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

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Message-ID: <alpine.DEB.2.21.1908202217150.2223@nanos.tec.linutronix.de>
Date:   Tue, 20 Aug 2019 22:22:07 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
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

On Mon, 19 Aug 2019, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@...utronix.de> wrote:
> >  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.

I kept the odd named expiries for the temporary union and then after the
patch which removes the abused struct task_cputime, I applied a separate
cleanup which looks similar to the above.

Just the names are a bit different and more aligned to what we have in
hrtimers:

struct posix_cputimer_base {
	u64		   	nextevt;
	struct timerqueue_head	tqhead;
};

and then have

struct posix_cputimers {
	struct posix_cputimer_base	bases[CPUCLOCK_MAX];
};

I'll send out a new version after doing some more testing.

Thanks,

	tglx