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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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