Re: [PATCH 3/3] resource: Introduce resource cache

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 12 Jun 2019 21:59:03 -0700 Nadav Amit <namit@vmware.com> wrote:

> For efficient search of resources, as needed to determine the memory
> type for dax page-faults, introduce a cache of the most recently used
> top-level resource. Caching the top-level should be safe as ranges in
> that level do not overlap (unlike those of lower levels).
> 
> Keep the cache per-cpu to avoid possible contention. Whenever a resource
> is added, removed or changed, invalidate all the resources. The
> invalidation takes place when the resource_lock is taken for write,
> preventing possible races.
> 
> This patch provides relatively small performance improvements over the
> previous patch (~0.5% on sysbench), but can benefit systems with many
> resources.

> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -53,6 +53,12 @@ struct resource_constraint {
>  
>  static DEFINE_RWLOCK(resource_lock);
>  
> +/*
> + * Cache of the top-level resource that was most recently use by
> + * find_next_iomem_res().
> + */
> +static DEFINE_PER_CPU(struct resource *, resource_cache);

A per-cpu cache which is accessed under a kernel-wide read_lock looks a
bit odd - the latency getting at that rwlock will swamp the benefit of
isolating the CPUs from each other when accessing resource_cache.

On the other hand, if we have multiple CPUs running
find_next_iomem_res() concurrently then yes, I see the benefit.  Has
the benefit of using a per-cpu cache (rather than a kernel-wide one)
been quantified?


> @@ -262,9 +268,20 @@ static void __release_child_resources(struct resource *r)
>  	}
>  }
>  
> +static void invalidate_resource_cache(void)
> +{
> +	int cpu;
> +
> +	lockdep_assert_held_exclusive(&resource_lock);
> +
> +	for_each_possible_cpu(cpu)
> +		per_cpu(resource_cache, cpu) = NULL;
> +}

All the calls to invalidate_resource_cache() are rather a
maintainability issue - easy to miss one as the code evolves.

Can't we just make find_next_iomem_res() smarter?  For example, start
the lookup from the cached point and if that failed, do a full sweep?

> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();
> +			invalidate_resource_cache();
> +	invalidate_resource_cache();
> +	invalidate_resource_cache();

Ow.  I guess the maintainability situation can be improved by renaming
resource_lock to something else (to avoid mishaps) then adding wrapper
functions.  But still.  I can't say this is a super-exciting patch :(