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

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

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%

where new:

 execlists_head_complete(struct intel_engine_execlists * const execlists,
-                       struct execlist_port * const port)
+                       struct execlist_port *port)
 {
        GEM_BUG_ON(port_index(port, execlists) != execlists->port_head);
        GEM_BUG_ON(!port_isset(port));
@@ -677,9 +677,10 @@ execlists_head_complete(struct intel_engine_execlists * const execlists,
 
        memset(port, 0, sizeof(*port));
 
-       execlists->port_head = port_head_add(execlists, 1);
+       port = execlists_port_next(execlists, port);
+       execlists->port_head = port - execlists->port;
 
-       return execlists_port_head(execlists);
+       return port;

-Mika

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