Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count

[Will Deacon]

On Thu, Sep 24, 2020 at 07:55:19PM +0800, Hou Tao wrote:
> The following is the newest performance data:
> 
> aarch64 host (4 sockets, 24 cores per sockets)
> 
> * v4.19.111
> 
> no writer, reader cn                                | 24        | 48        | 72        | 96
> rate of percpu_down_read/percpu_up_read per second  |
> default: use __this_cpu_inc|dec()                   | 166129572 | 166064100 | 165963448 | 165203565
> patched: use this_cpu_inc|dec()                     |  87727515 |  87698669 |  87675397 |  87337435
> modified: local_irq_save + __this_cpu_inc|dec()     |  15470357	|  15460642 |  15439423	|  15377199
> 
> * v4.19.111+ [1]
> 
> modified: use this_cpu_inc|dec() + LSE atomic       |   8224023 |   8079416 |	7883046 |   7820350
> 
> * 5.9-rc6
> 
> no writer, reader cn                                | 24        | 48        | 72        | 96
> rate of percpu_down_read/percpu_up_read per second  |
> reverted: use __this_cpu_inc|dec() + revert 91fc957c| 169664061 | 169481176 | 168493488 | 168844423
> reverted: use __this_cpu_inc|dec()                  |  78355071 |  78294285 |  78026030 |  77860492
> modified: use this_cpu_inc|dec() + no LSE atomic    |  64291101 |  64259867 |  64223206 |  63992316
> default: use this_cpu_inc|dec() + LSE atomic        |  16231421 |  16215618 |  16188581 |  15959290
> 
> It seems that enabling LSE atomic has a negative impact on performance under this test scenario.
> 
> And it is astonished to me that for my test scenario the performance of v5.9-rc6 is just one half of v4.19.
> The bisect finds the culprit is 91fc957c9b1d6 ("arm64/bpf: don't allocate BPF JIT programs in module memory").
> If reverting the patch brute-forcibly under 5.9-rc6 [2], the performance will be the same with
> v4.19.111 (169664061 vs 166129572). I have had the simplified test module [3] and .config attached [4],
> so could you please help to check what the problem is ?

I have no idea how that patch can be responsible for this :/ Have you
confirmed that the bisection is not bogus?

Ard, do you have any ideas?

Will