Re: [RFC] Reimplementation of linux dynamic percpu memory allocator

[Manfred Spraul]

Hi kiran,

> + * 
> + * Originally by Dipankar Sarma and Ravikiran Thirumalai,
> + * This reimplements alloc_percpu to make it 
> + * 1. Independent of slab/kmalloc
>  
Probably the right approach. slab should use per-cpu for it's internal 
head arrays, but I've never converted the slab code due to 
chicken-and-egg problems and due to the additional pointer dereference.

> + * Allocator is slow -- expected to be called during module/subsytem
> + * init. alloc_percpu can block.
>  
How slow is slow?
I think the block subsystem uses alloc_percpu for some statistics 
counters, i.e. one alloc during creation of a new disk. The slab 
implementation was really slow and that caused problems with LVM (or 
something like that) stress tests.

> +	/* Map pages for each cpu by splitting vm_struct for each cpu */
> +	for (i = 0; i < NR_CPUS; i++) {
> +		if (cpu_possible(i)) {
> +			tmppage = &blkp->pages[i*cpu_pages];
> +			tmp.addr = area->addr + i * PCPU_BLKSIZE;
> +			/* map_vm_area assumes a guard page of size PAGE_SIZE */
> +			tmp.size = PCPU_BLKSIZE + PAGE_SIZE; 
> +			if (map_vm_area(&tmp, PAGE_KERNEL, &tmppage))
> +				goto fail_map;
>  
That means no large pte entries for the per-cpu allocations, right?
I think that's a bad idea for non-numa systems. What about a fallback to 
simple getfreepages() for non-numa systems?

> + * This allocator is slow as we assume allocs to come
> + * by during boot/module init.
> + * Should not be called from interrupt context 
>  
"Must not" - it contains down() and thus can sleep.

--
   Manfred


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