Re: [PATCH 09/10] percpu: implement new dynamic percpu allocator

[Tejun Heo <tj@kernel.org>]

Hello, Rusty.

Rusty Russell wrote:
> On Wednesday 18 February 2009 22:34:35 Tejun Heo wrote:
>> Impact: new scalable dynamic percpu allocator which allows dynamic
>>         percpu areas to be accessed the same way as static ones
>>
>> Implement scalable dynamic percpu allocator which can be used for both
>> static and dynamic percpu areas.  This will allow static and dynamic
>> areas to share faster direct access methods.  This feature is optional
>> and enabled only when CONFIG_HAVE_DYNAMIC_PER_CPU_AREA is defined by
>> arch.  Please read comment on top of mm/percpu.c for details.
> 
> Hi Tejun,
> 
>    One question.  Are you thinking that to be defined by every SMP arch
> long-term?

Yeap, definitely.

> Because there are benefits in having &<percpuvar> == valid
> percpuptr, such as passing them around as parameters.  If so, IA64
> will want a dedicated per-cpu area for statics (tho it can probably
> just map it somehow, but it has to be 64k).

Hmmm...  Don't have much idea about ia64 and its magic 64k.  Can it
somehow be used for the first chunk?

>    It'd also be nice to use your generalised module_percpu allocator for the
> !CONFIG_HAVE_DYNAMIC_PER_CPU_AREA case, but doesn't really matter if that's
> temporary anyway.

Yeap, once the conversion is complete, the old allocator will go away
so there's no reason to put more work into it.

>> +#define PCPU_UNIT_PAGES_SHIFT	((int)__pcpu_unit_pages_shift)
>> +#define PCPU_UNIT_PAGES		((int)__pcpu_unit_pages)
>> +#define PCPU_UNIT_SHIFT		((int)__pcpu_unit_shift)
>> +#define PCPU_UNIT_SIZE		((int)__pcpu_unit_size)
>> +#define PCPU_CHUNK_SIZE		((int)__pcpu_chunk_size)
>> +#define PCPU_NR_SLOTS		((int)__pcpu_nr_slots)
> 
> These pseudo-constants seem like a really weird thing to do to me.

I explained this in the reply to Andrew's comment.  It's
non-really-constant-but-should-be-considered-so-by-users thing.  Is it
too weird?  Even if I add comment explaning it?

> And AFAICT you have the requirement that PCPU_UNIT_PAGES*PAGE_SIZE >=
> sizeof(.data.percpu).  Should probably note that somewhere.

__pcu_unit_pages_shift is adjusted automatically according to
sizeof(.data.percpu), so it will adapt as necessary.  After the
initial adjustment, it should be considered constant, so the above
seemingly weird hack.

>> +static DEFINE_MUTEX(pcpu_mutex);		/* one mutex to rule them all */
>> +static struct list_head *pcpu_slot;		/* chunk list slots */
>> +static struct rb_root pcpu_addr_root = RB_ROOT;	/* chunks by address */
> 
> rbtree might be overkill on first cut.  I'm bearing in mind that Christoph L
> had a nice patch to use dynamic percpu allocation in the sl*b allocators;
> which would mean this needs to only use get_free_page.

Hmmm... the reverse mapping can be piggy backed on vmalloc by adding a
private pointer to the vm_struct but rbtree isn't too difficult to use
so I just did it directly.  Nick, what do you think about adding
private field to vm_struct and providing a reverse map function?

As for the sl*b allocation thing, can you please explain in more
detail or point me to the patches / threads?

Thanks.  :-)

-- 
tejun