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Date:   Mon, 26 Sep 2022 15:27:18 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Uladzislau Rezki <urezki@gmail.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com,
        neeraj.iitr10@gmail.com, frederic@kernel.org, rostedt@goodmis.org
Subject: Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power
Message-ID: <20220926222718.GW4196@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20220922220104.2446868-1-joel@joelfernandes.org>
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 <YzH+XAVSlHpuWYoD@pc636>
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On Mon, Sep 26, 2022 at 09:32:44PM +0200, Uladzislau Rezki wrote:
> On Mon, Sep 26, 2022 at 10:48:46AM -0700, Paul E. McKenney wrote:
> > 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@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.
> > > 
> > > 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.
> > 
> > We do seem to be in need of some way to quickly and easily locate the
> > callback that needed to be _flush() due to a wakeup.
> >
> <snip>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index aeea9731ef80..fe1146d97f1a 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1771,7 +1771,7 @@ bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork)
>  
>         if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
>                 rwork->wq = wq;
> -               call_rcu(&rwork->rcu, rcu_work_rcufn);
> +               call_rcu_flush(&rwork->rcu, rcu_work_rcufn);
>                 return true;
>         }
>  
> <snip>
> 
> ?

This one does seem like a good candidate.  Someday there might need to
be a queue_rcu_work_flush() vs. queue_rcu_work(), but that needs to be
proven.

> But it does not fully solve my boot-up issue. Will debug tomorrow further.

Sounds good!

> > Might one more proactive approach be to use Coccinelle to locate such
> > callback functions?  We might not want -all- callbacks that do wakeups
> > to use call_rcu_flush(), but knowing which are which should speed up
> > slow-boot debugging by quite a bit.
> > 
> > Or is there a better way to do this?
> > 
> I am not sure what Coccinelle is. If we had something automated that measures
> a boot time and if needed does some profiling it would be good. Otherwise it
> is a manual debugging mainly, IMHO.

Coccinelle is sort of like a variant of the "sed" command that understands
C syntax.  It is useful for searching for patterns in Linux-kernel source
code.

But if you are able to easily find the call_rcu() invocations that are
slowing things down, maybe it is not needed.  For now, anyway.

							Thanx, Paul