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

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Date:   Wed, 8 Mar 2023 16:01:12 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.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 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?

Thanks.