linux-kernel - Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed

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Message-Id: <4069DA96-5E01-4E70-8A04-52543A43B3BC@joelfernandes.org>
Date:   Wed, 8 Mar 2023 07:09:42 -0800
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Qiuxu Zhuo <qiuxu.zhuo@...el.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        rcu@...r.kernel.org, urezki@...il.com
Subject: Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed



> On Mar 8, 2023, at 7:01 AM, Frederic Weisbecker <frederic@...nel.org> wrote:
> 
> On Wed, Mar 08, 2023 at 05:52:50AM -0800, Joel Fernandes wrote:
>> Just to add to previous reply:
>> 
>> One thing to consider is that it is more of a performance improvement for
>> booting in expedited mode to fallback to normal later, than a bug
>> fix. Repeated synchronize_rcu() can easily add 100s of milliseconds and to
>> remedy that — a conversion of the call from normal API to the expedited API
>> will not help.
> 
> 2 things to consider:
> 
> 1) Is it this about specific calls to synchronize_rcu() that repeat a lot
>   and thus create such measurable impact? If so the specific callsites should
>   be considered for a conversion.
> 
> 2) Is it about lots of different calls to synchronize_rcu() that gather a big
>   noise? Then the solution is different.
> 
> Again without proper analysis, what do we know?

Again, no one disputed that proper analysis is needed. That is obvious. I was just responding to your assumption that if boot is slow, user space will also be slow. That is not a good thing to conclude because there are many factors. Slowness at boot may be considered a bug, but slowness after boot may not be (say if the user care mores for power later).

On my side I am planning to dig deeper into our boot process, but it will take time. I hope Qiuxu can do the boot analysis on his side.

Thanks.

> 
> Thanks.