Re: [PATCH] locking/qspinlock: Use this_cpu_ptr instead of this_cpu_dec

[xinhui <xinhui.pan@linux.vnet.ibm.com>]

On 2016年06月04日 05:35, Peter Zijlstra wrote:
> On Fri, Jun 03, 2016 at 05:20:10PM -0400, Waiman Long wrote:
>> On 06/03/2016 05:48 AM, Pan Xinhui wrote:
>>> queued_spin_lock_slowpath should not worry about interrupt change
>>> node->count by accident because ->count is inc and dec when we
>>> enter/leave queued_spin_lock_slowpath.
>>>
>>> So this_cpu_dec() does some no point things here, lets use this_cpu_ptr
>>> for a small optimization.
>>>
>>> Signed-off-by: Pan Xinhui<xinhui.pan@linux.vnet.ibm.com>
>>> ---
>>>   kernel/locking/qspinlock.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>>> index 99f31e4..2b4daac 100644
>>> --- a/kernel/locking/qspinlock.c
>>> +++ b/kernel/locking/qspinlock.c
>>> @@ -492,7 +492,7 @@ release:
>>>   	/*
>>>   	 * release the node
>>>   	 */
>>> -	this_cpu_dec(mcs_nodes[0].count);
>>> +	this_cpu_ptr(&mcs_nodes[0])->count--;
>>>   }
>>>   EXPORT_SYMBOL(queued_spin_lock_slowpath);
>>>
>>
>> Is this going to generate better code for PPC? For x86, I think it will
>> cause more instruction to be issued.
>
yes, ppc will do some check when restore irq flags. it's really heavy.

and yes, such change cause more instructions to be issued.

there is a RELOC_HIDE macro in this_cpu_ptr and gcc can't do optimization.
How about ICC. I once used ICC when I was in intel but i forgot the result.

> It does; I think he wants __this_cpu_dec() instead, but the Changelog
> needs improvement to explain why that is ok.
>

oh, great. __this_cpu_dec() is fine. thanks
no any irq flags save/restore again. :)

thanks
xinhui