On 8/5/22 14:09, David Hildenbrand wrote: > On 05.08.22 13:49, Vlastimil Babka wrote: >> On 6/14/22 14:02, Kirill A. Shutemov wrote: >>> UEFI Specification version 2.9 introduces the concept of memory >>> acceptance. Some Virtual Machine platforms, such as Intel TDX or AMD >>> SEV-SNP, require memory to be accepted before it can be used by the >>> guest. Accepting happens via a protocol specific to the Virtual Machine >>> platform. >>> >>> There are several ways kernel can deal with unaccepted memory: >>> >>> 1. Accept all the memory during the boot. It is easy to implement and >>> it doesn't have runtime cost once the system is booted. The downside >>> is very long boot time. >>> >>> Accept can be parallelized to multiple CPUs to keep it manageable >>> (i.e. via DEFERRED_STRUCT_PAGE_INIT), but it tends to saturate >>> memory bandwidth and does not scale beyond the point. >>> >>> 2. Accept a block of memory on the first use. It requires more >>> infrastructure and changes in page allocator to make it work, but >>> it provides good boot time. >>> >>> On-demand memory accept means latency spikes every time kernel steps >>> onto a new memory block. The spikes will go away once workload data >>> set size gets stabilized or all memory gets accepted. >>> >>> 3. Accept all memory in background. Introduce a thread (or multiple) >>> that gets memory accepted proactively. It will minimize time the >>> system experience latency spikes on memory allocation while keeping >>> low boot time. >>> >>> This approach cannot function on its own. It is an extension of #2: >>> background memory acceptance requires functional scheduler, but the >>> page allocator may need to tap into unaccepted memory before that. >>> >>> The downside of the approach is that these threads also steal CPU >>> cycles and memory bandwidth from the user's workload and may hurt >>> user experience. >>> >>> Implement #2 for now. It is a reasonable default. Some workloads may >>> want to use #1 or #3 and they can be implemented later based on user's >>> demands. >>> >>> Support of unaccepted memory requires a few changes in core-mm code: >>> >>> - memblock has to accept memory on allocation; >>> >>> - page allocator has to accept memory on the first allocation of the >>> page; >>> >>> Memblock change is trivial. >>> >>> The page allocator is modified to accept pages on the first allocation. >>> The new page type (encoded in the _mapcount) -- PageUnaccepted() -- is >>> used to indicate that the page requires acceptance. >>> >>> Architecture has to provide two helpers if it wants to support >>> unaccepted memory: >>> >>> - accept_memory() makes a range of physical addresses accepted. >>> >>> - range_contains_unaccepted_memory() checks anything within the range >>> of physical addresses requires acceptance. >>> >>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> >>> Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> # memblock >>> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> >> >> Hmm I realize it's not ideal to raise this at v7, and maybe it was discussed >> before, but it's really not great how this affects the core page allocator >> paths. Wouldn't it be possible to only release pages to page allocator when >> accepted, and otherwise use some new per-zone variables together with the >> bitmap to track how much exactly is where to accept? Then it could be hooked >> in get_page_from_freelist() similarly to CONFIG_DEFERRED_STRUCT_PAGE_INIT - >> if we fail zone_watermark_fast() and there are unaccepted pages in the zone, >> accept them and continue. With a static key to flip in case we eventually >> accept everything. Because this is really similar scenario to the deferred >> init and that one was solved in a way that adds minimal overhead. > > I kind of like just having the memory stats being correct (e.g., free > memory) and acceptance being an internal detail to be triggered when > allocating pages -- just like the arch_alloc_page() callback. Hm, good point about the stats. Could be tweaked perhaps so it appears correct on the outside, but might be tricky. > I'm sure we could optimize for the !unaccepted memory via static keys > also in this version with some checks at the right places if we find > this to hurt performance? It would be great if we would at least somehow hit the necessary code only when dealing with a >=pageblock size block. The bitmap approach and accepting everything smaller uprofront actually seems rather compatible. Yet in the current patch we e.g. check PageUnaccepted(buddy) on every buddy size while merging. A list that sits besides the existing free_area, contains only >=pageblock order sizes of unaccepted pages (no migratetype distinguished) and we tap into it approximately before __rmqueue_fallback()? There would be some trickery around releasing zone-lock for doing accept_memory(), but should be manageable.
