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

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Sep 26, 2022 at 09:02:21PM +0000, Joel Fernandes wrote:
> On Mon, Sep 26, 2022 at 09:32:44PM +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.
> > > 
> > > 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>
> > 
> > ?
> > 
> > But it does not fully solve my boot-up issue. Will debug tomorrow further.
> 
> Ah, but at least its progress, thanks. Could you send me a patch to include
> in the next revision with details of this?
> 
> > > 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.
> 
> Paul, What about using a default-off kernel CONFIG that splats on all lazy
> call_rcu() callbacks that do a wake up. We could use the trace hooks to do it
> in kernel I think. I can talk to Steve to get ideas on how to do that but I
> think it can be done purely from trace events (we might need a new
> trace_end_invoke_callback to fire after the callback is invoked). Thoughts?

Could you look for wakeups invoked between trace_rcu_batch_start() and
trace_rcu_batch_end() that are not from interrupt context?  This would
of course need to be associated with a task rather than a CPU.

Note that you would need to check for wakeups from interrupt handlers
even with the extra trace_end_invoke_callback().  The window where an
interrupt handler could do a wakeup would be reduced, but not eliminated.

							Thanx, Paul