linux-kernel - Re: [PATCH v3 resend 0/6] Implement call_rcu

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Date:   Wed, 10 Aug 2022 23:22:13 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     linux-kernel@...r.kernel.org, rushikesh.s.kadam@...el.com,
        urezki@...il.com, neeraj.iitr10@...il.com, frederic@...nel.org,
        rostedt@...dmis.org, rcu@...r.kernel.org
Subject: Re: [PATCH v3 resend 0/6] Implement call_rcu_lazy() and miscellaneous
 fixes



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

Thanks,

 - Joel