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

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Date:   Mon, 16 May 2022 15:08:48 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Shakeel Butt <shakeelb@...gle.com>, Vasily Averin <vvs@...nvz.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Christoph Lameter <cl@...ux.com>, kernel@...nvz.org,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: Re: [PATCH] tracing: add ACCOUNT flag for allocations from marked
 slab caches

On Mon, May 16, 2022 at 11:41:27PM +0200, Vlastimil Babka wrote:
> On 5/16/22 21:10, Shakeel Butt wrote:
> > On Mon, May 16, 2022 at 11:53 AM Vasily Averin <vvs@...nvz.org> wrote:
> >>
> >> Slab 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.
> >>
> >> Signed-off-by: Vasily Averin <vvs@...nvz.org>
> >> ---
> >>  mm/slab.c | 3 +++
> >>  mm/slub.c | 3 +++
> >>  2 files changed, 6 insertions(+)
> >>
> >> diff --git a/mm/slab.c b/mm/slab.c
> >> index 0edb474edef1..4c3da8dfcbdb 100644
> >> --- a/mm/slab.c
> >> +++ b/mm/slab.c
> >> @@ -3492,6 +3492,9 @@ void *__kmem_cache_alloc_lru(struct kmem_cache *cachep, struct list_lru *lru,
> > 
> > What about kmem_cache_alloc_node()?
> > 
> >>  {
> >>         void *ret = slab_alloc(cachep, lru, flags, cachep->object_size, _RET_IP_);
> >>
> >> +       if (cachep->flags & SLAB_ACCOUNT)
> > 
> > Should this 'if' be unlikely() or should we trace cachep->flags
> > explicitly to avoid this branch altogether?
> 
> Hm I think ideally the tracepoint accepts cachep instead of current
> cachep->*size parameters and does the necessary extraction and
> modification in its fast_assign.

+1 for fast_assign

Changing flags just for tracing looks a bit excessive.