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Subject: Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking
 readers from consuming CPU) cause qemu boot slow
To:     Neeraj Upadhyay <quic_neeraju@quicinc.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>
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From:   "zhangfei.gao@foxmail.com" <zhangfei.gao@foxmail.com>
Date:   Tue, 14 Jun 2022 22:03:35 +0800
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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

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

Thanks