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SubjectRe: [PATCH v16 13/14] pvqspinlock: Improve slowpath performance by avoiding cmpxchg
On 04/29/2015 02:27 PM, Linus Torvalds wrote:
> On Wed, Apr 29, 2015 at 11:11 AM, Peter Zijlstra<peterz@infradead.org> wrote:
>> On Fri, Apr 24, 2015 at 02:56:42PM -0400, Waiman Long wrote:
>>> In the pv_scan_next() function, the slow cmpxchg atomic operation is
>>> performed even if the other CPU is not even close to being halted. This
>>> extra cmpxchg can harm slowpath performance.
>>>
>>> This patch introduces the new mayhalt flag to indicate if the other
>>> spinning CPU is close to being halted or not. The current threshold
>>> for x86 is 2k cpu_relax() calls. If this flag is not set, the other
>>> spinning CPU will have at least 2k more cpu_relax() calls before
>>> it can enter the halt state. This should give enough time for the
>>> setting of the locked flag in struct mcs_spinlock to propagate to
>>> that CPU without using atomic op.
>> Yuck! I'm not at all sure you can make assumptions like that. And the
>> worst part is, if it goes wrong the borkage is subtle and painful.\
> I have to agree with Peter.
>
> But it goes beyond this particular patch. Patterns like this:
>
> xchg(&pn->mayhalt, true);
>
> are just evil and disgusting. Even befoe this patch, that code had
>
> (void)xchg(&pn->state, vcpu_halted);
>
> which is *wrong* and should never be done.
>
> If you want it to be "set_mb()" (which sets a value and has a memory
> barrier), then use set_mb(). Yes, it happens to use a "xchg()" to do
> so, but dammit, it documents that whole "this is a memory barrier" in
> the name.
> Also, anybody who does this should damn well document why the memory
> barrier is needed. The xchg(&pn->state, vcpu_halted) at least is
> preceded by a comment about the barriers. The new mayhalt has no sane
> comment in it, and the reason seems to be that no sane comment is
> possible. The xchg() seems to be some black magic thing.
>
> Let's not introduce magic stuff in our locking primitives. At least
> not undocumented magic that makes no sense.
>
> Linus

Thanks for the comments. I will withdraw this patch and use set_mb() in
the code as suggested for better readability.

Cheers,
Longman


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