linux-kernel - Re: Perf and Hackbench results on my machine

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Date:   Mon, 11 Oct 2021 13:49:48 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Perf and Hackbench results on my machine

On Mon, Oct 11, 2021 at 10:33:02AM +0000, Hyeonggon Yoo wrote:
> Hello Vlastimil.
> 
> On Mon, Oct 11, 2021 at 09:21:01AM +0200, Vlastimil Babka wrote:
> > On 10/11/21 00:49, David Rientjes wrote:
> > > On Fri, 8 Oct 2021, Hyeonggon Yoo wrote:
> > > 
> > >> It's certain that an object will be not only read, but also
> > >> written after allocation.
> > >> 
> > > 
> > > Why is it certain?  I think perhaps what you meant to say is that if we 
> > > are doing any prefetching here, then access will benefit from prefetchw 
> > > instead of prefetch.  But it's not "certain" that allocated memory will be 
> > > accessed at all.
> > 
> > I think the primary reason there's a prefetch is freelist traversal. The
> > cacheline we prefetch will be read during the next allocation, so if we
> > expect there to be one soon, prefetch might help.
> 
> I agree that.
> 
> > That the freepointer is
> > part of object itself and thus the cache line will be probably accessed also
> > after the allocation, is secondary.
> 
> Right. it depends on cache line size and whether first cache line of an
> object is frequently accessed or not.

Not first cache line because free pointer is in the middle of object or
out of object area. my mistake.

> >> Use prefetchw instead of prefetchw. On supported architecture
> >
> > If we're using prefetchw instead of prefetchw, I think the diff would be
> > 0 lines changed :)
> >
> >> like x86, it helps to invalidate cache line when the object exists
> >> in other processors' cache.
> >>
> >> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
> >> ---
> >>  mm/slub.c | 7 +++----
> >>  1 file changed, 3 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/mm/slub.c b/mm/slub.c
> >> index 3d2025f7163b..2aca7523165e 100644
> >> --- a/mm/slub.c
> >> +++ b/mm/slub.c
> >> @@ -352,9 +352,9 @@ static inline void *get_freepointer(struct kmem_cache *s, void *object)
> >>  	return freelist_dereference(s, object + s->offset);
> >>  }
> >>
> >> -static void prefetch_freepointer(const struct kmem_cache *s, void *object)
> >> +static void prefetchw_freepointer(const struct kmem_cache *s, void *object)
>
> I wouldn't rename the function itself, unless we have both  variants for
> different situations (we don't). That it uses prefetchw() is internal detail
> at this point.

looks good. that is simpler.