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SubjectRe: [RFC PATCH 6/6] workqueue: use kmalloc_align() instead of hacking
On 03/20/2012 11:46 PM, Tejun Heo wrote:
> On Tue, Mar 20, 2012 at 06:21:24PM +0800, Lai Jiangshan wrote:
>> kmalloc_align() makes the code simpler.
>>
>> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
>> ---
>> kernel/workqueue.c | 23 +++++------------------
>> 1 files changed, 5 insertions(+), 18 deletions(-)
>>
>> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
>> index 5abf42f..beec5fd 100644
>> --- a/kernel/workqueue.c
>> +++ b/kernel/workqueue.c
>> @@ -2897,20 +2897,9 @@ static int alloc_cwqs(struct workqueue_struct *wq)
>>
>> if (!(wq->flags & WQ_UNBOUND))
>> wq->cpu_wq.pcpu = __alloc_percpu(size, align);
>> - else {
>> - void *ptr;
>> -
>> - /*
>> - * Allocate enough room to align cwq and put an extra
>> - * pointer at the end pointing back to the originally
>> - * allocated pointer which will be used for free.
>> - */
>> - ptr = kzalloc(size + align + sizeof(void *), GFP_KERNEL);
>> - if (ptr) {
>> - wq->cpu_wq.single = PTR_ALIGN(ptr, align);
>> - *(void **)(wq->cpu_wq.single + 1) = ptr;
>> - }
>> - }
>> + else
>> + wq->cpu_wq.single = kmalloc_align(size,
>> + GFP_KERNEL | __GFP_ZERO, align);
>>
>> /* just in case, make sure it's actually aligned */
>> BUG_ON(!IS_ALIGNED(wq->cpu_wq.v, align));
>> @@ -2921,10 +2910,8 @@ static void free_cwqs(struct workqueue_struct *wq)
>> {
>> if (!(wq->flags & WQ_UNBOUND))
>> free_percpu(wq->cpu_wq.pcpu);
>> - else if (wq->cpu_wq.single) {
>> - /* the pointer to free is stored right after the cwq */
>> - kfree(*(void **)(wq->cpu_wq.single + 1));
>> - }
>> + else if (wq->cpu_wq.single)
>> + kfree(wq->cpu_wq.single);
>
> Yes, this is hacky but I don't think building the whole
> kmalloc_align() for only this is a good idea. If the open coded hack
> bothers you just write a simplistic wrapper somewhere. We can make
> that better integrated / more efficient when there are multiple users
> of the interface, which I kinda doubt would happen. The reason why
> cwq requiring larger alignment is more historic than anything else
> after all.
>

Yes, I don't want to build a complex kmalloc_align(). But after I found
that SLAB/SLUB's kmalloc-objects are natural/automatic aligned to
a proper big power of two. I will do nothing if I introduce kmalloc_align()
except just care the debugging.

o SLAB/SLUB's kmalloc-objects are natural/automatic aligned.
o 70LOC in total, and about 90% are just renaming or wrapping.

I think it is a worth trade-off, it give us convenience and we pay
zero overhead(when runtime) and 70LOC(when coding, pay in a lump sum).

And kmalloc_align() can be used in the following case:
o a type object need to be aligned with cache-line for it contains a frequent
update-part and a frequent read-part.
o The total number of these objects in a given type is not much, creating
a new slab cache for a given type will be overkill.

This is a RFC patch and it seems mm gurus don't like it. I'm sorry I bother all of you.

Thanks,
Lai





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