Re: [patch V2 20/38] posix-cpu-timers: Provide array based access to expiry cache

[Frederic Weisbecker]

On Wed, Aug 21, 2019 at 09:09:07PM +0200, Thomas Gleixner 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.
> 
> Having the expiry cache array based allows further simplification of the
> expiry code.
> 
> To avoid an all in one cleanup which is hard to review add a temporary
> anonymous union into struct task_cputime which allows array based access to
> it. That requires to reorder the members. Add a build time sanity check to
> validate that the members are at the same place.
> 
> The union and the build time checks will be removed after conversion.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>