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Date: Mon, 26 Sep 2022 21:39:23 +0200
From: Uladzislau Rezki <urezki@...il.com>
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, paulmck@...nel.org,
rostedt@...dmis.org
Subject: Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power
On Sun, Sep 25, 2022 at 05:46:53PM +0000, Joel Fernandes wrote:
> Hi Vlad,
>
> On Sun, Sep 25, 2022 at 10:57:10AM +0200, Uladzislau Rezki wrote:
> > > Implement timer-based RCU lazy callback batching. The batch is flushed
> > > whenever a certain amount of time has passed, or the batch on a
> > > particular CPU grows too big. Also memory pressure will flush it in a
> > > future patch.
> > >
> > > To handle several corner cases automagically (such as rcu_barrier() and
> > > hotplug), we re-use bypass lists to handle lazy CBs. The bypass list
> > > length has the lazy CB length included in it. A separate lazy CB length
> > > counter is also introduced to keep track of the number of lazy CBs.
> > >
> > > v5->v6:
> > >
> > > [ Frederic Weisbec: Program the lazy timer only if WAKE_NOT, since other
> > > deferral levels wake much earlier so for those it is not needed. ]
> > >
> > > [ Frederic Weisbec: Use flush flags to keep bypass API code clean. ]
> > >
> > > [ Frederic Weisbec: Make rcu_barrier() wake up only if main list empty. ]
> > >
> > > [ Frederic Weisbec: Remove extra 'else if' branch in rcu_nocb_try_bypass(). ]
> > >
> > > [ Joel: Fix issue where I was not resetting lazy_len after moving it to rdp ]
> > >
> > > [ Paul/Thomas/Joel: Make call_rcu() default lazy so users don't mess up. ]
> > >
> > I think it make sense to add some data to the commit message
> > illustrating what this patch does.
>
> Sure, will do!
>
> > From my side i gave a try of this patch on my setup. Some data:
> >
> > <snip>
> > root@...38:/home/urezki/rcu_v6# ./perf_script_parser ./perf_v6.script | sort -nk 6 | grep rcu
> > name: rcuop/23 pid: 184 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/26 pid: 206 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/29 pid: 227 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/2 pid: 35 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/33 pid: 256 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/34 pid: 263 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/35 pid: 270 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/36 pid: 277 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/37 pid: 284 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/38 pid: 291 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/49 pid: 370 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/59 pid: 441 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/63 pid: 469 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuog/0 pid: 16 woken-up 2 interval: min 8034 max 8034 avg 4017
> > name: rcuog/24 pid: 191 woken-up 2 interval: min 7941 max 7941 avg 3970
> > name: rcuog/32 pid: 248 woken-up 2 interval: min 7542 max 7542 avg 3771
> > name: rcuog/48 pid: 362 woken-up 2 interval: min 8065 max 8065 avg 4032
> > name: rcuog/56 pid: 419 woken-up 2 interval: min 8076 max 8076 avg 4038
> > name: rcuop/21 pid: 170 woken-up 2 interval: min 13311438 max 13311438 avg 6655719
> > name: rcuog/16 pid: 134 woken-up 4 interval: min 8029 max 13303387 avg 3329863
> > name: rcuop/9 pid: 85 woken-up 4 interval: min 10007570 max 10007586 avg 7505684
> > name: rcuog/8 pid: 77 woken-up 8 interval: min 6240 max 10001242 avg 3753622
> > name: rcu_preempt pid: 15 woken-up 18 interval: min 6058 max 9999713 avg 2140788
> > name: test_rcu/0 pid: 1411 woken-up 10003 interval: min 165 max 19072 avg 4275
> > root@...38:/home/urezki/rcu_v6#
> >
> > root@...38:/home/urezki/rcu_v6# ./perf_script_parser ./perf_default.script | sort -nk 6 | grep rcu
> > name: rcuop/33 pid: 256 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuop/51 pid: 384 woken-up 1 interval: min 0 max 0 avg 0
> > name: rcuog/32 pid: 248 woken-up 2 interval: min 11927 max 11927 avg 5963
> > name: rcuop/63 pid: 469 woken-up 2 interval: min 23963 max 23963 avg 11981
> > name: rcuog/56 pid: 419 woken-up 3 interval: min 11132 max 23967 avg 11699
> > name: rcuop/50 pid: 377 woken-up 3 interval: min 8057 max 4944344 avg 1650800
> > name: rcuog/48 pid: 362 woken-up 8 interval: min 2712 max 37430015 avg 5298801
> > name: rcuop/16 pid: 135 woken-up 4790 interval: min 7340 max 16649 avg 8843
> > name: rcuog/16 pid: 134 woken-up 4792 interval: min 7368 max 16644 avg 8844
> > name: rcu_preempt pid: 15 woken-up 5302 interval: min 26 max 12179 avg 7994
> > name: test_rcu/0 pid: 1353 woken-up 10003 interval: min 169 max 18508 avg 4236
> > root@...38:/home/urezki/rcu_v6#
> > <snip>
> >
> > so it is obvious that the patch does the job.
>
> Thanks a lot for testing!
>
> > 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.
>
> > > 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.
> > ...
> > #else
> > ...
> > #endif
> >
> > >
> > > +/*
> > > + * LAZY_FLUSH_JIFFIES decides the maximum amount of time that
> > > + * can elapse before lazy callbacks are flushed. Lazy callbacks
> > > + * could be flushed much earlier for a number of other reasons
> > > + * however, LAZY_FLUSH_JIFFIES will ensure no lazy callbacks are
> > > + * left unsubmitted to RCU after those many jiffies.
> > > + */
> > > +#define LAZY_FLUSH_JIFFIES (10 * HZ)
> > > +static unsigned long jiffies_till_flush = LAZY_FLUSH_JIFFIES;
> > Make it configurable? I do not think you actually need 10 seconds here.
> > Reducing it will reduce a possibility to hit a low memory condition. 1
> > second would be far enough i think.
>
> Hmm, I can make the delay configurable but for now I'll keep this as default
> as all of our power testing has been done with that and I don't want risk
> losing the optimization.
>
Fine to me. Later on is OK.
>
> Honestly, I am not worried too about memory pressure as we have a shrinker
> which triggers flushes on the slightest hint of memory pressure. If it is not
> handling it properly, then we need to fix the shrinker.
>
Will not speculate here since i have not tested this patch enough with
different time out and a low mem. condition.
--
Uladzislau Rezki
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