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Thu, 10 Jun 2021 08:27:51 -0700 (PDT) Date: Thu, 10 Jun 2021 17:27:48 +0200 From: Daniel Vetter To: Matthew Brost Subject: Re: [Intel-gfx] [RFC PATCH 36/97] drm/i915/guc: Add non blocking CTB send function Message-ID: References: <20210525172121.GE14724@sdutt-i7> <0f26f76f-e066-fb23-a7b2-784bb8ee771d@linux.intel.com> <20210526181053.GA3435@sdutt-i7> <53613c13-1cab-b9bd-3922-0389600773ee@linux.intel.com> <20210527143514.GA24720@sdutt-i7> <828fe399-5319-78a9-c6e3-c0c027e08e9c@linux.intel.com> <20210607173101.GA11968@sdutt-i7> <2706c890-5145-4edb-acd1-b9862caba8cf@linux.intel.com> <20210609231023.GB5471@sdutt-i7> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210609231023.GB5471@sdutt-i7> X-Operating-System: Linux phenom 5.10.32scarlett+ X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Tvrtko Ursulin , intel-gfx , dri-devel , Jason Ekstrand , Daniel Vetter Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Wed, Jun 09, 2021 at 04:10:23PM -0700, Matthew Brost wrote: > On Tue, Jun 08, 2021 at 10:46:15AM +0200, Daniel Vetter wrote: > > On Tue, Jun 8, 2021 at 10:39 AM Tvrtko Ursulin > > wrote: > > > > > > > > > On 07/06/2021 18:31, Matthew Brost wrote: > > > > On Thu, May 27, 2021 at 04:11:50PM +0100, Tvrtko Ursulin wrote: > > > >> > > > >> On 27/05/2021 15:35, Matthew Brost wrote: > > > >>> On Thu, May 27, 2021 at 11:02:24AM +0100, Tvrtko Ursulin wrote: > > > >>>> > > > >>>> On 26/05/2021 19:10, Matthew Brost wrote: > > > >>>> > > > >>>> [snip] > > > >>>> > > > >>>>>>>>> +static int ct_send_nb(struct intel_guc_ct *ct, > > > >>>>>>>>> + const u32 *action, > > > >>>>>>>>> + u32 len, > > > >>>>>>>>> + u32 flags) > > > >>>>>>>>> +{ > > > >>>>>>>>> + struct intel_guc_ct_buffer *ctb = &ct->ctbs.send; > > > >>>>>>>>> + unsigned long spin_flags; > > > >>>>>>>>> + u32 fence; > > > >>>>>>>>> + int ret; > > > >>>>>>>>> + > > > >>>>>>>>> + spin_lock_irqsave(&ctb->lock, spin_flags); > > > >>>>>>>>> + > > > >>>>>>>>> + ret = ctb_has_room(ctb, len + 1); > > > >>>>>>>>> + if (unlikely(ret)) > > > >>>>>>>>> + goto out; > > > >>>>>>>>> + > > > >>>>>>>>> + fence = ct_get_next_fence(ct); > > > >>>>>>>>> + ret = ct_write(ct, action, len, fence, flags); > > > >>>>>>>>> + if (unlikely(ret)) > > > >>>>>>>>> + goto out; > > > >>>>>>>>> + > > > >>>>>>>>> + intel_guc_notify(ct_to_guc(ct)); > > > >>>>>>>>> + > > > >>>>>>>>> +out: > > > >>>>>>>>> + spin_unlock_irqrestore(&ctb->lock, spin_flags); > > > >>>>>>>>> + > > > >>>>>>>>> + return ret; > > > >>>>>>>>> +} > > > >>>>>>>>> + > > > >>>>>>>>> static int ct_send(struct intel_guc_ct *ct, > > > >>>>>>>>> const u32 *action, > > > >>>>>>>>> u32 len, > > > >>>>>>>>> @@ -473,6 +541,7 @@ static int ct_send(struct intel_guc_ct *ct, > > > >>>>>>>>> u32 response_buf_size, > > > >>>>>>>>> u32 *status) > > > >>>>>>>>> { > > > >>>>>>>>> + struct intel_guc_ct_buffer *ctb = &ct->ctbs.send; > > > >>>>>>>>> struct ct_request request; > > > >>>>>>>>> unsigned long flags; > > > >>>>>>>>> u32 fence; > > > >>>>>>>>> @@ -482,8 +551,20 @@ static int ct_send(struct intel_guc_ct *ct, > > > >>>>>>>>> GEM_BUG_ON(!len); > > > >>>>>>>>> GEM_BUG_ON(len & ~GUC_CT_MSG_LEN_MASK); > > > >>>>>>>>> GEM_BUG_ON(!response_buf && response_buf_size); > > > >>>>>>>>> + might_sleep(); > > > >>>>>>>> > > > >>>>>>>> Sleep is just cond_resched below or there is more? > > > >>>>>>>> > > > >>>>>>> > > > >>>>>>> Yes, the cond_resched. > > > >>>>>>> > > > >>>>>>>>> + /* > > > >>>>>>>>> + * We use a lazy spin wait loop here as we believe that if the CT > > > >>>>>>>>> + * buffers are sized correctly the flow control condition should be > > > >>>>>>>>> + * rare. > > > >>>>>>>>> + */ > > > >>>>>>>>> +retry: > > > >>>>>>>>> spin_lock_irqsave(&ct->ctbs.send.lock, flags); > > > >>>>>>>>> + if (unlikely(!ctb_has_room(ctb, len + 1))) { > > > >>>>>>>>> + spin_unlock_irqrestore(&ct->ctbs.send.lock, flags); > > > >>>>>>>>> + cond_resched(); > > > >>>>>>>>> + goto retry; > > > >>>>>>>>> + } > > > >>>>>>>> > > > >>>>>>>> If this patch is about adding a non-blocking send function, and below we can > > > >>>>>>>> see that it creates a fork: > > > >>>>>>>> > > > >>>>>>>> intel_guc_ct_send: > > > >>>>>>>> ... > > > >>>>>>>> if (flags & INTEL_GUC_SEND_NB) > > > >>>>>>>> return ct_send_nb(ct, action, len, flags); > > > >>>>>>>> > > > >>>>>>>> ret = ct_send(ct, action, len, response_buf, response_buf_size, &status); > > > >>>>>>>> > > > >>>>>>>> Then why is there a change in ct_send here, which is not the new > > > >>>>>>>> non-blocking path? > > > >>>>>>>> > > > >>>>>>> > > > >>>>>>> There is not a change to ct_send(), just to intel_guc_ct_send. > > > >>>>>> > > > >>>>>> I was doing by the diff which says: > > > >>>>>> > > > >>>>>> static int ct_send(struct intel_guc_ct *ct, > > > >>>>>> const u32 *action, > > > >>>>>> u32 len, > > > >>>>>> @@ -473,6 +541,7 @@ static int ct_send(struct intel_guc_ct *ct, > > > >>>>>> u32 response_buf_size, > > > >>>>>> u32 *status) > > > >>>>>> { > > > >>>>>> + struct intel_guc_ct_buffer *ctb = &ct->ctbs.send; > > > >>>>>> struct ct_request request; > > > >>>>>> unsigned long flags; > > > >>>>>> u32 fence; > > > >>>>>> @@ -482,8 +551,20 @@ static int ct_send(struct intel_guc_ct *ct, > > > >>>>>> GEM_BUG_ON(!len); > > > >>>>>> GEM_BUG_ON(len & ~GUC_CT_MSG_LEN_MASK); > > > >>>>>> GEM_BUG_ON(!response_buf && response_buf_size); > > > >>>>>> + might_sleep(); > > > >>>>>> + /* > > > >>>>>> + * We use a lazy spin wait loop here as we believe that if the CT > > > >>>>>> + * buffers are sized correctly the flow control condition should be > > > >>>>>> + * rare. > > > >>>>>> + */ > > > >>>>>> +retry: > > > >>>>>> spin_lock_irqsave(&ct->ctbs.send.lock, flags); > > > >>>>>> + if (unlikely(!ctb_has_room(ctb, len + 1))) { > > > >>>>>> + spin_unlock_irqrestore(&ct->ctbs.send.lock, flags); > > > >>>>>> + cond_resched(); > > > >>>>>> + goto retry; > > > >>>>>> + } > > > >>>>>> > > > >>>>>> So it looks like a change to ct_send to me. Is that wrong? > > > >>>> > > > >>>> What about this part - is the patch changing the blocking ct_send or not, > > > >>>> and if it is why? > > > >>>> > > > >>> > > > >>> Yes, ct_send() changes. Sorry for the confusion. > > > >>> > > > >>> This function needs to be updated to account for the H2G space and > > > >>> backoff if no space is available. > > > >> > > > >> Since this one is the sleeping path, it probably can and needs to be smarter > > > >> than having a cond_resched busy loop added. Like sleep and get woken up when > > > >> there is space. Otherwise it can degenerate to busy looping via contention > > > >> with the non-blocking path. > > > >> > > > > > > > > That screams over enginerring a simple problem to me. If the CT channel > > > > is full we are really in trouble anyways - i.e. the performance is going > > > > to terrible as we overwhelmed the GuC with traffic. That being said, > > > > > > Performance of what would be terrible? Something relating to submitting > > > new jobs to the GPU I guess. Or something SRIOV related as you hint below. > > > > > > But there is no real reason why CPU cycles/power should suffer if GuC is > > > busy. > > > > > > Okay, if it can't happen in real world then it's possibly passable as a > > > design of a communication interface. But to me it leaves a bad taste and > > > a doubt that there is this other aspect of the real world. And that is > > > when the unexpected happens. Even the most trivial things like a bug in > > > GuC firmware causes the driver to busy spin in there. So not much > > > happening on the machine but CPU cores pinned burning cycles in this > > > code. It's just lazy and not robust design. "Bug #nnnnn - High CPU usage > > > and GUI blocked - Solution: Upgrade GuC firmware and _reboot_ the > > > machine". Oh well.. > > > > > > At least I think the commit message should spell out clearly that a busy > > > looping path is being added to the sleeping send as a downside of > > > implementation choices. Still, for the record, I object to the design. > > > > > > > IGTs can do this but that really isn't a real world use case. For the > > > > real world, this buffer is large enough that it won't ever be full hence > > > > the comment + lazy spin loop. > > > > > > > > Next, it isn't like we get an interrupt or something when space > > > > becomes available so how would we wake this thread? Could we come up > > > > with a convoluted scheme where we insert ops that generated an interrupt > > > > at regular intervals, probably? Would it be super complicated, totally > > > > unnecessary, and gain use nothing - absolutely. > > > > > > > > Lastly, blocking CTBs really shouldn't ever be used. Certainly the > > > > submission code doesn't use these. I think SRIOV might, but those can > > > > probably be reworked too to use non-blocking. At some point we might > > > > want to scrub the driver and just delete the blocking path. > > > > I'd do an s/cond_resched()/msleep(1)/ and comment explaining why we > > just don't care about this. That checks of the cpu wasting in this > > case (GuC is overloaded, it wont come back anytime soon anyway) and > > explains why we really don't want to make this any more clever or > > complex code (because comment can explain why we wont hit this in > > actual real world usage except when something else is on fire already > > anyway). > > > > Sounds good. > > > If you want to go absolutely overkill and it's not too much work, make > > the msleep interruptible or check for signals, and bail out. That way > > the process can be made unstuck with ^C at least. > > This loop is already bound by a timer and if no forward progress is made > we pop out of this loop. It is assumed if this happens the GuC / GPU is > dead a and full GPU reset will have to be issued. A following patch > adds the timer, a bit later in submission section of the series a patch > is added to trigger the reset. Yeah timeout bail-out works too, and if you then switch it from timeout to also interruptible it shouldn't be much more code. It's just nice to not have any uninterruptible sleep. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch