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Date: Tue, 5 Jul 2022 10:34:15 +0800
From: Feng Tang <feng.tang@...el.com>
To: Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc: Christoph Lameter <cl@...two.de>,
Andrew Morton <akpm@...ux-foundation.org>,
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,
Robin Murphy <robin.murphy@....com>,
John Garry <john.garry@...wei.com>
Subject: Re: [PATCH v1] mm/slub: enable debugging memory wasting of kmalloc
On Mon, Jul 04, 2022 at 10:05:29AM +0000, Hyeonggon Yoo wrote:
> On Mon, Jul 04, 2022 at 01:56:00PM +0800, Feng Tang wrote:
> > On Sun, Jul 03, 2022 at 02:17:37PM +0000, Hyeonggon Yoo wrote:
> > > On Fri, Jul 01, 2022 at 11:04:51PM +0800, Feng Tang wrote:
> > > > Hi Christoph,
> > > >
> > > > On Fri, Jul 01, 2022 at 04:37:00PM +0200, Christoph Lameter wrote:
> > > > > On Fri, 1 Jul 2022, Feng Tang wrote:
> > > > >
> > > > > > 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)
> > > > > > {
> > > > >
> > > > > It would be good to avoid expanding the basic slab handling functions for
> > > > > kmalloc. Can we restrict the mods to the kmalloc related functions?
> > > >
> > > > Yes, this is the part that concerned me. I tried but haven't figured
> > > > a way.
> > > >
> > > > I started implemting it several month ago, and stuck with several
> > > > kmalloc APIs in a hacky way like dump_stack() when there is a waste
> > > > over 1/4 of the object_size of the kmalloc_caches[][].
> > > >
> > > > Then I found one central API which has all the needed info (object_size &
> > > > orig_size) that we can yell about the waste :
> > > >
> > > > static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_lru *lru,
> > > > gfp_t gfpflags, int node, unsigned long addr, size_t orig_size)
> > > >
> > > > which I thought could be still hacky, as the existing 'alloc_traces'
> > > > can't be resued which already has the count/call-stack info. Current
> > > > solution leverage it at the cost of adding 'orig_size' parameters, but
> > > > I don't know how to pass the 'waste' info through as track/location is
> > > > in the lowest level.
> > >
> > > If adding cost of orig_size parameter for non-debugging case is concern,
> > > what about doing this in userspace script that makes use of kmalloc
> > > tracepoints?
> > >
> > > kmalloc: call_site=tty_buffer_alloc+0x43/0x90 ptr=00000000b78761e1
> > > bytes_req=1056 bytes_alloc=2048 gfp_flags=GFP_ATOMIC|__GFP_NOWARN
> > > accounted=false
> > >
> > > calculating sum of (bytes_alloc - bytes_req) for each call_site
> > > may be an alternative solution.
> >
> > Yes, this is doable, and it will met some of the problems I met before,
> > one is there are currently 2 alloc path: kmalloc and kmalloc_node, also
> > we need to consider the free problem to calculate the real waste, and
> > the free trace point doesn't have size info (Yes, we could compare
> > the pointer with alloc path, and the user script may need to be more
> > complexer). That's why I love the current 'alloc_traces' interface,
> > which has the count (slove the free counting problem) and full call
> > stack info.
>
> Understood.
>
> > And for the extra parameter cost issue, I rethink about it, and we
> > can leverage the 'slab_alloc_node()' to solve it, and the patch is
> > much simpler now without adding a new parameter:
> >
> > ---
> > diff --git a/mm/slub.c b/mm/slub.c
> > index b1281b8654bd3..ce4568dbb0f2d 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 */
> > };
> >
> > @@ -3240,6 +3241,16 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_l
> > init = slab_want_init_on_alloc(gfpflags, s);
> >
> > out:
> > +
> > +#ifdef CONFIG_SLUB_DEBUG
> > + if (object && s->object_size != orig_size) {
> > + struct track *track;
> > +
> > + track = get_track(s, object, TRACK_ALLOC);
> > + track->waste = s->object_size - orig_size;
> > + }
> > +#endif
> > +
>
> This scares me. It does not check if the cache has
> SLAB_STORE_USER flag.
Yes, I missed that.
> Also CONFIG_SLUB_DEBUG is enabled by default, which means that
> it is still against not affecting non-debugging case.
Yes, logically these debug stuff can be put together in low-level
function.
> I like v1 more than modified version.
I see, thanks
- Feng
> Thanks,
> Hyeonggon
>
> > slab_post_alloc_hook(s, objcg, gfpflags, 1, &object, init);
> >
> > return object;
> > @@ -5092,6 +5103,7 @@ 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;
> > @@ -5142,7 +5154,7 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
> > {
> > long start, end, pos;
> > struct location *l;
> > - unsigned long caddr, chandle;
> > + unsigned long caddr, chandle, cwaste;
> > unsigned long age = jiffies - track->when;
> > depot_stack_handle_t handle = 0;
> >
> > @@ -5162,11 +5174,13 @@ 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;
> > + cwaste = l->waste;
> > + if ((track->addr == caddr) && (handle == chandle) &&
> > + (track->waste == cwaste)) {
> >
> > - l = &t->loc[pos];
> > l->count++;
> > if (track->when) {
> > l->sum_time += age;
> > @@ -5191,6 +5205,9 @@ 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 < cwaste)
> > + end = pos;
> > else
> > start = pos;
> > }
> > @@ -5214,6 +5231,7 @@ 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;
> > cpumask_clear(to_cpumask(l->cpus));
> > cpumask_set_cpu(track->cpu, to_cpumask(l->cpus));
> > nodes_clear(l->nodes);
> > @@ -6102,6 +6120,10 @@ 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",
> > + 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),
> >
> > Thanks,
> > Feng
> >
> > > Thanks,
> > > Hyeonggon
> > >
> > > > Thanks,
> > > > Feng
> > > >
> > > >
> > > >
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