On 6/25/19 4:06 AM, Richard W.M. Jones wrote: > On Wed, Jun 19, 2019 at 09:11:52PM -0500, Eric Blake wrote: >> When we are blocked waiting for POLLOUT during a request, and happen >> to receive notice of POLLIN instead, we know that the work done in >> response to POLLIN will be non-blocking (it returns to %.READY as soon >> as it would block, which in turn jumps right back into ISSUE_COMMAND >> because we have a pending request not fully sent yet). Since the >> jaunt through REPLY was non-blocking, it is unlikely that the POLLOUT >> situation has changed in the meantime, so if we use SET_NEXT_STATE() >> to step back into SEND_REQUEST, our recv() call will likely fail with >> EAGAIN, once again blocking us until our next POLLOUT. Although the >> wasted syscall is not on the hot-path (after all, we can't progress >> until data arrives from the server), it's slightly cleaner if we >> instead declare that we are already blocked. >>
>> if (h->wlen) {
>> if (h->in_write_payload)
>> - SET_NEXT_STATE(%SEND_WRITE_PAYLOAD);
>> + *next_state = %SEND_WRITE_PAYLOAD;
>> else
>> - SET_NEXT_STATE(%SEND_REQUEST);
>> + *next_state = %SEND_REQUEST;
>
> It would be nice to do this without fiddling with essentially an
> internal detail of the generated code.
>
> Could we add another macro, something like "SET_NEXT_STATE_AND_BLOCK"?
Yes, that's a nice idea, and easy enough to squash in.
>
> On the other hand if it's not on the hot path, maybe we shouldn't
> do this at all?
It wasn't on the hot path on any test I could come up with (where we
were waiting for the server anyway); but it may still be possible to
come up with a scenario where it matters more.
Should I push with this squashed in?
diff --git i/generator/generator w/generator/generator
index 34e70da..cbf4e59 100755
--- i/generator/generator
+++ w/generator/generator
@@ -2541,6 +2541,7 @@ let generate_lib_states_c () =
pr "%s\n" state_machine_prologue;
pr "\n";
pr "#define SET_NEXT_STATE(s) (*blocked = false, *next_state = (s))\n";
+ pr "#define SET_NEXT_STATE_AND_BLOCK(s) (*next_state = (s))\n";
pr "\n";
(* The state machine C code fragments. *)
diff --git i/generator/states-issue-command.c
w/generator/states-issue-command.c
index 9fc8c93..35f3c79 100644
--- i/generator/states-issue-command.c
+++ w/generator/states-issue-command.c
@@ -33,9 +33,9 @@
*/
if (h->wlen) {
if (h->in_write_payload)
- *next_state = %SEND_WRITE_PAYLOAD;
+ SET_NEXT_STATE_AND_BLOCK (%SEND_WRITE_PAYLOAD);
else
- *next_state = %SEND_REQUEST;
+ SET_NEXT_STATE_AND_BLOCK (%SEND_REQUEST);
return 0;
}
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
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