linux-kernel - Re: [patch 27/44] posix-cpu-timers: Provide array based access to expiry cache

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Date:   Mon, 19 Aug 2019 21:32:24 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: [patch 27/44] posix-cpu-timers: Provide array based access to
 expiry cache


* Thomas Gleixner <tglx@...utronix.de> wrote:

> Using struct task_cputime for the expiry cache is a pretty odd choice and
> comes with magic defines to rename the fields for usage in the expiry
> cache.
> 
> struct task_cputime is basically a u64 array with 3 members, but it has
> distinct members.
> 
> The expiry cache content is different than the content of task_cputime
> because
> 
>   expiry[PROF]  = task_cputime.stime + task_cputime.utime
>   expiry[VIRT]  = task_cputime.utime
>   expiry[SCHED] = task_cputime.sum_exec_runtime
> 
> So there is no direct mapping between task_cputime and the expiry cache and
> the #define based remapping is just a horrible hack.

>  struct posix_cputimers {
> -	struct task_cputime	cputime_expires;
> -	struct list_head	cpu_timers[CPUCLOCK_MAX];
> +	/* Temporary union until all users are cleaned up */
> +	union {
> +		struct task_cputime	cputime_expires;
> +		u64			expiries[CPUCLOCK_MAX];
> +	};
> +	struct list_head		cpu_timers[CPUCLOCK_MAX];
>  };

Could we please name this first_expiry[] or such, to make it clear that 
this is cached value of the first expiry of all timers of this process, 
instead of the rather vague 'expiries[]' naming?

Also, while at it, after the above temporary transition union, the final 
structure becomes:

 struct posix_cputimers {
       u64                     expiries[CPUCLOCK_MAX];
       struct list_head        cpu_timers[CPUCLOCK_MAX];
 };

Wouldn't it be more natural and easier to read to have the list head and 
the expiry together:

	struct posix_cputimer_list {
		u64				first_expiry;
		struct list_head		list;
	};

	struct posix_cputimers {
		struct posix_cputimer_list	timers[CPUCLOCK_MAX];
	};

?

This makes the array structure rather clear and the first_expiry field 
mostly self-documenting.

Thanks,

	Ingo