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

From: Neeraj Upadhyay <quic_neeraju@quicinc.com>
To: "zhangfei.gao@foxmail.com" <zhangfei.gao@foxmail.com>,
	Shameerali Kolothum Thodi  <shameerali.kolothum.thodi@huawei.com>,
	"paulmck@kernel.org" <paulmck@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Zhangfei Gao <zhangfei.gao@linaro.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"rcu@vger.kernel.org" <rcu@vger.kernel.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Matthew Wilcox <willy@infradead.org>,
	"mtosatti@redhat.com" <mtosatti@redhat.com>,
	Auger Eric <eric.auger@redhat.com>,
	"chenxiang (M)" <chenxiang66@hisilicon.com>
Subject: Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow
Date: Tue, 14 Jun 2022 19:44:27 +0530	[thread overview]
Message-ID: <88780123-32a3-ab34-cd3d-01e4a3421f03@quicinc.com> (raw)
In-Reply-To: <tencent_8FD344DA7FC376C7D1204604DA7689DA4906@qq.com>

On 6/14/2022 7:33 PM, 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());
>>
> 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
> 

Thanks for this data. Can you please share below test combo also?

7. 5.19-rc1 + fix5: fix2 + Neeraj's change of srcu_gp_end

8. 5.19-rc1 + fix6: fix3 + Neeraj's change of srcu_gp_end

Thanks
Neeraj

> More test details: https://docs.qq.com/doc/DRXdKalFPTVlUbFN5
> 
> Thanks
> 