Re: [PATCH] mm: percpu: Add PCPU_FC_FIXED to pcpu_fc for setting fixed pcpu_atom_size.

Tejun Heo
2012-04-25T22:24:35Z

Hello, ShouX.

On Wed, Apr 25, 2012 at 04:49:28PM +0800, ShuoX Liu wrote:
> From: ShuoX Liu <shuox.liu@intel.com>
>
> We are enabling Android on Medfield. On i386, if the board has more
> physical memory, it means the vmalloc space is small. If the vmalloc
> space is big, it means physical memory is small. Dynamic percpu
> allocation is based on VM space. On i386, by default, the chunk size
> is 4MB. As vmalloc space is <= 128M, percpu allocation often fails.

Can you please provide more details - which kernel/user split was
used, how much memory was there and so on? Also, can you please
attach boot log and the output of "cat /proc/vmallocinfo"?

> If using PERCPU_FC_PAGE, system can't go to deep sleep states.

Why?

> diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
> index 71f4727..824bc41 100644
> --- a/arch/x86/kernel/setup_percpu.c
> +++ b/arch/x86/kernel/setup_percpu.c
> @@ -185,9 +185,13 @@ void __init setup_per_cpu_areas(void)
> #endif
> rc = -EINVAL;
> if (pcpu_chosen_fc != PCPU_FC_PAGE) {
> - const size_t atom_size = cpu_has_pse ? PMD_SIZE : PAGE_SIZE;
> + size_t atom_size;
> const size_t dyn_size = PERCPU_MODULE_RESERVE +
> PERCPU_DYNAMIC_RESERVE - PERCPU_FIRST_CHUNK_RESERVE;
> + if (pcpu_chosen_fc == PCPU_FC_FIXED && pcpu_atom_size)
> + atom_size = pcpu_atom_size;
> + else
> + atom_size = cpu_has_pse ? PMD_SIZE : PAGE_SIZE;
>
> rc = pcpu_embed_first_chunk(PERCPU_FIRST_CHUNK_RESERVE,
> dyn_size, atom_size,

Umm... this is way too hacky. atom_size can't be an arbitrary value
and the param is meaningful only to x86 yet defined globally. Also,
while atom_size has effect on vmalloc area usage, way more important
factor is distance between units. What we probably need to do is
tighten the rejection criteria of pcpu_embed_first_chunk() and fix
whatever problem FC_PAGE is causing.

Thanks.

--
tejun