Re: [PATCH 11/11] x86,rcu: use percpu rcu_preempt_depth

Lai Jiangshan
2019-11-01T15:47:28Z

On 2019/11/1 9:13 下午, Peter Zijlstra wrote:
> On Fri, Nov 01, 2019 at 05:58:16AM -0700, Paul E. McKenney wrote:
>> On Thu, Oct 31, 2019 at 10:08:06AM +0000, Lai Jiangshan wrote:
>>> +/* We mask the RCU_NEED_SPECIAL bit so that it return real depth */
>>> +static __always_inline int rcu_preempt_depth(void)
>>> +{
>>> + return raw_cpu_read_4(__rcu_preempt_depth) & ~RCU_NEED_SPECIAL;
>>
>> Why not raw_cpu_generic_read()?
>>
>> OK, OK, I get that raw_cpu_read_4() translates directly into an "mov"
>> instruction on x86, but given that x86 percpu_from_op() is able to
>> adjust based on operand size, why doesn't something like raw_cpu_read()
>> also have an x86-specific definition that adjusts based on operand size?
>
> The reason for preempt.h was header recursion hell.

Oh, I didn't notice. May we can use raw_cpu_generic_read
for rcu here, I will have a try.

Thanks
Lai.

>
>>> +}
>>> +
>>> +static __always_inline void rcu_preempt_depth_set(int pc)
>>> +{
>>> + int old, new;
>>> +
>>> + do {
>>> + old = raw_cpu_read_4(__rcu_preempt_depth);
>>> + new = (old & RCU_NEED_SPECIAL) |
>>> + (pc & ~RCU_NEED_SPECIAL);
>>> + } while (raw_cpu_cmpxchg_4(__rcu_preempt_depth, old, new) != old);
>>
>> Ummm...
>>
>> OK, as you know, I have long wanted _rcu_read_lock() to be inlineable.
>> But are you -sure- that an x86 cmpxchg is faster than a function call
>> and return? I have strong doubts on that score.
>
> This is a regular CMPXCHG instruction, not a LOCK prefixed one, and that
> should make all the difference
>
>> Plus multiplying the x86-specific code by 26 doesn't look good.
>>
>> And the RCU read-side nesting depth really is a per-task thing. Copying
>> it to and from the task at context-switch time might make sense if we
>> had a serious optimization, but it does not appear that we do.
>>
>> You original patch some years back, ill-received though it was at the
>> time, is looking rather good by comparison. Plus it did not require
>> architecture-specific code!
>
> Right, so the per-cpu preempt_count code relies on the preempt_count
> being invariant over context switches. That means we never have to
> save/restore the thing.
>
> For (preemptible) rcu, this is 'obviously' not the case.
>
> That said, I've not looked over this patch series, I only got 1 actual
> patch, not the whole series, and I've not had time to go dig out the
> rest..
>