From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 4145B299A81; Fri, 27 Mar 2026 03:20:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774581614; cv=none; b=F7VOgLYkQL3IgbesauHfORoGeBcs28U3KtA+/AAs9CEyTtjfpzMOy+uQrk6VD/FC1pNswhZVesBX1B0ZROYE2vwR/k1cJNguMzV7KPXHe2hz/tioonZFIT1HJb2muepdF6qx0RG+QyTmxywEMHBaGixT8jbvcUp73xTsYsyleK0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774581614; c=relaxed/simple; bh=3rohkKHMZQSs2BVw73LamMIlHPpx5pWmvpls8oac/OA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=StwV8roI2qNJvatUFFH3F6NiPw1+qnfCmo7dqPihJCjLOKbo6AtnXuC634YJFjz6YtAipC0Svio6DSf6VUBkO4gIH/kE1GobzeBj+Z9iKE/HbFl/9st1+fBTTzRKe6jTcnoyigr7wKYqQs+hw7P7Y0k6Xz+t6bQEui5xAppXkiU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Z0XL+JbA; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Z0XL+JbA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 60289C116C6; Fri, 27 Mar 2026 03:20:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774581613; bh=3rohkKHMZQSs2BVw73LamMIlHPpx5pWmvpls8oac/OA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Z0XL+JbAwseDsRa2gIRo+mhPVfWP36nkJqE9DU2VW9+LjgoUeic597AbmvhNA6gAF HbiLKzocmNLQvYO68jqA5sLxqkVn6sIOLGp4lSw9cLWRj9sbHJdwSS6eGf6GwvdYoS 9sTACWuIhhiYse1jfPgrtkX7weuRp2pY7y9uZZtCKIc3AngL8U9iFjW75nRjtN4XqI i7SXgX39UBWVHal+BXyxk0AtmR7y9T73E/Q/G8vi9/tqmL7mgrTtrAkQ9bZnsQyHgj ann5mgDwKks8CFCOZe/A0IUP/OPBcE8EV8fQiJ87xnuiJYDqiCWO/OLmUEuxn0TnkJ jVLcZyqZb1AUg== Date: Fri, 27 Mar 2026 12:20:11 +0900 From: "Harry Yoo (Oracle)" To: "Vlastimil Babka (SUSE)" Cc: Aishwarya Rambhadran , Vlastimil Babka , Petr Tesarik , Christoph Lameter , David Rientjes , Roman Gushchin , Hao Li , Andrew Morton , Uladzislau Rezki , "Liam R. Howlett" , Suren Baghdasaryan , Sebastian Andrzej Siewior , Alexei Starovoitov , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, bpf@vger.kernel.org, kasan-dev@googlegroups.com, kernel test robot , stable@vger.kernel.org, "Paul E. McKenney" , ryan.roberts@arm.com Subject: Re: [REGRESSION] slab: replace cpu (partial) slabs with sheaves Message-ID: References: <20260123-sheaves-for-all-v4-0-041323d506f7@suse.cz> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Mar 26, 2026 at 03:42:02PM +0100, Vlastimil Babka (SUSE) wrote: > On 3/26/26 13:43, Aishwarya Rambhadran wrote: > > Hi Vlastimil, Harry, > > Hi! Hi! > > We have observed few kernel performance benchmark regressions, > > mainly in perf & vmalloc workloads, when comparing v6.19 mainline > > kernel results against later releases in the v7.0 cycle. > > Independent bisections on different machines consistently point > > to commits within the slab percpu sheaves series. However, towards > > the end of the bisection, the signal becomes less clear, so it's > > not yet certain which specific commit within the series is the > > root cause. > > > > The workloads were triggered on AWS Graviton3 (arm64) & AWS Intel > > Sapphire Rapids (x86_64) systems in which the regressions are > > reproducible across different kernel release candidates. > > (R)/(I) mean statistically significant regression/improvement, > > where "statistically significant" means the 95% confidence > > intervals do not overlap”. > > > > Below given are the performance benchmark results generated by > > Fastpath Tool, for different kernel -rc versions relative to the > > base version v6.19, executed on the mentioned SUTs. The perf/ > > syscall benchmarks (execve/fork) regress consistently by ~6–11% on > > both arm64 and x86_64 across v7.0-rc1 to rc5, while vmalloc > > workloads show smaller but stable regressions (~2–10%), particularly > > in kvfree_rcu paths. > > > > Regressions on AWS Intel Sapphire Rapids (x86_64) : > > The table formatting is broken for me, can you resend it please? Maybe a > .txt attachment would work better. A quick manual re-formatting with a hope that your monitor is wide enough to cover it :) Regressions on AWS Intel Sapphire Rapids (x86_64) : +-----------------+----------------------------------------------------------+-----------------+-------------+-------------+---------------------------+-------------+-------------+-------------+ | Benchmark | Result Class | 6-19-0 (base) | 7-0-0-rc1 | 7-0-0-rc2 | 7-0-0-rc2-gaf4e9ef3d784 | 7-0-0-rc3 | 7-0-0-rc4 | 7-0-0-rc5 | +=================+==========================================================+=================+=============+=============+==========================+=============+=============+=============+ | micromm/vmalloc | kvfree_rcu_1_arg_vmalloc_test: p:1, h:0, l:500000 (usec) | 262605.17 | -4.94% | -7.48% | (R) -8.11% | -4.51% | -6.23% | -3.47% | | | kvfree_rcu_2_arg_vmalloc_test: p:1, h:0, l:500000 (usec) | 253198.67 | -7.56% | (R) -10.57% | (R) -10.13% | (R) -7.07% | -6.37% | -6.55% | | | pcpu_alloc_test: p:1, h:0, l:500000 (usec) | 197904.67 | -2.07% | -3.38% | -2.07% | -2.97% | (R) -4.30% | -3.39% | | | random_size_align_alloc_test: p:1, h:0, l:500000 (usec) | 1707089.83 | -2.63% | (R) -3.69% | (R) -3.25% | (R) -2.87% | -2.22% | (R) -3.63% | +-----------------+----------------------------------------------------------+-----------------+-------------+-------------+--------------------------+-------------+-------------+-------------+ | perf/syscall | execve (ops/sec) | 1202.92 | (R) -7.15% | (R) -7.05% | (R) -7.03% | (R) -7.93% | (R) -6.51% | (R) -7.36% | | | fork (ops/sec) | 996.00 | (R) -9.00% | (R) -10.27% | (R) -9.92% | (R) -11.19% | (R) -10.69% | (R) -10.28% | +-----------------+----------------------------------------------------------+-----------------+-------------+-------------+--------------------------+-------------+-------------+-------------+ Regressions on AWS Graviton3 (arm64) : +-----------------+----------------------------------------------------------+-----------------+-------------+-------------+--------------------------+-------------+-------------+-------------+ | Benchmark | Result Class | 6-19-0 (base) | 7-0-0-rc1 | 7-0-0-rc2 | 7-0-0-rc2-gaf4e9ef3d784 | 7-0-0-rc3 | 7-0-0-rc4 | 7-0-0-rc5 | +=================+==========================================================+=================+=============+=============+==========================+=============+=============+=============+ | micromm/vmalloc | fix_size_alloc_test: p:1, h:0, l:500000 (usec) | 320101.50 | (R) -4.72% | (R) -3.81% | (R) -5.05% | -3.06% | -3.16% | (R) -3.91% | | | fix_size_alloc_test: p:4, h:0, l:500000 (usec) | 522072.83 | (R) -2.15% | -1.25% | (R) -2.16% | (R) -2.13% | -2.10% | -1.82% | | | fix_size_alloc_test: p:16, h:0, l:500000 (usec) | 1041640.33 | -0.50% | (R) -2.04% | -1.43% | -0.69% | -1.78% | (R) -2.03% | | | fix_size_alloc_test: p:256, h:1, l:100000 (usec) | 2255794.00 | -1.51% | (R) -2.24% | (R) -2.33% | -1.14% | -0.94% | -1.60% | | | kvfree_rcu_1_arg_vmalloc_test: p:1, h:0, l:500000 (usec) | 343543.83 | (R) -4.50% | (R) -3.54% | (R) -5.00% | (R) -4.88% | (R) -4.01% | (R) -5.54% | | | kvfree_rcu_2_arg_vmalloc_test: p:1, h:0, l:500000 (usec) | 342290.33 | (R) -5.15% | (R) -3.24% | (R) -3.76% | (R) -5.37% | (R) -3.74% | (R) -5.51% | | | random_size_align_alloc_test: p:1, h:0, l:500000 (usec) | 1209666.83 | -2.43% | -2.09% | -1.19% | (R) -4.39% | -1.81% | -3.15% | +-----------------+----------------------------------------------------------+-----------------+-------------+-------------+--------------------------+-------------+-------------+-------------+ | perf/syscall | execve (ops/sec) | 1219.58 | | (R) -8.12% | (R) -7.37% | (R) -7.60% | (R) -7.86% | (R) -7.71% | | | fork (ops/sec) | 863.67 | | (R) -7.24% | (R) -7.07% | (R) -6.42% | (R) -6.93% | (R) -6.55% | +-----------------+----------------------------------------------------------+-----------------+-------------+-------------+--------------------------+-------------+-------------+-------------+ > > The details of latest bisections that were carried out for the above > > listed regressions, are given below : > > -Graviton3 (arm64) > >  good: v6.19 (05f7e89ab973) > >  bad:  v7.0-rc2 (11439c4635ed) > >  workload: perf/syscall (execve) > >  bisected to: f1427a1d6415 (“slab: make percpu sheaves compatible with > >  kmalloc_nolock()/kfree_nolock()”) > > > > -Sapphire Rapids (x86_64) > >  good: v6.19 (05f7e89ab973) > >  bad:  v7.0-rc3 (1f318b96cc84) > >  workload: perf/syscall (fork) > >  bisected to: f1427a1d6415 (“slab: make percpu sheaves compatible with > >  kmalloc_nolock()/kfree_nolock()”) > > > > -Graviton3 (arm64) > >  good: v6.19 (05f7e89ab973) > >  bad:  v7.0-rc3 (1f318b96cc84) > >  workload: perf/syscall (execve) > >  bisected to: f3421f8d154c (“slab: introduce percpu sheaves bootstrap”) > > Yeah none of these are likely to introduce the regression. Agreed. > We've seen other reports from e.g. lkp pointing to later commits that remove > the cpu (partial) slabs. The theory is that on benchmarks that stress vma > and maple node caches (fork and execve are likely those), the introduction > of sheaves in 6.18 (for those caches only) resulted in ~doubled percpu > caching capacity (and likely associated performance increase) - by sheaves > backed by cpu (partial) slabs,. Removing the latter then looks like a > regression in isolation in the 7.0 series. Yeah, going through a comparison similar to what Hao Li did [1] a while ago might confirm the theory. [1] https://lore.kernel.org/linux-mm/pdmjsvpkl5nsntiwfwguplajq27ak3xpboq3ab77zrbu763pq7@la3hyiqigpir > > I'm aware that some fixes for the sheaves series have already been > > merged around v7.0-rc3; however, these do not appear to resolve the > > regressions described above completely. Are there additional fixes or > > follow-ups in progress that I should evaluate? I can investigate > > further and provide additional data, if that would be useful. > > We have some followups planned for 7.1 that would make a difference for > systems with memoryless nodes. That would mean "numactl -H" shows nodes that > have cpus but no memory, or that memory is all ZONE_MOVABLE and not ZONE_NORMAL. In any case having numactl -H for those machines would be helpful! -- Cheers, Harry / Hyeonggon