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,
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.
Matt
> Regards,
>
> Tvrtko
