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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 19, 2018 at 08:50:38AM -0400, Johannes Weiner wrote:
> 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...

Yes.. a worthy goal :-)

> But don't we flush that delta out and update the timestamp on every
> tick?

Indeed.

> entity_tick() does update_curr(). That might be too expensive :(

Well, since you already do all this accounting on every enqueue/dequeue,
this can run many thousands of times per tick already, so once per tick
doesn't sound bad.

However, I just realized this might not in fact work, because
curr->se.exec_start is per task, and you really want something per-cpu
for this.

Bah, if only perf had a useful tool to report on data layout instead of
this c2c crap.. :-( The thinking being that we could maybe find a
usage-hole (a data member that is not in fact used) near something we
already touch for writing.