mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-06 02:24:14 +08:00
3cdd868ec6
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>
120 lines
5.4 KiB
ReStructuredText
120 lines
5.4 KiB
ReStructuredText
============================
|
|
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.
|