linux-kernel - Re: [PATCH v7 00/11] rcu: call

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Date:   Fri, 7 Oct 2022 16:40:11 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, frederic@...nel.org, rostedt@...dmis.org,
        youssefesmat@...gle.com, surenb@...gle.com
Subject: Re: [PATCH v7 00/11] rcu: call_rcu() power improvements

On Thu, Oct 06, 2022 at 11:55:43AM -0700, Paul E. McKenney wrote:
> On Tue, Oct 04, 2022 at 02:41:46AM +0000, Joel Fernandes (Google) wrote:
> > v7 version of RCU lazy patches based on rcu/next branch.
> > 
> > To facilitate easier merge, I dropped tracing and other patches and just
> > implemented the new changes. I will post the tracing patches later along with
> > rcutop as I need to add new tracepoints that Frederic suggested.
> > 
> > Main recent changes:
> > 1. rcu_barrier() wake up only for lazy bypass list.
> > 2. Make all call_rcu() default-lazy and add call_rcu_flush() API.
> > 3. Take care of some callers using call_rcu_flush() API.
> > 4. Several refactorings suggested by Paul/Frederic.
> > 5. New call_rcu() to call_rcu_flush() conversions by Joel/Vlad/Paul.
> > 
> > I am seeing good performance and power with these patches on real ChromeOS x86
> > asymmetric hardware.
> > 
> > Earlier cover letter with lots of details is here:
> > https://lore.kernel.org/all/20220901221720.1105021-1-joel@joelfernandes.org/
> > 
> > List of recent changes:
> >     
> >     [ 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. ]
> >     
> >     [ Paul/Frederic : Cosmetic changes, split out wakeup of nocb thread. ]
> >     
> >     [ Vlad/Joel : More call_rcu -> flush conversions ]
> 
> Thank you for your continued work on this!
> 
> I pulled these into an experimental branch, applied Uladzislau's
> Tested-by and ran a quick round of rcutorture.
> 
> From TREE02, TREE03, and TREE09 I got this:
> 
> In file included from kernel/rcu/tree.c:68:
> kernel/rcu/tree.h:449:13: error: ‘wake_nocb_gp’ used but never defined [-Werror]
>   449 | static bool wake_nocb_gp(struct rcu_data *rdp, bool force);
>       |             ^~~~~~~~~~~~
> 
> One could argue that this is not a big deal, except that Linus gets a
> bit tetchy when this sort of thing shows up in mainline.
> 
Sorry. I have not tested TREE02, TREE03 and TREE09 scenarios. My goal
was to check below functionalities:

- call_rcu_flush() does not introduce any delays once it is queued
- call_rcu() does not apply pressure on the RCU-machinery from wake-up point of view
- boot-time is not degraded
- synchronize_rcu() and rcu_barrier() work as expected
- if bypass consists of lazy callbacks the *flush() one has to initiate the offloading

--
Uladzislau Rezki