linux-kernel - Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall

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Message-ID: <20230830010444.GA33437@google.com>
Date:   Wed, 30 Aug 2023 01:04:44 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Huacai Chen <chenhuacai@...nel.org>
Cc:     paulmck@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Z qiang <qiang.zhang1211@...il.com>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Binbin Zhou <zhoubinbin@...ngson.cn>
Subject: Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()

On Mon, Aug 28, 2023 at 11:12:49PM +0800, Huacai Chen wrote:
> Maybe putting
> nr_fqs_jiffies_stall before jiffies_force_qs is better, because I
> think putting an 'int' between two 'long' is wasting space. :)

Though it is a decent suggestion, moving it before jiffies_force_qs does not
make the structure smaller. Trying to outsmart the compiler seems not a good
idea when it is not really packing the structure.

Besides, I am not too worried about 4-byte holes considering the structure is
full of them and that ofl_lock is ____cacheline_internodealigned_in_smp which
adds a giant 55 byte hole to the structure. That should keep you at night if
you are worried about 4 byte holes.

And the total size of the structure on a 64-bit build is 3776 bytes so 4
bytes is about 1/10th of a percent. Puny.

So I'd rather leave nr_fqs_jiffies_stall where it is especially considering
it is more readable where it is. :-)

thanks,

 - Joel

> 
> Huacai
> 
> >
> > Thanks.