linux-kernel - Re: [PATCH v2] tracing: add ACCOUNT flag for allocations from marked slab caches

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Date:   Tue, 17 May 2022 21:29:01 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     YoMccU66auLAPEHa@...per.infradead.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>, kernel@...nvz.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v2] tracing: add ACCOUNT flag for allocations from marked
 slab caches

On Tue, May 17, 2022 at 08:59:31PM +0900, Hyeonggon Yoo wrote:
> On Tue, May 17, 2022 at 12:44:14PM +0300, Vasily Averin wrote:
> > dSlab caches marked with SLAB_ACCOUNT force accounting for every
> > allocation from this cache even if __GFP_ACCOUNT flag is not passed.
> > Unfortunately, at the moment this flag is not visible in ftrace output,
> > and this makes it difficult to analyze the accounted allocations.
> > 
> > This patch adds the __GFP_ACCOUNT flag for allocations from slab caches
> > marked with SLAB_ACCOUNT to the ftrace output
> > ---
> > v2:
> >  1) handle kmem_cache_alloc_node() too, thanks to Shakeel
> >  2) rework kmem_cache_alloc* tracepoints to use cachep instead
> >     of current cachep->*size parameters. Now kmalloc[_node] and
> >     kmem_cache_alloc[_node] tracepoints do not use common template
> > 
> > NB: kmem_cache_alloc_node tracepoint in SLOB cannot be switched to cachep,
> >     therefore it was replaced by kmalloc_node tracepoint.
> > ---
> > VvS: is this acceptable? Maybe I should split this patch?
> > 
> > Signed-off-by: Vasily Averin <vvs@...nvz.org>
> > ---
> >  include/trace/events/kmem.h | 82 +++++++++++++++++++++++++++----------
> >  mm/slab.c                   |  7 +---
> >  mm/slab_common.c            |  7 ++--
> >  mm/slob.c                   | 10 ++---
> >  mm/slub.c                   |  6 +--
> >  5 files changed, 71 insertions(+), 41 deletions(-)
> > 
> > diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
> > index 71c141804222..3b4f96e4a607 100644
> > --- a/include/trace/events/kmem.h
> > +++ b/include/trace/events/kmem.h
> > @@ -9,7 +9,7 @@
> >  #include <linux/tracepoint.h>
> >  #include <trace/events/mmflags.h>
> >  
> > -DECLARE_EVENT_CLASS(kmem_alloc,
> > +TRACE_EVENT(kmalloc,
> >  
> >  	TP_PROTO(unsigned long call_site,
> >  		 const void *ptr,
> > @@ -43,23 +43,41 @@ DECLARE_EVENT_CLASS(kmem_alloc,
> >  		show_gfp_flags(__entry->gfp_flags))
> >  );
> >  
> > -DEFINE_EVENT(kmem_alloc, kmalloc,
> > +TRACE_EVENT(kmem_cache_alloc,
> >  
> > -	TP_PROTO(unsigned long call_site, const void *ptr,
> > -		 size_t bytes_req, size_t bytes_alloc, gfp_t gfp_flags),
> > +	TP_PROTO(unsigned long call_site,
> > +		 const void *ptr,
> > +		 struct kmem_cache *s,
> > +		 gfp_t gfp_flags),
> >  
> > -	TP_ARGS(call_site, ptr, bytes_req, bytes_alloc, gfp_flags)
> > -);
> > +	TP_ARGS(call_site, ptr, s, gfp_flags),
> >  
> > -DEFINE_EVENT(kmem_alloc, kmem_cache_alloc,
> > +	TP_STRUCT__entry(
> > +		__field(	unsigned long,	call_site	)
> > +		__field(	const void *,	ptr		)
> > +		__field(	size_t,		bytes_req	)
> > +		__field(	size_t,		bytes_alloc	)
> > +		__field(	unsigned long,	gfp_flags	)
> > +	),
> >  
> > -	TP_PROTO(unsigned long call_site, const void *ptr,
> > -		 size_t bytes_req, size_t bytes_alloc, gfp_t gfp_flags),
> > +	TP_fast_assign(
> > +		__entry->call_site	= call_site;
> > +		__entry->ptr		= ptr;
> > +		__entry->bytes_req	= s->object_size;
> > +		__entry->bytes_alloc	= s->size;
> > +		__entry->gfp_flags	= (__force unsigned long)gfp_flags |
> > +				(s->flags & SLAB_ACCOUNT ? __GFP_ACCOUNT : 0);
> > +	),
> 
> This is a bit of lie. SLAB_ACCOUNT is not a gfp flag.
>

Maybe it is not a problem since the functionalities of SLAB_ACCOUNT and
__GFP_ACCOUNT are similar.
 
> IMO the problem here is that we don't know which cache kernel is allocating
> from. What about just printing name of cache and remove bytes_req,
> bytes_alloc?

Is it a problem? Because we have changed the behavior to users. Should
we treat the tracepoint as a stable API to users? If so, I think we
should not change the parameter of this tracepoint.  Maybe I am wrong,
just some thoughts from me.

Thanks.

> 
> And then you can check if the cache uses SLAB_ACCOUNT or not.
>