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

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Message-ID: <87663566-ed67-fee8-4598-13591f2f072b@joelfernandes.org>
Date:   Wed, 10 Aug 2022 22:31:56 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, frederic@...nel.org, paulmck@...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 10:23 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.

- Joel