Date:   Fri, 1 Jul 2022 18:29:20 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, dave.hansen@...el.com
Subject: Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc

On Fri, Jul 01, 2022 at 10:23:30AM +0800, Feng Tang wrote:
> 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@...el.com>
> > > ---
> > > 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.

Let's see how others think :)
I'm okay with current patch.

> > >  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;
>

Yes. Exactly. :)

Thanks,
Hyeonggon

> > 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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