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

[Paul E. McKenney]

On Tue, Jun 14, 2022 at 10:03:35PM +0800, zhangfei.gao@xxxxxxxxxxx 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.)

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

And thank you for these details.

							Thanx, Paul