On an ARMv7 platform with 32M pageblock(MAX_ORDER 14), we observed a huge number of retries of CMA allocation (1k+) during booting when allocating one page for each of 3 mmc instance probe.
This is caused by CMA now supports cocurrent allocation since commit a4efc174b382 ("mm/cma.c: remove redundant cma_mutex lock"). The pageblock we tried to allocate may have already been acquired and isolated by others, then cma_alloc() will retry the next area of the same size by bitmap_no + mask + 1. However, the pageblock order could be big and pageblock_nr_pages is huge (e.g. 8192), then keep retrying in a small step become meaningless because it's likely known to fail again due to within the same pageblock.
Instread of looping in the same pageblock and wasting CPU mips, especially for big pageblock system (e.g. 16M or 32M), we try the next pageblock directly.
Doing this way can greatly mitigate the situtation.
Below is the original error log during booting: [ 2.004804] cma: cma_alloc(cma (ptrval), count 1, align 0) [ 2.010318] cma: cma_alloc(cma (ptrval), count 1, align 0) [ 2.010776] cma: cma_alloc(): memory range at (ptrval) is busy, retrying [ 2.010785] cma: cma_alloc(): memory range at (ptrval) is busy, retrying [ 2.010793] cma: cma_alloc(): memory range at (ptrval) is busy, retrying [ 2.010800] cma: cma_alloc(): memory range at (ptrval) is busy, retrying [ 2.010807] cma: cma_alloc(): memory range at (ptrval) is busy, retrying [ 2.010814] cma: cma_alloc(): memory range at (ptrval) is busy, retrying .... (+1K retries)
After fix, the 1200+ reties can be reduced to 0. Another test running 8 VPU decoder in parallel shows that 1500+ retries dropped to ~145.
IOW this patch can improve the CMA allocation speed a lot when there're enough CMA memory by reducing retries significantly.
Cc: Andrew Morton akpm@linux-foundation.org Cc: Marek Szyprowski m.szyprowski@samsung.com Cc: Lecopzer Chen lecopzer.chen@mediatek.com Cc: David Hildenbrand david@redhat.com Cc: Vlastimil Babka vbabka@suse.cz CC: stable@vger.kernel.org # 5.11+ Fixes: a4efc174b382 ("mm/cma.c: remove redundant cma_mutex lock") Signed-off-by: Dong Aisheng aisheng.dong@nxp.com --- mm/cma.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/mm/cma.c b/mm/cma.c index 1c13a729d274..108a1ceacbe7 100644 --- a/mm/cma.c +++ b/mm/cma.c @@ -500,7 +500,9 @@ struct page *cma_alloc(struct cma *cma, unsigned long count, trace_cma_alloc_busy_retry(cma->name, pfn, pfn_to_page(pfn), count, align); /* try again with a bit different memory target */ - start = bitmap_no + mask + 1; + start = ALIGN(bitmap_no + mask + 1, + pageblock_nr_pages >> cma->order_per_bit); + }
trace_cma_alloc_finish(cma->name, pfn, page, count, align);