On Wed, Dec 02, 2020 at 04:49:15PM +0100, Michal Hocko wrote: > On Wed 02-12-20 10:14:41, David Hildenbrand wrote: > > On 01.12.20 18:51, Minchan Kim wrote: > > > There is a need for special HW to require bulk allocation of > > > high-order pages. For example, 4800 * order-4 pages, which > > > would be minimum, sometimes, it requires more. > > > > > > To meet the requirement, a option reserves 300M CMA area and > > > requests the whole 300M contiguous memory. However, it doesn't > > > work if even one of those pages in the range is long-term pinned > > > directly or indirectly. The other option is to ask higher-order > > > > My latest knowledge is that pages in the CMA area are never long term > > pinned. > > > > https://lore.kernel.org/lkml/20201123090129.GD27488@xxxxxxxxxxxxxx/ > > > > "gup already tries to deal with long term pins on CMA regions and migrate > > to a non CMA region. Have a look at __gup_longterm_locked." > > > > We should rather identify ways how that is still possible and get rid of > > them. > > > > > > Now, short-term pinnings and PCP are other issues where > > alloc_contig_range() could be improved (e.g., in contrast to a FAST > > mode, a HARD mode which temporarily disables the PCP, ...). > > Agreed! > > > > size (e.g., 2M) than requested order(64K) repeatedly until driver > > > could gather necessary amount of memory. Basically, this approach > > > makes the allocation very slow due to cma_alloc's function > > > slowness and it could be stuck on one of the pageblocks if it > > > encounters unmigratable page. > > > > > > To solve the issue, this patch introduces cma_alloc_bulk. > > > > > > int cma_alloc_bulk(struct cma *cma, unsigned int align, > > > bool fast, unsigned int order, size_t nr_requests, > > > struct page **page_array, size_t *nr_allocated); > > > > > > Most parameters are same with cma_alloc but it additionally passes > > > vector array to store allocated memory. What's different with cma_alloc > > > is it will skip pageblocks without waiting/stopping if it has unmovable > > > page so that API continues to scan other pageblocks to find requested > > > order page. > > > > > > cma_alloc_bulk is best effort approach in that it skips some pageblocks > > > if they have unmovable pages unlike cma_alloc. It doesn't need to be > > > perfect from the beginning at the cost of performance. Thus, the API > > > takes "bool fast parameter" which is propagated into alloc_contig_range to > > > avoid significat overhead functions to inrecase CMA allocation success > > > ratio(e.g., migration retrial, PCP, LRU draining per pageblock) > > > at the cost of less allocation success ratio. If the caller couldn't > > > allocate enough, they could call it with "false" to increase success ratio > > > if they are okay to expense the overhead for the success ratio. > > > > Just so I understand what the idea is: > > > > alloc_contig_range() sometimes fails on CMA regions when trying to > > allocate big chunks (e.g., 300M). Instead of tackling that issue, you > > rather allocate plenty of small chunks, and make these small allocations > > fail faster/ make the allocations less reliable. Correct? > > > > I don't really have a strong opinion on that. Giving up fast rather than > > trying for longer sounds like a useful thing to have - but I wonder if > > it's strictly necessary for the use case you describe. > > > > I'd like to hear Michals opinion on that. > > Well, what I can see is that this new interface is an antipatern to our > allocation routines. We tend to control allocations by gfp mask yet you > are introducing a bool parameter to make something faster... What that > really means is rather arbitrary. Would it make more sense to teach > cma_alloc resp. alloc_contig_range to recognize GFP_NOWAIT, GFP_NORETRY resp. > GFP_RETRY_MAYFAIL instead? If we use cma_alloc, that interface requires "allocate one big memory chunk". IOW, return value is just struct page and expected that the page is a big contiguos memory. That means it couldn't have a hole in the range. However the idea here, what we asked is much smaller chunk rather than a big contiguous memory so we could skip some of pages if they are randomly pinned(long-term/short-term whatever) and search other pages in the CMA area to avoid long stall. Thus, it couldn't work with exising cma_alloc API with simple gfp_mak. > > I am not deeply familiar with the cma allocator so sorry for a > potentially stupid question. Why does a bulk interface performs better > than repeated calls to cma_alloc? Is this because a failure would help > to move on to the next pfn range while a repeated call would have to > deal with the same range? Yub, true with other overheads(e.g., migration retrial, waiting writeback PCP/LRU draining IPI) > > > > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx> > > > --- > > > include/linux/cma.h | 5 ++ > > > include/linux/gfp.h | 2 + > > > mm/cma.c | 126 ++++++++++++++++++++++++++++++++++++++++++-- > > > mm/page_alloc.c | 19 ++++--- > > > 4 files changed, 140 insertions(+), 12 deletions(-) > > > > -- > Michal Hocko > SUSE Labs
