Re: [PATCH 2/2] drm/i915: Move execlists port head instead of memmoving array

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Mika Kuoppala (2017-11-22 13:52:09)
> Chris Wilson <chris@chris-wilson.co.uk> writes:
> 
> > Quoting Mika Kuoppala (2017-10-31 15:27:34)
> >> From: Mika Kuoppala <mika.kuoppala@intel.com>
> >> 
> >> As all our access to execlist ports are through head and tail
> >> helpers, we can now move the head instead of memmoving the array.
> >> 
> >> v2: use memset (Chris)
> >> 
> >> Cc: Michał Winiarski <michal.winiarski@intel.com>
> >> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> >> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> >> Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/intel_ringbuffer.h | 10 +++++-----
> >>  1 file changed, 5 insertions(+), 5 deletions(-)
> >> 
> >> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> >> index 387667fe50d3..011c4b8f1339 100644
> >> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> >> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> >> @@ -611,13 +611,13 @@ static inline struct execlist_port *
> >>  execlists_head_complete(struct intel_engine_execlists * const execlists,
> >>                         struct execlist_port * const port)
> >>  {
> >> -       const unsigned int m = execlists->port_mask;
> >> -
> >> -       GEM_BUG_ON(port_index(port, execlists) != 0);
> >> +       GEM_BUG_ON(port_index(port, execlists) != execlists->port_head);
> >> +       GEM_BUG_ON(!port_isset(port));
> >>         GEM_BUG_ON(!execlists_is_active(execlists, EXECLISTS_ACTIVE_USER));
> >>  
> >> -       memmove(port, port + 1, m * sizeof(struct execlist_port));
> >> -       memset(port + m, 0, sizeof(struct execlist_port));
> >> +       memset(port, 0, sizeof(*port));
> >> +
> >> +       execlists->port_head = port_head_add(execlists, 1);
> >
> > Ok, I would have gone for
> >
> > port = port_next(port);
> > execlists->port_head = port - execlists->port;
> > return port;
> >
> > That to me looks more natural advance of port as we complete the
> > requests, and matches the loop in the irq handler.
> >
> > Care to crunch the numbers and see which gcc favours?
> 
> gcc (Ubuntu 6.3.0-12ubuntu2) 6.3.0 20170406:
> 
> add/remove: 0/0 grow/shrink: 3/0 up/down: 219/0 (219)
> function                                     old     new   delta
> execlists_submission_tasklet                2405    2525    +120
> execlists_cancel_port_requests               315     376     +61
> guc_submission_tasklet                      1643    1681     +38
> Total: Before=1168854, After=1169073, chg +0.02%
> 
> gcc (Ubuntu 7.2.0-8ubuntu3) 7.2.0:
> 
> add/remove: 0/0 grow/shrink: 3/0 up/down: 158/0 (158)
> function                                     old     new   delta
> execlists_submission_tasklet                2383    2460     +77
> execlists_cancel_port_requests               344     401     +57
> guc_submission_tasklet                      1684    1708     +24
> Total: Before=1164662, After=1164820, chg +0.01%

Ok, have a
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris
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