trace doc: convert trace/events-kmem.txt to rst format

This converts the plain text documentation to reStructuredText format and
add it into Sphinx TOC tree. No essential content change.

Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 119 insertions, 0 deletions

diff --git a/Documentation/trace/events-kmem.rst b/Documentation/trace/events-kmem.rst
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+============================
+Subsystem Trace Points: kmem
+============================
+
+The kmem tracing system captures events related to object and page allocation
+within the kernel. Broadly speaking there are five major subheadings.
+
+  - Slab allocation of small objects of unknown type (kmalloc)
+  - Slab allocation of small objects of known type
+  - Page allocation
+  - Per-CPU Allocator Activity
+  - External Fragmentation
+
+This document describes what each of the tracepoints is and why they
+might be useful.
+
+1. Slab allocation of small objects of unknown type
+===================================================
+::
+
+  kmalloc		call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s
+  kmalloc_node	call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s node=%d
+  kfree		call_site=%lx ptr=%p
+
+Heavy activity for these events may indicate that a specific cache is
+justified, particularly if kmalloc slab pages are getting significantly
+internal fragmented as a result of the allocation pattern. By correlating
+kmalloc with kfree, it may be possible to identify memory leaks and where
+the allocation sites were.
+
+
+2. Slab allocation of small objects of known type
+=================================================
+::
+
+  kmem_cache_alloc	call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s
+  kmem_cache_alloc_node	call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s node=%d
+  kmem_cache_free		call_site=%lx ptr=%p
+
+These events are similar in usage to the kmalloc-related events except that
+it is likely easier to pin the event down to a specific cache. At the time
+of writing, no information is available on what slab is being allocated from,
+but the call_site can usually be used to extrapolate that information.
+
+3. Page allocation
+==================
+::
+
+  mm_page_alloc		  page=%p pfn=%lu order=%d migratetype=%d gfp_flags=%s
+  mm_page_alloc_zone_locked page=%p pfn=%lu order=%u migratetype=%d cpu=%d percpu_refill=%d
+  mm_page_free		  page=%p pfn=%lu order=%d
+  mm_page_free_batched	  page=%p pfn=%lu order=%d cold=%d
+
+These four events deal with page allocation and freeing. mm_page_alloc is
+a simple indicator of page allocator activity. Pages may be allocated from
+the per-CPU allocator (high performance) or the buddy allocator.
+
+If pages are allocated directly from the buddy allocator, the
+mm_page_alloc_zone_locked event is triggered. This event is important as high
+amounts of activity imply high activity on the zone->lock. Taking this lock
+impairs performance by disabling interrupts, dirtying cache lines between
+CPUs and serialising many CPUs.
+
+When a page is freed directly by the caller, the only mm_page_free event
+is triggered. Significant amounts of activity here could indicate that the
+callers should be batching their activities.
+
+When pages are freed in batch, the also mm_page_free_batched is triggered.
+Broadly speaking, pages are taken off the LRU lock in bulk and
+freed in batch with a page list. Significant amounts of activity here could
+indicate that the system is under memory pressure and can also indicate
+contention on the zone->lru_lock.
+
+4. Per-CPU Allocator Activity
+=============================
+::
+
+  mm_page_alloc_zone_locked	page=%p pfn=%lu order=%u migratetype=%d cpu=%d percpu_refill=%d
+  mm_page_pcpu_drain		page=%p pfn=%lu order=%d cpu=%d migratetype=%d
+
+In front of the page allocator is a per-cpu page allocator. It exists only
+for order-0 pages, reduces contention on the zone->lock and reduces the
+amount of writing on struct page.
+
+When a per-CPU list is empty or pages of the wrong type are allocated,
+the zone->lock will be taken once and the per-CPU list refilled. The event
+triggered is mm_page_alloc_zone_locked for each page allocated with the
+event indicating whether it is for a percpu_refill or not.
+
+When the per-CPU list is too full, a number of pages are freed, each one
+which triggers a mm_page_pcpu_drain event.
+
+The individual nature of the events is so that pages can be tracked
+between allocation and freeing. A number of drain or refill pages that occur
+consecutively imply the zone->lock being taken once. Large amounts of per-CPU
+refills and drains could imply an imbalance between CPUs where too much work
+is being concentrated in one place. It could also indicate that the per-CPU
+lists should be a larger size. Finally, large amounts of refills on one CPU
+and drains on another could be a factor in causing large amounts of cache
+line bounces due to writes between CPUs and worth investigating if pages
+can be allocated and freed on the same CPU through some algorithm change.
+
+5. External Fragmentation
+=========================
+::
+
+  mm_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
+
+External fragmentation affects whether a high-order allocation will be
+successful or not. For some types of hardware, this is important although
+it is avoided where possible. If the system is using huge pages and needs
+to be able to resize the pool over the lifetime of the system, this value
+is important.
+
+Large numbers of this event implies that memory is fragmenting and
+high-order allocations will start failing at some time in the future. One
+means of reducing the occurrence of this event is to increase the size of
+min_free_kbytes in increments of 3*pageblock_size*nr_online_nodes where
+pageblock_size is usually the size of the default hugepage size.