Re: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO

[Johannes Weiner]

On Thu, Jul 19, 2018 at 11:26:14AM +0200, Peter Zijlstra wrote:
> On Wed, Jul 18, 2018 at 02:03:18PM +0200, Peter Zijlstra wrote:
> 
> > Leaving us just 5 bytes short of needing a single cacheline :/
> > 
> > struct ponies {
> >         unsigned int               tasks[3];                                             /*     0    12 */
> >         unsigned int               cpu_state:2;                                          /*    12:30  4 */
> >         unsigned int               io_state:2;                                           /*    12:28  4 */
> >         unsigned int               mem_state:2;                                          /*    12:26  4 */
> > 
> >         /* XXX 26 bits hole, try to pack */
> > 
> >         /* typedef u64 */ long long unsigned int     last_time;                          /*    16     8 */
> >         /* typedef u64 */ long long unsigned int     some_time[3];                       /*    24    24 */
> >         /* typedef u64 */ long long unsigned int     full_time[2];                       /*    48    16 */
> >         /* --- cacheline 1 boundary (64 bytes) --- */
> >         /* typedef u64 */ long long unsigned int     nonidle_time;                       /*    64     8 */
> > 
> >         /* size: 72, cachelines: 2, members: 8 */
> >         /* bit holes: 1, sum bit holes: 26 bits */
> >         /* last cacheline: 8 bytes */
> > };
> > 
> > ARGGH!
> 
> It _might_ be possible to use curr->se.exec_start for last_time if you
> very carefully audit and place the hooks. I've not gone through it in
> detail, but it might just work.

Hnngg, and chop off an entire cacheline...

But don't we flush that delta out and update the timestamp on every
tick? entity_tick() does update_curr(). That might be too expensive :(