From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from abb.hmeau.com (abb.hmeau.com [180.181.231.80]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AD35F47C0F2; Fri, 2 Oct 2026 10:13:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=180.181.231.80 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790936000; cv=none; b=CUm5+AnAqkm3T1tvxsFV8+274Ny42VEDT+3RrWdIFSAhb2aovWkcdual1n7e0yFg6nOuYcaeZgI5GVJ99omqv0iPFcUurumimHUv62dY/JLQWFQ3hTbWzxuEoNMtegA8zWVgdeE0zlYVTHD8GNNBhbyb6atO6Aa078VLLWo1sDI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790936000; c=relaxed/simple; bh=1K1pSu9xqd/QnEsydj1SEWL1rT4GROdwGnFU9QKa0DY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=KyXwjBPoANX9NbC+EopU+T9ca/JkEP22G3VWvDJ7snPGHF5pW1e0FDkKLRwL/pddN8jmwFegXMYMOCSGUnhCCdWkygIsC+5qiKMrKSsbxF/9pk9zvuT2Pz4CI0fjX9aMMrv2j7A6XJa/8Q0MxxwSVmjYrXU9LSEXYSGp2N6QGHU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=gondor.apana.org.au; spf=pass smtp.mailfrom=gondor.apana.org.au; dkim=pass (2048-bit key) header.d=gondor.apana.org.au header.i=@gondor.apana.org.au header.b=sg2LVVh2; arc=none smtp.client-ip=180.181.231.80 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=gondor.apana.org.au Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gondor.apana.org.au Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gondor.apana.org.au header.i=@gondor.apana.org.au header.b="sg2LVVh2" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=gondor.apana.org.au; s=h01; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:cc:to:subject:message-id:date: from:content-type:reply-to; bh=scpWT3qLm4oe8br1I0C/RtwU+L9zN4X8RYPTjpix0os=; b=sg2LVVh2tTbTrV9FU/THA4LSi2WrnA0DfNzGBYaok+5ogNhNmbRUDma3AHAvTvGRpxZjudsRSWW 21gZ2WzTOD9VOPcbBBn9+1u0lcJS9TrEfH+umdyNmD3Wpupg/wFhpbbai0ADBouZvB6ezfTcX9Zuc Wks9GVcmErchDOqDn16aH9DGJlW2QqPvpIfQ8Uqxkvmi5VUwvv0yflyOcadOB+l0opZH9MIPri+IY 7l2ldOFzDAKZ9aIxe18mR664w6WHYRhPfMl6NV29MA1KUJUw/+TjM/IhDt2TGFbbWTeugXrUzumnG /cDmz9UnfEuDbs72+ZBERZl0WPx8T1IqiqfQ==; Received: from loth.rohan.me.apana.org.au ([192.168.167.2]) by formenos.hmeau.com with smtp (Exim 4.98.2 #2 (Debian)) id 1xCaGI-000000001hp-1cbu; Fri, 02 Oct 2026 18:13:07 +0800 Received: by loth.rohan.me.apana.org.au (sSMTP sendmail emulation); Fri, 02 Oct 2026 20:13:06 +1000 Date: Fri, 2 Oct 2026 20:13:06 +1000 From: Herbert Xu To: Usama Arif Cc: Andrew Morton , justinstitt@google.com, linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, morbo@google.com, nathan@kernel.org, ndesaulniers@google.com, Thomas Graf , nickolay.lysenko@gmail.com, peterz@infradead.org, rostedt@goodmis.org, sched-ext@lists.linux.dev, tj@kernel.org Subject: Re: [PATCH] rhashtable: specialize default comparison for constant parameters Message-ID: References: <20260924184733.2317353-1-usama.arif@linux.dev> Precedence: bulk X-Mailing-List: linux-crypto@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260924184733.2317353-1-usama.arif@linux.dev> On Thu, Sep 24, 2026 at 11:47:33AM -0700, Usama Arif wrote: > The inline lookup and insert helpers receive the rhashtable parameters by > value. With a static const parameter block, key_offset and key_len are > compile-time constants at the call site. > > The default comparison throws that information away by reading both fields > back from ht->p. Fixed-size keys consequently load the parameters and call > bcmp() for every object in the bucket, even when the compiler could use > scalar comparisons instead. > > The commit "sched_ext: Specialize the DSQ hashtable compare" [1] resulted > in a 2.9x faster DSQ lookup after replacing this path for its u64 key > with a scalar comparison. Doing that through obj_cmpfn requires every > fixed-key user to provide its own callback. > > Pass the call-site parameters to rhashtable_compare(), as > rht_key_get_hash() already does. Use them when key_len is a compile-time > constant. Retain the ht->p path for dynamic parameter blocks. A constant > zero key_len continues to take the runtime length from ht->p.key_len. > > This specializes the default comparison for all fixed-size users. With > Clang 22 on x86-64, bcmp() disappears from sched_ext, mac80211, NFSd, > TIPC, NFQUEUE, VFS superblock, pidfs, SysV IPC and hardware-breakpoint > table paths. The affected build_policy.o, sta_info.o and nfsd filecache.o > text shrinks by 512, 400 and 352 bytes respectively. Four- and eight-byte > keys become direct scalar comparisons; six-byte MAC addresses become a > four-byte and a two-byte comparison. > > Measure the VFS case with ustat() on a mounted ramfs. This exercises > user_get_super() and its super_dev lookup. Across ten interleaved baseline > and patched VM boot pairs, with five 3 million call samples per boot, the > average per-boot median latency drops from 477.2 to 462.6 ns per call, or > 3.0%. All ten pairs improved. > > No functional change intended. > > [1] https://lore.kernel.org/all/20260921171928.1639407-1-usama.arif@linux.dev/ > > Suggested-by: Mykola Lysenko > Signed-off-by: Usama Arif > --- > include/linux/rhashtable.h | 20 +++++++++++++++----- > lib/rhashtable.c | 5 +++-- > 2 files changed, 18 insertions(+), 7 deletions(-) Patch applied. Thanks. -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt