Date:   Mon, 26 Sep 2022 16:57:55 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, rushikesh.s.kadam@...el.com,
        neeraj.iitr10@...il.com, frederic@...nel.org, rostedt@...dmis.org
Subject: Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power

On Mon, Sep 26, 2022 at 07:44:19PM -0400, Joel Fernandes wrote:
> 
> 
> > On Sep 26, 2022, at 6:35 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
> > 
> > On Mon, Sep 26, 2022 at 08:54:27PM +0000, Joel Fernandes wrote:
> >> Hi Vlad,
> >> 
> >> On Mon, Sep 26, 2022 at 09:39:23PM +0200, Uladzislau Rezki wrote:
> >> [...]
> >>>>> On my KVM machine the boot time is affected:
> >>>>> 
> >>>>> <snip>
> >>>>> [    2.273406] e1000 0000:00:03.0 eth0: Intel(R) PRO/1000 Network Connection
> >>>>> [   11.945283] e1000 0000:00:03.0 ens3: renamed from eth0
> >>>>> [   22.165198] sr 1:0:0:0: [sr0] scsi3-mmc drive: 4x/4x cd/rw xa/form2 tray
> >>>>> [   22.165206] cdrom: Uniform CD-ROM driver Revision: 3.20
> >>>>> [   32.406981] sr 1:0:0:0: Attached scsi CD-ROM sr0
> >>>>> [  104.115418] process '/usr/bin/fstype' started with executable stack
> >>>>> [  104.170142] EXT4-fs (sda1): mounted filesystem with ordered data mode. Quota mode: none.
> >>>>> [  104.340125] systemd[1]: systemd 241 running in system mode. (+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN -PCRE2 default-hierarchy=hybrid)
> >>>>> [  104.340193] systemd[1]: Detected virtualization kvm.
> >>>>> [  104.340196] systemd[1]: Detected architecture x86-64.
> >>>>> [  104.359032] systemd[1]: Set hostname to <pc638>.
> >>>>> [  105.740109] random: crng init done
> >>>>> [  105.741267] systemd[1]: Reached target Remote File Systems.
> >>>>> <snip>
> >>>>> 
> >>>>> 2 - 11 and second delay is between 32 - 104. So there are still users which must
> >>>>> be waiting for "RCU" in a sync way.
> >>>> 
> >>>> I was wondering if you can compare boot logs and see which timestamp does the
> >>>> slow down start from. That way, we can narrow down the callback. Also another
> >>>> idea is, add "trace_event=rcu:rcu_callback,rcu:rcu_invoke_callback
> >>>> ftrace_dump_on_oops" to the boot params, and then manually call
> >>>> "tracing_off(); panic();" from the code at the first printk that seems off in
> >>>> your comparison of good vs bad. For example, if "crng init done" timestamp is
> >>>> off, put the "tracing_off(); panic();" there. Then grab the serial console
> >>>> output to see what were the last callbacks that was queued/invoked.
> >> 
> >> Would you be willing to try these steps? Meanwhile I will try on my side as
> >> well with the .config you sent me in another email.
> >> 
> >>>>>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> >>>>>> index 08605ce7379d..40ae36904825 100644
> >>>>>> --- a/include/linux/rcupdate.h
> >>>>>> +++ b/include/linux/rcupdate.h
> >>>>>> @@ -108,6 +108,13 @@ static inline int rcu_preempt_depth(void)
> >>>>>> 
> >>>>>> #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
> >>>>>> 
> >>>>>> +#ifdef CONFIG_RCU_LAZY
> >>>>>> +void call_rcu_flush(struct rcu_head *head, rcu_callback_t func);
> >>>>>> +#else
> >>>>>> +static inline void call_rcu_flush(struct rcu_head *head,
> >>>>>> +        rcu_callback_t func) {  call_rcu(head, func); }
> >>>>>> +#endif
> >>>>>> +
> >>>>>> /* Internal to kernel */
> >>>>>> void rcu_init(void);
> >>>>>> extern int rcu_scheduler_active;
> >>>>>> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> >>>>>> index f53ad63b2bc6..edd632e68497 100644
> >>>>>> --- a/kernel/rcu/Kconfig
> >>>>>> +++ b/kernel/rcu/Kconfig
> >>>>>> @@ -314,4 +314,12 @@ config TASKS_TRACE_RCU_READ_MB
> >>>>>>      Say N here if you hate read-side memory barriers.
> >>>>>>      Take the default if you are unsure.
> >>>>>> 
> >>>>>> +config RCU_LAZY
> >>>>>> +    bool "RCU callback lazy invocation functionality"
> >>>>>> +    depends on RCU_NOCB_CPU
> >>>>>> +    default n
> >>>>>> +    help
> >>>>>> +      To save power, batch RCU callbacks and flush after delay, memory
> >>>>>> +      pressure or callback list growing too big.
> >>>>>> +
> >>>>>> 
> >>>>> Do you think you need this kernel option? Can we just consider and make
> >>>>> it a run-time configurable? For example much more users will give it a try,
> >>>>> so it will increase a coverage. By default it can be off.
> >>>>> 
> >>>>> Also you do not need to do:
> >>>>> 
> >>>>> #ifdef LAZY
> >>>> 
> >>>> How does the "LAZY" macro end up being runtime-configurable? That's static /
> >>>> compile time. Did I miss something?
> >>>> 
> >>> I am talking about removing if:
> >>> 
> >>> config RCU_LAZY
> >>> 
> >>> we might run into issues related to run-time switching though.
> >> 
> >> When we started off, Paul said he wanted it kernel CONFIGurable. I will defer
> >> to Paul on a decision for that. I prefer kernel CONFIG so people don't forget
> >> to pass a boot param.
> > 
> > I am fine with a kernel boot parameter for this one.  You guys were the
> > ones preferring Kconfig options.  ;-)
> 
> Yes I still prefer that.. ;-)
> 
> > But in that case, the CONFIG_RCU_NOCB_CPU would come into play to handle
> > the case where there is no bypass.
> 
> If you don’t mind, let’s do both like we did for NOCB_CPU_ALL. In which case, Vlad since this was your suggestion, would you be so kind to send a patch adding a boot parameter on top of the series? ;-). I’ll include it in the next version. I’d suggest keep the boot param default off and add a CONFIG option that forces the boot param to be turned on.

NOCB_CPU_ALL?  If you are thinking in terms of laziness/flushing being
done on a per-CPU basis among the rcu_nocbs CPUs, that sounds like
something for later.

Are you thinking in terms of Kconfig options that allow: (1) No laziness.
(2) Laziness on all rcu_nocbs CPUs, but only if specified by a boot
parameter.  (3) Laziness on all rcu_nocbs CPUs regardless of boot
parameter.  I could get behind that.

							Thanx, Paul

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