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

Ingo Molnar
2019-08-19T19:32:30Z

* Thomas Gleixner <tglx@linutronix.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