linux-kernel - Re: [PATCH] rhashtable: add likely() to __rht

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Message-ID: <CADxym3b=_gb6o7xyozXekF4RbUoFe4h=zfegFuARFWqeWaisaQ@mail.gmail.com>
Date: Tue, 23 Sep 2025 20:10:30 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: NeilBrown <neil@...wn.name>
Cc: Herbert Xu <herbert@...dor.apana.org.au>, tgraf@...g.ch, 
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rhashtable: add likely() to __rht_ptr()

On Tue, Sep 23, 2025 at 7:31 PM NeilBrown <neilb@...mail.net> wrote:
>
> On Tue, 23 Sep 2025, Menglong Dong wrote:
> > On Tue, Sep 23, 2025 at 2:36 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
> > >
> > > Menglong Dong <menglong8.dong@...il.com> wrote:
> > > > In the fast path, the value of "p" in __rht_ptr() should be valid.
> > > > Therefore, wrap it with a "likely". The performance increasing is tiny,
> > > > but it's still worth to do it.
> > > >
> > > > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > > > ---
> > > > include/linux/rhashtable.h | 5 +++--
> > > > 1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > It's not obvious that rht_ptr would be non-NULL.  It depends on the
> > > work load.  For example, if you're doing a lookup where most keys
> > > are non-existent then it would most likely be NULL.
> >
> > Yeah, I see. In my case, the usage of the rhashtable will be:
> > add -> lookup, and rht_ptr is alway non-NULL. You are right,
> > it can be NULL in other situations, and it's not a good idea to
> > use likely() here ;)
>
> Have you measured a performance increase?  How tiny is it?

Hi. It is a bit difficult to accurately measure the performance
improvement. I use it in the bpf global trampoline in [1], and
the performance of the bpf bench testing increases from
135M/s to 136M/s when I add the likely() to the __rht_ptr().

https://lore.kernel.org/bpf/20250703121521.1874196-2-dongml2@chinatelecom.cn/
[1]

>
> It might conceivably make sense to have a rhashtable_lookup_likely() and
> rhashtable_lookup_unlikely(), but concrete evidence of the benefit would
> be needed.

I think such rhashtable_lookup_likely/rhashtable_lookup_unlikely make
sense (to me at least), and I'm sure the evidence above is enough ;)

Thanks!
Menglong Dong

>
> Thanks,
> NeilBrown