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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: stable@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! > > > 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. > > > +-----------------+----------------------------------------------------------+-----------------+-------------+-------------+---------------------------+-------------+-------------+-------------+ > > | 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. > 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. > > A regression of vmalloc related to kvfree_rcu might be new. Although if it's > kvfree_rcu() of vmalloc'd objects, it would be weird. More likely they are > kvmalloc'd but small enough to be actually kmalloc'd? What are the details > of that test? > static int kvfree_rcu_2_arg_vmalloc_test(void) { struct test_kvfree_rcu *p; int i; for (i = 0; i < test_loop_count; i++) { p = vmalloc(1 * PAGE_SIZE); if (!p) return -1; p->array[0] = 'a'; kvfree_rcu(p, rcu); } return 0; } static bool kfree_rcu_sheaf(void *obj) { struct kmem_cache *s; struct slab *slab; if (is_vmalloc_addr(obj)) return false; slab = virt_to_slab(obj); if (unlikely(!slab)) return false; s = slab->slab_cache; if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())) return __kfree_rcu_sheaf(s, obj); return false; } it does not go via sheaf since it is a vmalloc address. -- Uladzislau Rezki