Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc

From: Feng Tang
Date: Thu Jun 30 2022 - 22:23:40 EST

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In reply to: Hyeonggon Yoo: "Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc"
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Hi Hyeonggon,

Thanks for the review!

On Thu, Jun 30, 2022 at 11:38:26PM +0900, Hyeonggon Yoo wrote:
> 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?

Yes, you are right.

> 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.)

Yes, the field itself is a 'waste' for non-kmalloc objects :) I do
have another patch to add an option for this

+config SLUB_DEBUG_KMALLOC_WASTE
+ bool "Enable kmalloc memory waste debug"
+ depends on SLUB_DEBUG && DEBUG_FS
...

And didn't post it due to the same debugging thought as you. And I can
add it back if it's really necessary.

> > 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?

Good catch! should be s->object_size. thanks!

> > 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.)

Yes, with the patch, we found quite some cases that the same caller
requests different sizes.

> because the array is sorted by caller address, but not sorted by waste.

In the patch we have in add_location():

+ if ((track->addr == caddr) && (handle == chandle) &&
+ (track->waste == l->waste)) {

Do you mean the following is missed?

--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5176,6 +5176,8 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
end = pos;
else if (track->addr == caddr && handle < chandle)
end = pos;
+ else if (track->addr == caddr && handle == chandle && track->waste < l->waste)
+ end = pos;
else
start = pos;

> 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?

Sure, will follow current style.

Thanks,
Feng

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Previous message: Judy Hsiao: "[PATCH v1] FROMGIT: ASoC: rockchip: i2s: Fix error code when fail to read I2S_CLR"
In reply to: Hyeonggon Yoo: "Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc"
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