Date:	Fri, 20 Feb 2009 12:02:37 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
	hpa@...or.com, jeremy@...p.org, cpw@....com, mingo@...e.hu,
	tony.luck@...el.com, Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 09/10] percpu: implement new dynamic percpu allocator

Oops, forgot to cc Nick.  cc'ing and quoting whole body.

Tejun Heo wrote:
> 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
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