mm/slub: enable debugging memory wasting of kmalloc

ftang1 · tehcaster · commit 6edf2576a6cc · 2022-09-23T12:32:45.000+02:00
kmalloc's API family is critical for mm, with one nature that it will round up the request size to a fixed one (mostly power of 2). Say when user requests memory for '2^n + 1' bytes, actually 2^(n+1) bytes could be allocated, so in worst case, there is around 50% memory space waste. The wastage is not a big issue for requests that get allocated/freed quickly, but may cause problems with objects that have longer life time. We've met a kernel boot OOM panic (v5.10), and from the dumped slab info: [ 26.062145] kmalloc-2k 814056KB 814056KB From debug we found there are huge number of 'struct iova_magazine', whose size is 1032 bytes (1024 + 8), so each allocation will waste 1016 bytes. Though the issue was solved by giving the right (bigger) size of RAM, it is still nice to optimize the size (either use a kmalloc friendly size or create a dedicated slab for it). And from lkml archive, there was another crash kernel OOM case [1] back in 2019, which seems to be related with the similar slab waste situation, as the log is similar: [ 4.332648] iommu: Adding device 0000:20:02.0 to group 16 [ 4.338946] swapper/0 invoked oom-killer: gfp_mask=0x6040c0(GFP_KERNEL|__GFP_COMP), nodemask=(null), order=0, oom_score_adj=0 ... [ 4.857565] kmalloc-2048 59164KB 59164KB The crash kernel only has 256M memory, and 59M is pretty big here. (Note: the related code has been changed and optimised in recent kernel [2], these logs are just picked to demo the problem, also a patch changing its size to 1024 bytes has been merged) So add an way to track each kmalloc's memory waste info, and leverage the existing SLUB debug framework (specifically SLUB_STORE_USER) to show its call stack of original allocation, so that user can evaluate the waste situation, identify some hot spots and optimize accordingly, for a better utilization of memory. The waste info is integrated into existing interface: '/sys/kernel/debug/slab/kmalloc-xx/alloc_traces', one example of 'kmalloc-4k' after boot is: 126 ixgbe_alloc_q_vector+0xbe/0x830 [ixgbe] waste=233856/1856 age=280763/281414/282065 pid=1330 cpus=32 nodes=1 __kmem_cache_alloc_node+0x11f/0x4e0 __kmalloc_node+0x4e/0x140 ixgbe_alloc_q_vector+0xbe/0x830 [ixgbe] ixgbe_init_interrupt_scheme+0x2ae/0xc90 [ixgbe] ixgbe_probe+0x165f/0x1d20 [ixgbe] local_pci_probe+0x78/0xc0 work_for_cpu_fn+0x26/0x40 ... which means in 'kmalloc-4k' slab, there are 126 requests of 2240 bytes which got a 4KB space (wasting 1856 bytes each and 233856 bytes in total), from ixgbe_alloc_q_vector(). And when system starts some real workload like multiple docker instances, there could are more severe waste. [1]. https://lkml.org/lkml/2019/8/12/266 [2]. https://lore.kernel.org/lkml/2920df89-9975-5785-f79b-257d3052dfaf@huawei.com/ [Thanks Hyeonggon for pointing out several bugs about sorting/format] [Thanks Vlastimil for suggesting way to reduce memory usage of orig_size and keep it only for kmalloc objects] Signed-off-by: Feng Tang <feng.tang@intel.com> Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: Robin Murphy <robin.murphy@arm.com> Cc: John Garry <john.garry@huawei.com> Cc: Kefeng Wang <wangkefeng.wang@huawei.com> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
diff --git a/Documentation/mm/slub.rst b/Documentation/mm/slub.rst
@@ -400,21 +400,30 @@ information:
     allocated objects. The output is sorted by frequency of each trace.
 
     Information in the output:
-    Number of objects, allocating function, minimal/average/maximal jiffies since alloc,
-    pid range of the allocating processes, cpu mask of allocating cpus, and stack trace.
+    Number of objects, allocating function, possible memory wastage of
+    kmalloc objects(total/per-object), minimal/average/maximal jiffies
+    since alloc, pid range of the allocating processes, cpu mask of
+    allocating cpus, numa node mask of origins of memory, and stack trace.
 
     Example:::
 
-    1085 populate_error_injection_list+0x97/0x110 age=166678/166680/166682 pid=1 cpus=1::
-	__slab_alloc+0x6d/0x90
-	kmem_cache_alloc_trace+0x2eb/0x300
-	populate_error_injection_list+0x97/0x110
-	init_error_injection+0x1b/0x71
-	do_one_initcall+0x5f/0x2d0
-	kernel_init_freeable+0x26f/0x2d7
-	kernel_init+0xe/0x118
-	ret_from_fork+0x22/0x30
-
+    338 pci_alloc_dev+0x2c/0xa0 waste=521872/1544 age=290837/291891/293509 pid=1 cpus=106 nodes=0-1
+        __kmem_cache_alloc_node+0x11f/0x4e0
+        kmalloc_trace+0x26/0xa0
+        pci_alloc_dev+0x2c/0xa0
+        pci_scan_single_device+0xd2/0x150
+        pci_scan_slot+0xf7/0x2d0
+        pci_scan_child_bus_extend+0x4e/0x360
+        acpi_pci_root_create+0x32e/0x3b0
+        pci_acpi_scan_root+0x2b9/0x2d0
+        acpi_pci_root_add.cold.11+0x110/0xb0a
+        acpi_bus_attach+0x262/0x3f0
+        device_for_each_child+0xb7/0x110
+        acpi_dev_for_each_child+0x77/0xa0
+        acpi_bus_attach+0x108/0x3f0
+        device_for_each_child+0xb7/0x110
+        acpi_dev_for_each_child+0x77/0xa0
+        acpi_bus_attach+0x108/0x3f0
 
 2. free_traces::
 
diff --git a/include/linux/slab.h b/include/linux/slab.h
@@ -29,6 +29,8 @@
 #define SLAB_RED_ZONE		((slab_flags_t __force)0x00000400U)
 /* DEBUG: Poison objects */
 #define SLAB_POISON		((slab_flags_t __force)0x00000800U)
+/* Indicate a kmalloc slab */
+#define SLAB_KMALLOC		((slab_flags_t __force)0x00001000U)
 /* Align objs on cache lines */
 #define SLAB_HWCACHE_ALIGN	((slab_flags_t __force)0x00002000U)
 /* Use GFP_DMA memory */
diff --git a/mm/slab_common.c b/mm/slab_common.c
@@ -649,7 +649,8 @@ struct kmem_cache *__init create_kmalloc_cache(const char *name,
 	if (!s)
 		panic("Out of memory when creating slab %s\n", name);
 
-	create_boot_cache(s, name, size, flags, useroffset, usersize);
+	create_boot_cache(s, name, size, flags | SLAB_KMALLOC, useroffset,
+								usersize);
 	kasan_cache_create_kmalloc(s);
 	list_add(&s->list, &slab_caches);
 	s->refcount = 1;
diff --git a/mm/slub.c b/mm/slub.c
