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Date:   Wed, 10 Aug 2022 20:46:25 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com,
        urezki@gmail.com, neeraj.iitr10@gmail.com, frederic@kernel.org,
        rostedt@goodmis.org, rcu@vger.kernel.org
Subject: Re: [PATCH v3 resend 0/6] Implement call_rcu_lazy() and
 miscellaneous fixes
Message-ID: <20220811034625.GX2125313@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
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On Wed, Aug 10, 2022 at 11:22:13PM -0400, Joel Fernandes wrote:
> 
> 
> On 8/10/2022 10:51 PM, Paul E. McKenney wrote:
> > On Wed, Aug 10, 2022 at 10:31:56PM -0400, Joel Fernandes wrote:
> >>
> >>
> >> On 8/10/2022 10:23 PM, Joel Fernandes wrote:
> >>>
> >>>
> >>> On 8/8/2022 11:45 PM, Joel Fernandes (Google) wrote:
> >>>> Just a refresh of v3 with one additional debug patch. v3's cover letter is here:
> >>>>  https://lore.kernel.org/all/20220713213237.1596225-1-joel@joelfernandes.org/
> >>>>
> >>>> I just started working on this again while I have some time during paternity
> >>>> leave ;-) So I thought I'll just send it out again. No other changes other
> >>>> than that 1 debug patch I added on the top.
> >>>>
> >>>> Next I am going to go refine the power results as mentioned in Paul's comments
> >>>> on the last cover letter.
> >>>
> >>> Side note: Here is another big selling point for call_rcu_lazy().
> >>> Instead of _lazy(), if you just increased jiffies_till_first_fqs, and
> >>> slowed *all* call_rcu() down to achieve the same effect, that would
> >>> affect percpu refcounters switching to atomic-mode, for example.
> >>>
> >>> They switch to atomic mode by calling __percpu_ref_switch_mode() which
> >>> is called by percpu_ref_switch_to_atomic_sync().>
> >>> This will slow this call down for the full lazy duration which will slow
> >>> down suspend in blk_pre_runtime_suspend().
> >>
> >> Correction while I am going on the record (got to be careful these
> >> days). It *might* slow down RCU for the full lazy duration, unless of
> >> course a fly-by rescue call_rcu() comes in.
> > 
> > Just unload a module, which if I remember correctly invokes rcu_barrier().
> > Lots of rescue callbacks.  ;-)
> 
> Haha. Yes I suppose the per-cpu atomic switch paths can also invoke
> rcu_barrier() but I suspect somebody might complain about IPIs :-P

There is always a critic!  ;-)

							Thanx, Paul