Re: [PATCH] drm/i915/execlists: Micro-optimise "idle" context switch

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Hi Chris,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on drm-intel/for-linux-next]
[also build test ERROR on v4.18 next-20180817]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Chris-Wilson/drm-i915-execlists-Micro-optimise-idle-context-switch/20180817-205726
base:   git://anongit.freedesktop.org/drm-intel for-linux-next
config: i386-randconfig-x005-201832 (attached as .config)
compiler: gcc-7 (Debian 7.3.0-16) 7.3.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All errors (new ones prefixed by >>):

   drivers/gpu/drm/i915/intel_lrc.c: In function 'execlists_dequeue':
>> drivers/gpu/drm/i915/intel_lrc.c:730:9: error: implicit declaration of function 'intel_engine_signaled'; did you mean 'intel_engine_is_idle'? [-Werror=implicit-function-declaration]
            intel_engine_signaled(engine,
            ^~~~~~~~~~~~~~~~~~~~~
            intel_engine_is_idle
   cc1: some warnings being treated as errors

vim +730 drivers/gpu/drm/i915/intel_lrc.c

   580	
   581	static void execlists_dequeue(struct intel_engine_cs *engine)
   582	{
   583		struct intel_engine_execlists * const execlists = &engine->execlists;
   584		struct execlist_port *port = execlists->port;
   585		const struct execlist_port * const last_port =
   586			&execlists->port[execlists->port_mask];
   587		struct i915_request *last = port_request(port);
   588		struct rb_node *rb;
   589		bool submit = false;
   590	
   591		/*
   592		 * Hardware submission is through 2 ports. Conceptually each port
   593		 * has a (RING_START, RING_HEAD, RING_TAIL) tuple. RING_START is
   594		 * static for a context, and unique to each, so we only execute
   595		 * requests belonging to a single context from each ring. RING_HEAD
   596		 * is maintained by the CS in the context image, it marks the place
   597		 * where it got up to last time, and through RING_TAIL we tell the CS
   598		 * where we want to execute up to this time.
   599		 *
   600		 * In this list the requests are in order of execution. Consecutive
   601		 * requests from the same context are adjacent in the ringbuffer. We
   602		 * can combine these requests into a single RING_TAIL update:
   603		 *
   604		 *              RING_HEAD...req1...req2
   605		 *                                    ^- RING_TAIL
   606		 * since to execute req2 the CS must first execute req1.
   607		 *
   608		 * Our goal then is to point each port to the end of a consecutive
   609		 * sequence of requests as being the most optimal (fewest wake ups
   610		 * and context switches) submission.
   611		 */
   612	
   613		if (last) {
   614			/*
   615			 * Don't resubmit or switch until all outstanding
   616			 * preemptions (lite-restore) are seen. Then we
   617			 * know the next preemption status we see corresponds
   618			 * to this ELSP update.
   619			 */
   620			GEM_BUG_ON(!execlists_is_active(execlists,
   621							EXECLISTS_ACTIVE_USER));
   622			GEM_BUG_ON(!port_count(&port[0]));
   623	
   624			/*
   625			 * If we write to ELSP a second time before the HW has had
   626			 * a chance to respond to the previous write, we can confuse
   627			 * the HW and hit "undefined behaviour". After writing to ELSP,
   628			 * we must then wait until we see a context-switch event from
   629			 * the HW to indicate that it has had a chance to respond.
   630			 */
   631			if (!execlists_is_active(execlists, EXECLISTS_ACTIVE_HWACK))
   632				return;
   633	
   634			if (need_preempt(engine, last, execlists->queue_priority)) {
   635				inject_preempt_context(engine);
   636				return;
   637			}
   638	
   639			/*
   640			 * In theory, we could coalesce more requests onto
   641			 * the second port (the first port is active, with
   642			 * no preemptions pending). However, that means we
   643			 * then have to deal with the possible lite-restore
   644			 * of the second port (as we submit the ELSP, there
   645			 * may be a context-switch) but also we may complete
   646			 * the resubmission before the context-switch. Ergo,
   647			 * coalescing onto the second port will cause a
   648			 * preemption event, but we cannot predict whether
   649			 * that will affect port[0] or port[1].
   650			 *
   651			 * If the second port is already active, we can wait
   652			 * until the next context-switch before contemplating
   653			 * new requests. The GPU will be busy and we should be
   654			 * able to resubmit the new ELSP before it idles,
   655			 * avoiding pipeline bubbles (momentary pauses where
   656			 * the driver is unable to keep up the supply of new
   657			 * work). However, we have to double check that the
   658			 * priorities of the ports haven't been switch.
   659			 */
   660			if (port_count(&port[1]))
   661				return;
   662	
   663			/*
   664			 * WaIdleLiteRestore:bdw,skl
   665			 * Apply the wa NOOPs to prevent
   666			 * ring:HEAD == rq:TAIL as we resubmit the
   667			 * request. See gen8_emit_breadcrumb() for
   668			 * where we prepare the padding after the
   669			 * end of the request.
   670			 */
   671			last->tail = last->wa_tail;
   672		}
   673	
   674		while ((rb = rb_first_cached(&execlists->queue))) {
   675			struct i915_priolist *p = to_priolist(rb);
   676			struct i915_request *rq, *rn;
   677	
   678			list_for_each_entry_safe(rq, rn, &p->requests, sched.link) {
   679				/*
   680				 * Can we combine this request with the current port?
   681				 * It has to be the same context/ringbuffer and not
   682				 * have any exceptions (e.g. GVT saying never to
   683				 * combine contexts).
   684				 *
   685				 * If we can combine the requests, we can execute both
   686				 * by updating the RING_TAIL to point to the end of the
   687				 * second request, and so we never need to tell the
   688				 * hardware about the first.
   689				 */
   690				if (last &&
   691				    !can_merge_ctx(rq->hw_context, last->hw_context)) {
   692					/*
   693					 * If we are on the second port and cannot
   694					 * combine this request with the last, then we
   695					 * are done.
   696					 */
   697					if (port == last_port) {
   698						__list_del_many(&p->requests,
   699								&rq->sched.link);
   700						goto done;
   701					}
   702	
   703					/*
   704					 * If GVT overrides us we only ever submit
   705					 * port[0], leaving port[1] empty. Note that we
   706					 * also have to be careful that we don't queue
   707					 * the same context (even though a different
   708					 * request) to the second port.
   709					 */
   710					if (ctx_single_port_submission(last->hw_context) ||
   711					    ctx_single_port_submission(rq->hw_context)) {
   712						__list_del_many(&p->requests,
   713								&rq->sched.link);
   714						goto done;
   715					}
   716	
   717					GEM_BUG_ON(last->hw_context == rq->hw_context);
   718	
   719					/*
   720					 * Avoid reloading the previous context if we
   721					 * know it has just completed and we want
   722					 * to switch over to a new context. The CS
   723					 * interrupt is likely waiting for us to
   724					 * release the local irq lock and so we will
   725					 * proceed with the submission momentarily,
   726					 * which is quicker than reloading the context
   727					 * on the gpu.
   728					 */
   729					if (!submit &&
 > 730					    intel_engine_signaled(engine,
   731								  last->global_seqno)) {
   732						GEM_BUG_ON(!list_is_first(&rq->sched.link,
   733									  &p->requests));
   734						return;
   735					}
   736	
   737					if (submit)
   738						port_assign(port, last);
   739					port++;
   740	
   741					GEM_BUG_ON(port_isset(port));
   742				}
   743	
   744				INIT_LIST_HEAD(&rq->sched.link);
   745				__i915_request_submit(rq);
   746				trace_i915_request_in(rq, port_index(port, execlists));
   747				last = rq;
   748				submit = true;
   749			}
   750	
   751			rb_erase_cached(&p->node, &execlists->queue);
   752			INIT_LIST_HEAD(&p->requests);
   753			if (p->priority != I915_PRIORITY_NORMAL)
   754				kmem_cache_free(engine->i915->priorities, p);
   755		}
   756	
   757	done:
   758		/*
   759		 * Here be a bit of magic! Or sleight-of-hand, whichever you prefer.
   760		 *
   761		 * We choose queue_priority such that if we add a request of greater
   762		 * priority than this, we kick the submission tasklet to decide on
   763		 * the right order of submitting the requests to hardware. We must
   764		 * also be prepared to reorder requests as they are in-flight on the
   765		 * HW. We derive the queue_priority then as the first "hole" in
   766		 * the HW submission ports and if there are no available slots,
   767		 * the priority of the lowest executing request, i.e. last.
   768		 *
   769		 * When we do receive a higher priority request ready to run from the
   770		 * user, see queue_request(), the queue_priority is bumped to that
   771		 * request triggering preemption on the next dequeue (or subsequent
   772		 * interrupt for secondary ports).
   773		 */
   774		execlists->queue_priority =
   775			port != execlists->port ? rq_prio(last) : INT_MIN;
   776	
   777		if (submit) {
   778			port_assign(port, last);
   779			execlists_submit_ports(engine);
   780		}
   781	
   782		/* We must always keep the beast fed if we have work piled up */
   783		GEM_BUG_ON(rb_first_cached(&execlists->queue) &&
   784			   !port_isset(execlists->port));
   785	
   786		/* Re-evaluate the executing context setup after each preemptive kick */
   787		if (last)
   788			execlists_user_begin(execlists, execlists->port);
   789	
   790		/* If the engine is now idle, so should be the flag; and vice versa. */
   791		GEM_BUG_ON(execlists_is_active(&engine->execlists,
   792					       EXECLISTS_ACTIVE_USER) ==
   793			   !port_isset(engine->execlists.port));
   794	}
   795	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

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