1 Subsystem Trace Points: kmem 2 3The tracing system kmem captures events related to object and page allocation 4within the kernel. Broadly speaking there are four major subheadings. 5 6 o Slab allocation of small objects of unknown type (kmalloc) 7 o Slab allocation of small objects of known type 8 o Page allocation 9 o Per-CPU Allocator Activity 10 o External Fragmentation 11 12This document will describe what each of the tracepoints are and why they 13might be useful. 14 151. Slab allocation of small objects of unknown type 16=================================================== 17kmalloc call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s 18kmalloc_node call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s node=%d 19kfree call_site=%lx ptr=%p 20 21Heavy activity for these events may indicate that a specific cache is 22justified, particularly if kmalloc slab pages are getting significantly 23internal fragmented as a result of the allocation pattern. By correlating 24kmalloc with kfree, it may be possible to identify memory leaks and where 25the allocation sites were. 26 27 282. Slab allocation of small objects of known type 29================================================= 30kmem_cache_alloc call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s 31kmem_cache_alloc_node call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s node=%d 32kmem_cache_free call_site=%lx ptr=%p 33 34These events are similar in usage to the kmalloc-related events except that 35it is likely easier to pin the event down to a specific cache. At the time 36of writing, no information is available on what slab is being allocated from, 37but the call_site can usually be used to extrapolate that information 38 393. Page allocation 40================== 41mm_page_alloc page=%p pfn=%lu order=%d migratetype=%d gfp_flags=%s 42mm_page_alloc_zone_locked page=%p pfn=%lu order=%u migratetype=%d cpu=%d percpu_refill=%d 43mm_page_free_direct page=%p pfn=%lu order=%d 44mm_pagevec_free page=%p pfn=%lu order=%d cold=%d 45 46These four events deal with page allocation and freeing. mm_page_alloc is 47a simple indicator of page allocator activity. Pages may be allocated from 48the per-CPU allocator (high performance) or the buddy allocator. 49 50If pages are allocated directly from the buddy allocator, the 51mm_page_alloc_zone_locked event is triggered. This event is important as high 52amounts of activity imply high activity on the zone->lock. Taking this lock 53impairs performance by disabling interrupts, dirtying cache lines between 54CPUs and serialising many CPUs. 55 56When a page is freed directly by the caller, the mm_page_free_direct event 57is triggered. Significant amounts of activity here could indicate that the 58callers should be batching their activities. 59 60When pages are freed using a pagevec, the mm_pagevec_free is 61triggered. Broadly speaking, pages are taken off the LRU lock in bulk and 62freed in batch with a pagevec. Significant amounts of activity here could 63indicate that the system is under memory pressure and can also indicate 64contention on the zone->lru_lock. 65 664. Per-CPU Allocator Activity 67============================= 68mm_page_alloc_zone_locked page=%p pfn=%lu order=%u migratetype=%d cpu=%d percpu_refill=%d 69mm_page_pcpu_drain page=%p pfn=%lu order=%d cpu=%d migratetype=%d 70 71In front of the page allocator is a per-cpu page allocator. It exists only 72for order-0 pages, reduces contention on the zone->lock and reduces the 73amount of writing on struct page. 74 75When a per-CPU list is empty or pages of the wrong type are allocated, 76the zone->lock will be taken once and the per-CPU list refilled. The event 77triggered is mm_page_alloc_zone_locked for each page allocated with the 78event indicating whether it is for a percpu_refill or not. 79 80When the per-CPU list is too full, a number of pages are freed, each one 81which triggers a mm_page_pcpu_drain event. 82 83The individual nature of the events are so that pages can be tracked 84between allocation and freeing. A number of drain or refill pages that occur 85consecutively imply the zone->lock being taken once. Large amounts of PCP 86refills and drains could imply an imbalance between CPUs where too much work 87is being concentrated in one place. It could also indicate that the per-CPU 88lists should be a larger size. Finally, large amounts of refills on one CPU 89and drains on another could be a factor in causing large amounts of cache 90line bounces due to writes between CPUs and worth investigating if pages 91can be allocated and freed on the same CPU through some algorithm change. 92 935. External Fragmentation 94========================= 95mm_page_alloc_extfrag page=%p pfn=%lu alloc_order=%d fallback_order=%d pageblock_order=%d alloc_migratetype=%d fallback_migratetype=%d fragmenting=%d change_ownership=%d 96 97External fragmentation affects whether a high-order allocation will be 98successful or not. For some types of hardware, this is important although 99it is avoided where possible. If the system is using huge pages and needs 100to be able to resize the pool over the lifetime of the system, this value 101is important. 102 103Large numbers of this event implies that memory is fragmenting and 104high-order allocations will start failing at some time in the future. One 105means of reducing the occurange of this event is to increase the size of 106min_free_kbytes in increments of 3*pageblock_size*nr_online_nodes where 107pageblock_size is usually the size of the default hugepage size. 108