Messages in this thread | | | Date | Wed, 20 Mar 2024 10:46:54 -0700 | From | Yury Norov <> | Subject | Re: [PATCH] cpumask: Create dedicated kmem cache for cpumask var |
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On Wed, Mar 20, 2024 at 10:03:02AM +0100, Rasmus Villemoes wrote: > On 19/03/2024 13.24, Dawei Li wrote: > > alloc_cpumask_var_node() and friends allocate cpumask var dynamically > > for CONFIG_CPUMASK_OFFSTACK=y kernel. The allocated size of cpumask var > > is cpumask_size(), which is runtime constant after nr_cpu_ids is > > freezed. > > > > Create a dedicated kmem cache for dynamic allocation of cpumask var. > > Why?
Hi Dawei,
Agree with Rasmus. CPUMASK_OFFSTACK=y is quite a rare configuration, normally disabled. Can you show the architecture that you're working with and how the cache you're adding affects performance.
> > The window for creation of cache is somewhat narrow: > > - After last update of nr_cpu_ids(via set_nr_cpu_ids()) > > - After kmem cache is available. > > - Before any alloc_cpumask_var_node() invocations(sched_init() e.g).
Not only narrow but also not uniform across platforms. For example, on XEN xen_smp_count_cpus() may adjust nr_cpu_ids. I don't think that people runn XEN with CPUMASK_OFFSTACK=y, but you have to make sure that your cache is always created before the 1st allocation.
> OK, so this sounds somewhat fragile. It's maybe correct, but I fail to > see what is gained by this, and the commit message does not provide any > hints.
Agree. To make it less vulnerable, you have to enforce something like:
bool cpumask_cache_used = false;
static inline void set_nr_cpu_ids(unsigned int nr) { if (WARN_ON(cpumask_cache_used)) return; nr_cpu_ids = nr; cpumask_cache_destroy(); cpumask_cache_init() }
bool alloc_cpumask_var_node() { cpumask_cache_used = true; *mask = kmalloc_node(cpumask_size(), flags, node); ... }
But at the very first, we need to understand under which scenarios the new cache would benefit performance?
Thnaks, Yury benefits performance
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