linux-kernel - Re: [PATCH] mm, slub: print CPU id on slab OOM

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Message-ID: <6951700d-b6c0-b9b7-6587-1823a9d8c63d@google.com>
Date: Sun, 11 Aug 2024 13:21:52 -0700 (PDT)
From: David Rientjes <rientjes@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
cc: Axel Rasmussen <axelrasmussen@...gle.com>, 
    Andrew Morton <akpm@...ux-foundation.org>, 
    Christoph Lameter <cl@...ux.com>, Hyeonggon Yoo <42.hyeyoo@...il.com>, 
    Joonsoo Kim <iamjoonsoo.kim@....com>, Pekka Enberg <penberg@...nel.org>, 
    Roman Gushchin <roman.gushchin@...ux.dev>, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org
Subject: Re: [PATCH] mm, slub: print CPU id on slab OOM

On Sun, 11 Aug 2024, Vlastimil Babka wrote:

> > diff --git a/mm/slub.c b/mm/slub.c
> > index c9d8a2497fd6..7148047998de 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3422,7 +3422,8 @@ slab_out_of_memory(struct kmem_cache *s, gfp_t gfpflags, int nid)
> >  	if ((gfpflags & __GFP_NOWARN) || !__ratelimit(&slub_oom_rs))
> >  		return;
> >  
> > -	pr_warn("SLUB: Unable to allocate memory on node %d, gfp=%#x(%pGg)\n",
> > +	pr_warn("SLUB: Unable to allocate memory for CPU %u on node %d, gfp=%#x(%pGg)\n",
> 
> BTW, wouldn't "on CPU" be more correct, as "for CPU" might be misleading
> that we are somehow constrained to that CPU?
> 

Agreed.

When I suggested this patch, I was trying to ascertain whether something 
was really wonky based on some logs that we were seeing.

  node 0: slabs: 223, objs: 11819, free: 0
  node 1: slabs: 951, objs: 50262, free: 218

This is for a NUMA_NO_NODE allocation, so I wanted to know if the cpu was 
on node 0 or node 1.

Even with the patch, that requires knowing the cpu-to-node mapping.  If we 
add the CPU output here, we likely also want to print out cpu_to_node().

> > +		preemptible() ? raw_smp_processor_id() : smp_processor_id(),
> 
> Also could we just use raw_smp_processor_id() always here? I don't see
> this has any advantage or am I missing something?
> 

This matches my understanding as well.