linux-kernel - Re: Subject: [PATCH] [PATCH] time: tick-sched: use bool for tick

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Date:   Tue, 10 Apr 2018 16:12:00 +0800
From:   yuankuiz@...eaurora.org
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-pm@...r.kernel.org, rjw@...ysocki.net, fweisbec@...il.com,
        peterz@...radead.org, aulmck@...ux.vnet.ibm.com, mingo@...nel.org,
        len.brown@...el.com, linux-kernel@...r.kernel.org
Subject: Re: Subject: [PATCH] [PATCH] time: tick-sched: use bool for
 tick_stopped

On 2018-04-10 03:55 PM, Thomas Gleixner wrote:
> On Tue, 10 Apr 2018, yuankuiz@...eaurora.org wrote:
> 
>> From: John Zhao <yuankuiz@...eaurora.org>
>> 
>> Variable tick_stopped returned by tick_nohz_tick_stopped
>> can have only true / forse values. Since the return type
>> of the tick_nohz_tick_stopped is also bool, variable
>> tick_stopped nice to have data type as bool in place of int.
> 
> The data type is not int.
[ZJ] Yes. Per newest tip in branch of linux-pm-cpuidle, it is unsigned 
int with 1 bit in width.
There is typo in commit message, "int" at here should be "unsigned int"

> It's part of an integer bitfield and occupies
> exactly one bit of storage, while bool has an architecture dependend 
> size
> and is at least 1 byte, i.e. 8 bit. So with alignment effects you grew 
> the
> size of the data structure and therefore the cache foot print.
[ZJ] So, 1 bit in width is specified as:
+	bool				tick_stopped	: 1;
      This patch is based on the linux-pm-cpuidle branch which has
already gathered available space in the structure of tick_sched.

> 
> This is not about 'nice to have' ....
> 
> Thanks,
> 
> 	tglx