Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power

Uladzislau Rezki
2022-09-25T08:57:23Z

> 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.

From my side i gave a try of this patch on my setup. Some data:

<snip>
root@pc638:/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@pc638:/home/urezki/rcu_v6#

root@pc638:/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@pc638:/home/urezki/rcu_v6#
<snip>

so it is obvious that the patch does the job.

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.

> 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
...
#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.

--
Uladzislau Rezki