introducing new physical memory notifiers to platforms lacking
dynamic ACPI events would be highly advantageous while maintaining
existing user-friendly interface.
Thanks
On 15.11.23 15:20, Sumanth Korikkar wrote:
> On Tue, Nov 14, 2023 at 07:39:40PM +0100, David Hildenbrand wrote:
>> On 14.11.23 19:02, Sumanth Korikkar wrote:
>>> Implement MEM_PHYS_ONLINE and MEM_PHYS_OFFLINE memory notifiers on s390
>>>
> ...
>>> arch/s390/mm/init.c | 16 +++++++++++++++-
>>> drivers/s390/char/sclp_cmd.c | 33 ++++++++++++++++++++++++++++++---
>>> 2 files changed, 45 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
>>> index 8d9a60ccb777..db505ed590b2 100644
>>> --- a/arch/s390/mm/init.c
>>> +++ b/arch/s390/mm/init.c
>>> @@ -288,6 +288,12 @@ int arch_add_memory(int nid, u64 start, u64 size,
>>> rc = vmem_add_mapping(start, size);
>>> if (rc)
>>> return rc;
>>> + /*
>>> + * If MHP_MEMMAP_ON_MEMORY is enabled, perform __add_pages() during memory
>>> + * onlining phase
>>> + */
>>> + if (params->altmap)
>>> + return 0;
>>
>>
>> So we'd have added memory blocks without a memmap? Sorry, but this seems to
>> further hack into the s390x direction.
>
> This new approach has the advantage that we do not need to allocate any
> additional memory during online phase, neither for direct mapping page
> tables nor struct pages, so that memory hotplug can never fail.
Right, just like any other architecture that (triggered by whatever
mechanism) ends up calling add_memory() and friends.
>
> The old approach (without altmap) is already a hack, because we add
> the memmap / struct pages, but for memory that is not really accessible.
Yes, it's disgusting. And you still allocate other things like memory
block devices or the identify map.
> And with all the disadvantage of pre-allocating struct pages from system
> memory.
Jep. It never should have been done like that.
>
> The new approach allows to better integrate s390 to the existing
> interface, and also make use of altmap support, which would eliminate
> the major disadvantage of the old behaviour. So from s390 perspective,
> this new mechanism would be preferred, provided that there is no
> functional issue with the "added memory blocks without a memmap"
> approach.
It achieves that by s390x specific hacks in common code :) Instead of
everybody else that simply uses add_memory() and friends.
>
> Do you see any functional issues, e.g. conflict with common
> code?
I don't see functional issues right now, just the way it is done to
implement support for a new feature is a hack IMHO. Replacing hack #1 by
hack #2 is not really something reasonable. Let's try to remove hacks.
>>
>> Maybe s390x should just provide a dedicate interface to add these memory
>> blocks instead of adding them during boot and then relying on the old way of
>> using online/offline set them online/offline.
>
> Existing behavior:
> The current 'lsmem -a' command displays both online and standby memory.
>
> interface changes:
> If a new interface is introduced and standby memory is no longer listed,
> the following consequences might occur:
>
> 1. Running 'lsmem -a' would only show online memory, potentially leading
> to user complaints.
That's why the new, clean way of doing it will require a world switch.
If the admin wants the benefits of altmap/memmap allocation, it can be
enabled.
> 2. standby memory addition would need:
> * echo "standby memory addr" > /sys/devices/system/memory/probe
> As far as I understand, this interface is already deprecated.
It should actually be an s390x specific interface where people are able
to query the standby ranges, and request to add/remove them. There,
s390x can perform checks and setup everything accordingly before calling
add_memory() and have the memory onlined.
We do have something comparable with the dax/kmem infrastructure: users
configure the available memory to hotplug, and then hotplug it. Tooling
onlines that memory automatically.
Ideally they will add ranges, not memory blocks.
>
> 3. To remove standby memory, a new interface probe_remove is needed
> * echo "standby memory addr" > /sys/devices/system/memory/probe_remove
>
Similarly, an s390x specific interface that performs checks and properly
tears everything s390x-specifc down -- for example, turning system RAM
into standby RAM again.
> 4. Users may express a need to identify standby memory addresses,
> resulting in the creation of another interface to list these standby
> memory ranges.
Exactly. Memory that is not added to the system that does not consume
any resources, but can be added on demand using an interface that is not
the second stage (onlining/offlining) of memory hot(un)plug.
--
Cheers,
David / dhildenb
>>>
>>> Maybe s390x should just provide a dedicate interface to add these memory
>>> blocks instead of adding them during boot and then relying on the old way of
>>> using online/offline set them online/offline.
>>
>> Existing behavior:
>> The current 'lsmem -a' command displays both online and standby memory.
>>
>> interface changes:
>> If a new interface is introduced and standby memory is no longer listed,
>> the following consequences might occur:
>>
>> 1. Running 'lsmem -a' would only show online memory, potentially leading
>> to user complaints.
>
> That's why the new, clean way of doing it will require a world switch.
> If the admin wants the benefits of altmap/memmap allocation, it can be
> enabled.
BTW, thinking about it, I guess one could teach lsmem (and maybe chmem)
to consult additional interfaces on s390x to show standby memory that's
not added to the system yet.
--
Cheers,
David / dhildenb
On Thu, 16 Nov 2023 20:16:02 +0100
David Hildenbrand <[email protected]> wrote:
> On 15.11.23 15:20, Sumanth Korikkar wrote:
> > On Tue, Nov 14, 2023 at 07:39:40PM +0100, David Hildenbrand wrote:
> >> On 14.11.23 19:02, Sumanth Korikkar wrote:
> >>> Implement MEM_PHYS_ONLINE and MEM_PHYS_OFFLINE memory notifiers on s390
> >>>
> > ...
> >>> arch/s390/mm/init.c | 16 +++++++++++++++-
> >>> drivers/s390/char/sclp_cmd.c | 33 ++++++++++++++++++++++++++++++---
> >>> 2 files changed, 45 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
> >>> index 8d9a60ccb777..db505ed590b2 100644
> >>> --- a/arch/s390/mm/init.c
> >>> +++ b/arch/s390/mm/init.c
> >>> @@ -288,6 +288,12 @@ int arch_add_memory(int nid, u64 start, u64 size,
> >>> rc = vmem_add_mapping(start, size);
> >>> if (rc)
> >>> return rc;
> >>> + /*
> >>> + * If MHP_MEMMAP_ON_MEMORY is enabled, perform __add_pages() during memory
> >>> + * onlining phase
> >>> + */
> >>> + if (params->altmap)
> >>> + return 0;
> >>
> >>
> >> So we'd have added memory blocks without a memmap? Sorry, but this seems to
> >> further hack into the s390x direction.
> >
> > This new approach has the advantage that we do not need to allocate any
> > additional memory during online phase, neither for direct mapping page
> > tables nor struct pages, so that memory hotplug can never fail.
>
> Right, just like any other architecture that (triggered by whatever
> mechanism) ends up calling add_memory() and friends.
Just for better understanding, are page tables for identity and also
vmemmap mapping not allocated from system memory on other archs? I.e.
no altmap support for that, only for struct pages (so far)?
>
> >
> > The old approach (without altmap) is already a hack, because we add
> > the memmap / struct pages, but for memory that is not really accessible.
>
> Yes, it's disgusting. And you still allocate other things like memory
> block devices or the identify map.
I would say it is special :-). And again, for understanding, all other
things apart from struct pages, still would need to be allocated from
system memory on other archs?
Of course, struct pages would be by far the biggest part, so having
altmap support for that helps a lot. Doing the other allocations also
via altmap would feel natural, but it is not possible yet, or did we
miss something?
>
> > And with all the disadvantage of pre-allocating struct pages from system
> > memory.
>
> Jep. It never should have been done like that.
At that time, it gave the benefit over all others, that we do not need
to allocate struct pages from system memory, at the time of memory online,
when memory pressure might be high and such allocations might fail.
I guess you can say that it should have been done "right" at that time,
e.g. by already adding something like altmap support, instead of our own
hack.
>
> >
> > The new approach allows to better integrate s390 to the existing
> > interface, and also make use of altmap support, which would eliminate
> > the major disadvantage of the old behaviour. So from s390 perspective,
> > this new mechanism would be preferred, provided that there is no
> > functional issue with the "added memory blocks without a memmap"
> > approach.
>
> It achieves that by s390x specific hacks in common code :) Instead of
> everybody else that simply uses add_memory() and friends.
>
> >
> > Do you see any functional issues, e.g. conflict with common
> > code?
>
> I don't see functional issues right now, just the way it is done to
> implement support for a new feature is a hack IMHO. Replacing hack #1 by
> hack #2 is not really something reasonable. Let's try to remove hacks.
Ok, sounds reasonable, let's try that. Introducing some new s390-specific
interface also feels a bit hacky, or ugly, but we'll see if we find a
way so that it is only "special" :-)
Reminds me a bit of that "probe" attribute, that also was an arch-specific
hack initially, IIRC, and is now to be deprecated...
[catching up on mails]
>>> This new approach has the advantage that we do not need to allocate any
>>> additional memory during online phase, neither for direct mapping page
>>> tables nor struct pages, so that memory hotplug can never fail.
>>
>> Right, just like any other architecture that (triggered by whatever
>> mechanism) ends up calling add_memory() and friends.
>
> Just for better understanding, are page tables for identity and also
> vmemmap mapping not allocated from system memory on other archs? I.e.
> no altmap support for that, only for struct pages (so far)?
Yes, only the actual "memmap ("struct page")" comes from altmap space,
everything else comes from the buddy during memory hotplug.
>
>>
>>>
>>> The old approach (without altmap) is already a hack, because we add
>>> the memmap / struct pages, but for memory that is not really accessible.
>>
>> Yes, it's disgusting. And you still allocate other things like memory
>> block devices or the identify map.
>
> I would say it is special :-). And again, for understanding, all other
:)
> things apart from struct pages, still would need to be allocated from
> system memory on other archs?
Yes!
>
> Of course, struct pages would be by far the biggest part, so having
> altmap support for that helps a lot. Doing the other allocations also
> via altmap would feel natural, but it is not possible yet, or did we
> miss something?
The tricky part is making sure ahead of time that that we set aside the
required number of pageblocks, to properly control during memory
onlining what to set aside and what to expose to the buddy.
See mhp_supports_memmap_on_memory() /
memory_block_memmap_on_memory_pages() for the dirty details :)
>
>>
>>> And with all the disadvantage of pre-allocating struct pages from system
>>> memory.
>>
>> Jep. It never should have been done like that.
>
> At that time, it gave the benefit over all others, that we do not need
> to allocate struct pages from system memory, at the time of memory online,
> when memory pressure might be high and such allocations might fail.
Agreed. Having the memmap already around can be helpful. But not for all
standby memory, that's just pure waste.
... but as memory onlining is triggered by user space, it's likely that
that user space cannot even make progress (e.g., start a process to set
memory online) to actually trigger memory onlining in serious low-memory
situations.
>
> I guess you can say that it should have been done "right" at that time,
> e.g. by already adding something like altmap support, instead of our own
> hack.
Probably yes. IMHO, relying on the existing memory block interface was
the low hanging fruit. Now, s390x is just special.
>
>>
>>>
>>> The new approach allows to better integrate s390 to the existing
>>> interface, and also make use of altmap support, which would eliminate
>>> the major disadvantage of the old behaviour. So from s390 perspective,
>>> this new mechanism would be preferred, provided that there is no
>>> functional issue with the "added memory blocks without a memmap"
>>> approach.
>>
>> It achieves that by s390x specific hacks in common code :) Instead of
>> everybody else that simply uses add_memory() and friends.
>>
>>>
>>> Do you see any functional issues, e.g. conflict with common
>>> code?
>>
>> I don't see functional issues right now, just the way it is done to
>> implement support for a new feature is a hack IMHO. Replacing hack #1 by
>> hack #2 is not really something reasonable. Let's try to remove hacks.
>
> Ok, sounds reasonable, let's try that. Introducing some new s390-specific
> interface also feels a bit hacky, or ugly, but we'll see if we find a
> way so that it is only "special" :-)
As proposed in my other mail, I think there are ways to make s390x happy
first and look into a cleaner approach long-term.
> Reminds me a bit of that "probe" attribute, that also was an arch-specific
> hack initially, IIRC, and is now to be deprecated...
Yeah, that was for interfaces where the kernel has absolutely no clue
where/what/how memory gets hotplugged. ARM64 without ACPI.
s390x is completely different though: you know exactly which standby
memory exists, where it resides, in which granularity in can be
enabled/disabled, ... you don't have to play dangerous "I'm pretty sure
there is memory out there although nobody can check so I crash the
kernel" games.
--
Cheers,
David / dhildenb