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.
Regards,
Tvrtko
