From: Vlastimil Babka vbabka@suse.cz Subject: mm, slub: better heuristic for number of cpus when calculating slab order

When creating a new kmem cache, SLUB determines how large the slab pages will based on number of inputs, including the number of CPUs in the system. Larger slab pages mean that more objects can be allocated/free from per-cpu slabs before accessing shared structures, but also potentially more memory can be wasted due to low slab usage and fragmentation. The rough idea of using number of CPUs is that larger systems will be more likely to benefit from reduced contention, and also should have enough memory to spare.

Number of CPUs used to be determined as nr_cpu_ids, which is number of possible cpus, but on some systems many will never be onlined, thus commit 045ab8c9487b ("mm/slub: let number of online CPUs determine the slub page order") changed it to nr_online_cpus(). However, for kmem caches created early before CPUs are onlined, this may lead to permamently low slab page sizes.

Vincent reports a regression [1] of hackbench on arm64 systems:

I'm facing significant performances regression on a large arm64 server system (224 CPUs). Regressions is also present on small arm64 system (8 CPUs) but in a far smaller order of magnitude

On 224 CPUs system : 9 iterations of hackbench -l 16000 -g 16 v5.11-rc4 : 9.135sec (+/- 0.45%) v5.11-rc4 + revert this patch: 3.173sec (+/- 0.48%) v5.10: 3.136sec (+/- 0.40%)

Mel reports a regression [2] of hackbench on x86_64, with lockstat suggesting page allocator contention:

i.e. the patch incurs a 7% to 32% performance penalty. This bisected cleanly yesterday when I was looking for the regression and then found the thread.

Numerous caches change size. For example, kmalloc-512 goes from order-0 (vanilla) to order-2 with the revert.

So mostly this is down to the number of times SLUB calls into the page allocator which only caches order-0 pages on a per-cpu basis.

Clearly num_online_cpus() doesn't work too early in bootup. We could change the order dynamically in a memory hotplug callback, but runtime order changing for existing kmem caches has been already shown as dangerous, and removed in 32a6f409b693 ("mm, slub: remove runtime allocation order changes"). It could be resurrected in a safe manner with some effort, but to fix the regression we need something simpler.

We could use num_present_cpus() that should be the number of physically present CPUs even before they are onlined. That would work for PowerPC [3], which triggered the original commit, but that still doesn't work on arm64 [4] as explained in [5].

So this patch tries to determine the best available value without specific arch knowledge.

- num_present_cpus() if the number is larger than 1, as that means the arch is likely setting it properly

- nr_cpu_ids otherwise

This should fix the reported regressions while also keeping the effect of 045ab8c9487b for PowerPC systems. It's possible there are configurations where num_present_cpus() is 1 during boot while nr_cpu_ids is at the same time bloated, so these (if they exist) would keep the large orders based on nr_cpu_ids as was before 045ab8c9487b.

[1] https://lore.kernel.org/linux-mm/CAKfTPtA_JgMf_+zdFbcb_V9rM7JBWNPjAz9irgwFj7... [2] https://lore.kernel.org/linux-mm/20210128134512.GF3592@techsingularity.net/ [3] https://lore.kernel.org/linux-mm/20210123051607.GC2587010@in.ibm.com/ [4] https://lore.kernel.org/linux-mm/CAKfTPtAjyVmS5VYvU6DBxg4-JEo5bdmWbngf-03YsY... [5] https://lore.kernel.org/linux-mm/20210126230305.GD30941@willie-the-truck/

Link: https://lkml.kernel.org/r/20210208134108.22286-1-vbabka@suse.cz Fixes: 045ab8c9487b ("mm/slub: let number of online CPUs determine the slub page order") Signed-off-by: Vlastimil Babka vbabka@suse.cz Reported-by: Vincent Guittot vincent.guittot@linaro.org Reported-by: Mel Gorman mgorman@techsingularity.net Tested-by: Vincent Guittot vincent.guittot@linaro.org Cc: Catalin Marinas catalin.marinas@arm.com Cc: Aneesh Kumar K.V aneesh.kumar@linux.ibm.com Cc: Bharata B Rao bharata@linux.ibm.com Cc: Christoph Lameter cl@linux.com Cc: Roman Gushchin guro@fb.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Joonsoo Kim iamjoonsoo.kim@lge.com Cc: Jann Horn jannh@google.com Cc: Michal Hocko mhocko@kernel.org Cc: David Rientjes rientjes@google.com Cc: Shakeel Butt shakeelb@google.com Cc: Will Deacon will@kernel.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/slub.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-)

--- a/mm/slub.c~mm-slub-better-heuristic-for-number-of-cpus-when-calculating-slab-order +++ a/mm/slub.c @@ -3423,6 +3423,7 @@ static inline int calculate_order(unsign unsigned int order; unsigned int min_objects; unsigned int max_objects; + unsigned int nr_cpus;

/* * Attempt to find best configuration for a slab. This @@ -3433,8 +3434,21 @@ static inline int calculate_order(unsign * we reduce the minimum objects required in a slab. */ min_objects = slub_min_objects; - if (!min_objects) - min_objects = 4 * (fls(num_online_cpus()) + 1); + if (!min_objects) { + /* + * Some architectures will only update present cpus when + * onlining them, so don't trust the number if it's just 1. But + * we also don't want to use nr_cpu_ids always, as on some other + * architectures, there can be many possible cpus, but never + * onlined. Here we compromise between trying to avoid too high + * order on systems that appear larger than they are, and too + * low order on systems that appear smaller than they are. + */ + nr_cpus = num_present_cpus(); + if (nr_cpus <= 1) + nr_cpus = nr_cpu_ids; + min_objects = 4 * (fls(nr_cpus) + 1); + } max_objects = order_objects(slub_max_order, size); min_objects = min(min_objects, max_objects);

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