Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow

["zhangfei.gao@foxmail.com" <zhangfei.gao@foxmail.com>]

On 2022/6/14 下午10:17, Paul E. McKenney wrote:
> On Tue, Jun 14, 2022 at 10:03:35PM +0800, zhangfei.gao@foxmail.com wrote:
>>
>> On 2022/6/14 下午8:19, Neeraj Upadhyay wrote:
>>>> 5.18-rc4 based               ~8sec
>>>>
>>>> 5.19-rc1                     ~2m43sec
>>>>
>>>> 5.19-rc1+fix1                 ~19sec
>>>>
>>>> 5.19-rc1-fix2                 ~19sec
>>>>
>>> If you try below diff on top of either 5.19-rc1+fix1 or 5.19-rc1-fix2 ;
>>> does it show any difference in boot time?
>>>
>>> --- a/kernel/rcu/srcutree.c
>>> +++ b/kernel/rcu/srcutree.c
>>> @@ -706,7 +706,7 @@ static void srcu_schedule_cbs_snp(struct srcu_struct
>>> *ssp, struct srcu_node *snp
>>>    */
>>>   static void srcu_gp_end(struct srcu_struct *ssp)
>>>   {
>>> -       unsigned long cbdelay;
>>> +       unsigned long cbdelay = 1;
>>>          bool cbs;
>>>          bool last_lvl;
>>>          int cpu;
>>> @@ -726,7 +726,9 @@ static void srcu_gp_end(struct srcu_struct *ssp)
>>>          spin_lock_irq_rcu_node(ssp);
>>>          idx = rcu_seq_state(ssp->srcu_gp_seq);
>>>          WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
>>> -       cbdelay = !!srcu_get_delay(ssp);
>>> +       if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq),
>>> READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
>>> +               cbdelay = 0;
>>> +
>>>          WRITE_ONCE(ssp->srcu_last_gp_end, ktime_get_mono_fast_ns());
> Thank you both for the testing and the proposed fix!
>
>> Test here:
>> qemu: https://github.com/qemu/qemu/tree/stable-6.1
>> kernel:
>> https://github.com/Linaro/linux-kernel-uadk/tree/uacce-devel-5.19-srcu-test
>> (in case test patch not clear, push in git tree)
>>
>> Hardware: aarch64
>>
>> 1. 5.18-rc6
>> real    0m8.402s
>> user    0m3.015s
>> sys     0m1.102s
>>
>> 2. 5.19-rc1
>> real    2m41.433s
>> user    0m3.097s
>> sys     0m1.177s
>>
>> 3. 5.19-rc1 + fix1 from Paul
>> real    2m43.404s
>> user    0m2.880s
>> sys     0m1.214s
>>
>> 4. 5.19-rc1 + fix2: fix1 + Remove "if (!jbase)" block
>> real    0m15.262s
>> user    0m3.003s
>> sys     0m1.033s
>>
>> When build kernel in the meantime, load time become longer.
>>
>> 5. 5.19-rc1 + fix3: fix1 + SRCU_MAX_NODELAY_PHASE 1000000
>> real    0m15.215s
>> user    0m2.942s
>> sys    0m1.172s
>>
>> 6. 5.19-rc1 + fix4: fix1 + Neeraj's change of srcu_gp_end　
>> real    1m23.936s
>> user    0m2.969s
>> sys    0m1.181s
> And thank you for the testing!
>
> Could you please try fix3 + Neeraj's change of srcu_gp_end?
>
> That is, fix1 + SRCU_MAX_NODELAY_PHASE 1000000 + Neeraj's change of
> srcu_gp_end.
>
> Also, at what value of SRCU_MAX_NODELAY_PHASE do the boot
> times start rising?  This is probably best done by starting with
> SRCU_MAX_NODELAY_PHASE=100000 and dividing by (say) ten on each run
> until boot time becomes slow, followed by a binary search between the
> last two values.  (The idea is to bias the search so that fast boot
> times are the common case.)

SRCU_MAX_NODELAY_PHASE 100 becomes slower.


8. 5.19-rc1 + fix6: fix4 + SRCU_MAX_NODELAY_PHASE 1000000

real 0m11.154s ~12s

user 0m2.919s

sys 0m1.064s



9. 5.19-rc1 + fix7: fix4 + SRCU_MAX_NODELAY_PHASE 10000

real 0m11.258s

user 0m3.113s

sys 0m1.073s



10. 5.19-rc1 + fix8: fix4 + SRCU_MAX_NODELAY_PHASE 100

real 0m30.053s ~ 32s

user 0m2.827s

sys 0m1.161s



By the way, if build kernel on the board in the meantime (using memory), 
time become much longer.

real 1m2.763s



11. 5.19-rc1 + fix9: fix4 + SRCU_MAX_NODELAY_PHASE 1000

real 0m11.443s

user 0m3.022s

sys 0m1.052s


Thanks

>
>> More test details: https://docs.qq.com/doc/DRXdKalFPTVlUbFN5
> And thank you for these details.
>
> 							Thanx, Paul