Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc
From: Hyeonggon Yoo
Date: Thu Jun 30 2022 - 10:39:46 EST
On Thu, Jun 30, 2022 at 09:47:15AM +0800, Feng Tang wrote:
> kmalloc's API family is critical for mm, with one shortcoming that
> its object size is fixed to be power of 2. When user requests memory
> for '2^n + 1' bytes, actually 2^(n+1) bytes will be allocated, so
> in worst case, there is around 50% memory space waste.
>
> We've met a kernel boot OOM panic, 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 is solved by giving the right(bigger)
> size of RAM, it is still better 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.
>
> So add an way to track each kmalloc's memory waste info, and leverage
> the existing SLUB debug framework to show its call stack info, 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+0xa5/0x4a0 [ixgbe] waste: 233856/1856 age=1493302/1493830/1494358 pid=1284 cpus=32 nodes=1
> __slab_alloc.isra.86+0x52/0x80
> __kmalloc_node+0x143/0x350
> ixgbe_alloc_q_vector+0xa5/0x4a0 [ixgbe]
> ixgbe_init_interrupt_scheme+0x1a6/0x730 [ixgbe]
> ixgbe_probe+0xc8e/0x10d0 [ixgbe]
> local_pci_probe+0x42/0x80
> work_for_cpu_fn+0x13/0x20
> process_one_work+0x1c5/0x390
> worker_thread+0x1b9/0x360
> kthread+0xe6/0x110
> ret_from_fork+0x1f/0x30
>
> 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). And when system starts some real
> workload like multiple docker instances, there are more
> severe waste.
>
> [1]. https://lkml.org/lkml/2019/8/12/266
>
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> ---
> Note:
> * this is based on linux-next tree with tag next-20220628
So this makes use of the fact that orig_size differ from
s->object_size when allocated from kmalloc, and for non-kmalloc
caches it doesn't track waste because s->object_size == orig_size.
Am I following?
And then it has overhead of 'waste' field for every non-kmalloc objects
because track is saved per object. Also the field is not used at free.
(Maybe that would be okay as it's only for debugging, just noting.)
> mm/slub.c | 45 ++++++++++++++++++++++++++++++---------------
> 1 file changed, 30 insertions(+), 15 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 26b00951aad1..bc4f9d4fb1e2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -271,6 +271,7 @@ struct track {
> #endif
> int cpu; /* Was running on cpu */
> int pid; /* Pid context */
> + unsigned long waste; /* memory waste for a kmalloc-ed object */
> unsigned long when; /* When did the operation occur */
> };
>
> @@ -747,6 +748,7 @@ static inline depot_stack_handle_t set_track_prepare(void)
>
> static void set_track_update(struct kmem_cache *s, void *object,
> enum track_item alloc, unsigned long addr,
> + unsigned long waste,
> depot_stack_handle_t handle)
> {
> struct track *p = get_track(s, object, alloc);
> @@ -758,14 +760,16 @@ static void set_track_update(struct kmem_cache *s, void *object,
> p->cpu = smp_processor_id();
> p->pid = current->pid;
> p->when = jiffies;
> + p->waste = waste;
> }
>
> static __always_inline void set_track(struct kmem_cache *s, void *object,
> - enum track_item alloc, unsigned long addr)
> + enum track_item alloc, unsigned long addr,
> + unsigned long waste)
> {
> depot_stack_handle_t handle = set_track_prepare();
>
> - set_track_update(s, object, alloc, addr, handle);
> + set_track_update(s, object, alloc, addr, waste, handle);
> }
>
> static void init_tracking(struct kmem_cache *s, void *object)
> @@ -1325,7 +1329,9 @@ static inline int alloc_consistency_checks(struct kmem_cache *s,
>
> static noinline int alloc_debug_processing(struct kmem_cache *s,
> struct slab *slab,
> - void *object, unsigned long addr)
> + void *object, unsigned long addr,
> + unsigned long waste
> + )
> {
> if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> if (!alloc_consistency_checks(s, slab, object))
> @@ -1334,7 +1340,7 @@ static noinline int alloc_debug_processing(struct kmem_cache *s,
>
> /* Success perform special debug activities for allocs */
> if (s->flags & SLAB_STORE_USER)
> - set_track(s, object, TRACK_ALLOC, addr);
> + set_track(s, object, TRACK_ALLOC, addr, waste);
> trace(s, slab, object, 1);
> init_object(s, object, SLUB_RED_ACTIVE);
> return 1;
> @@ -1398,6 +1404,7 @@ static noinline int free_debug_processing(
> int ret = 0;
> depot_stack_handle_t handle = 0;
>
> + /* TODO: feng: we can slab->waste -= track?) or in set_track */
> if (s->flags & SLAB_STORE_USER)
> handle = set_track_prepare();
>
> @@ -1418,7 +1425,7 @@ static noinline int free_debug_processing(
> }
>
> if (s->flags & SLAB_STORE_USER)
> - set_track_update(s, object, TRACK_FREE, addr, handle);
> + set_track_update(s, object, TRACK_FREE, addr, 0, handle);
> trace(s, slab, object, 0);
> /* Freepointer not overwritten by init_object(), SLAB_POISON moved it */
> init_object(s, object, SLUB_RED_INACTIVE);
> @@ -2905,7 +2912,7 @@ static inline void *get_freelist(struct kmem_cache *s, struct slab *slab)
> * already disabled (which is the case for bulk allocation).
> */
> static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> - unsigned long addr, struct kmem_cache_cpu *c)
> + unsigned long addr, struct kmem_cache_cpu *c, unsigned int orig_size)
> {
> void *freelist;
> struct slab *slab;
> @@ -3048,7 +3055,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> check_new_slab:
>
> if (kmem_cache_debug(s)) {
> - if (!alloc_debug_processing(s, slab, freelist, addr)) {
> + if (!alloc_debug_processing(s, slab, freelist, addr, s->object_size - orig_size)) {
> /* Slab failed checks. Next slab needed */
> goto new_slab;
> } else {
> @@ -3102,7 +3109,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> * pointer.
> */
> static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> - unsigned long addr, struct kmem_cache_cpu *c)
> + unsigned long addr, struct kmem_cache_cpu *c, unsigned int orig_size)
> {
> void *p;
>
> @@ -3115,7 +3122,7 @@ static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> c = slub_get_cpu_ptr(s->cpu_slab);
> #endif
>
> - p = ___slab_alloc(s, gfpflags, node, addr, c);
> + p = ___slab_alloc(s, gfpflags, node, addr, c, orig_size);
> #ifdef CONFIG_PREEMPT_COUNT
> slub_put_cpu_ptr(s->cpu_slab);
> #endif
> @@ -3206,7 +3213,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_l
> */
> if (IS_ENABLED(CONFIG_PREEMPT_RT) ||
> unlikely(!object || !slab || !node_match(slab, node))) {
> - object = __slab_alloc(s, gfpflags, node, addr, c);
> + object = __slab_alloc(s, gfpflags, node, addr, c, orig_size);
> } else {
> void *next_object = get_freepointer_safe(s, object);
>
> @@ -3709,7 +3716,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> * of re-populating per CPU c->freelist
> */
> p[i] = ___slab_alloc(s, flags, NUMA_NO_NODE,
> - _RET_IP_, c);
> + _RET_IP_, c, size);
This looks wrong. size here is size of array.
Maybe just s->object_size instead of size?
> if (unlikely(!p[i]))
> goto error;
>
> @@ -5068,6 +5075,7 @@ struct location {
> depot_stack_handle_t handle;
> unsigned long count;
> unsigned long addr;
> + unsigned long waste;
> long long sum_time;
> long min_time;
> long max_time;
> @@ -5138,11 +5146,12 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
> if (pos == end)
> break;
>
> - caddr = t->loc[pos].addr;
> - chandle = t->loc[pos].handle;
> - if ((track->addr == caddr) && (handle == chandle)) {
> + l = &t->loc[pos];
> + caddr = l->addr;
> + chandle = l->handle;
> + if ((track->addr == caddr) && (handle == chandle) &&
> + (track->waste == l->waste)) {
>
> - l = &t->loc[pos];
> l->count++;
> if (track->when) {
> l->sum_time += age;
> @@ -5190,6 +5199,7 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
> l->min_pid = track->pid;
> l->max_pid = track->pid;
> l->handle = handle;
> + l->waste = track->waste;
I think this may be fooled when there are different wastes values
from same caller (i.e. when a kmalloc_track_caller() is used.)
because the array is sorted by caller address, but not sorted by waste.
And writing this I noticed that it already can be fooled now :)
It's also not sorted by handle.
> cpumask_clear(to_cpumask(l->cpus));
> cpumask_set_cpu(track->cpu, to_cpumask(l->cpus));
> nodes_clear(l->nodes);
> @@ -6078,6 +6088,11 @@ static int slab_debugfs_show(struct seq_file *seq, void *v)
> else
> seq_puts(seq, "<not-available>");
>
> +
> + if (l->waste)
> + seq_printf(seq, " waste: %lu/%lu",
Maybe waste=%lu/%lu like others?
> + l->count * l->waste, l->waste);
> +
> if (l->sum_time != l->min_time) {
> seq_printf(seq, " age=%ld/%llu/%ld",
> l->min_time, div_u64(l->sum_time, l->count),
> --
> 2.27.0
>
--
Thanks,
Hyeonggon