Re: [PATCH] poll: prevent busy-waiting

[Paolo Bonzini]

Here is what I applied.  Thanks!

2012-05-21  Paolo Bonzini  <address@hidden>

        poll/select: prevent busy-waiting.  SwitchToThread() only gives away
        the rest of the current time slice to another thread in the current
        process. So if the thread that feeds the file decscriptor we're
        polling is not in the current process, we get busy-waiting.
        * lib/poll.c: Use SleepEx(1, TRUE) instead of SwitchToThread().
        Patch from Theodore Leblond.
        * lib/select.c: Split polling out of the loop that sets the output
        fd_sets.  Check for zero result and loop if the wait timeout is
        infinite.
---
 ChangeLog    |   12 ++++++++++++
 lib/poll.c   |    2 +-
 lib/select.c |   60 ++++++++++++++++++++++++++++++++++++++++------------------
 3 files changed, 55 insertions(+), 19 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 7dfccd7..a7fcb3f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,15 @@
+2012-05-21  Paolo Bonzini  <address@hidden>
+
+       poll/select: prevent busy-waiting.  SwitchToThread() only gives away
+       the rest of the current time slice to another thread in the current
+       process. So if the thread that feeds the file decscriptor we're
+       polling is not in the current process, we get busy-waiting.
+       * lib/poll.c: Use SleepEx(1, TRUE) instead of SwitchToThread().
+       Patch from Theodore Leblond.
+       * lib/select.c: Split polling out of the loop that sets the output
+       fd_sets.  Check for zero result and loop if the wait timeout is
+       infinite.
+
 2012-05-18  Paul Eggert  <address@hidden>
 
        crypto: fix bug in large buffer handling
diff --git a/lib/poll.c b/lib/poll.c
index 3071b12..5ad9d86 100644
--- a/lib/poll.c
+++ b/lib/poll.c
@@ -598,7 +598,7 @@ restart:
 
   if (!rc && timeout == INFTIM)
     {
-      SwitchToThread();
+      SleepEx (1, TRUE);
       goto restart;
     }
 
diff --git a/lib/select.c b/lib/select.c
index 377d549..e7ca97c 100644
--- a/lib/select.c
+++ b/lib/select.c
@@ -385,6 +385,10 @@ rpl_select (int nfds, fd_set *rfds, fd_set *wfds, fd_set 
*xfds,
         }
     }
 
+  /* Place a sentinel at the end of the array.  */
+  handle_array[nhandles] = NULL;
+
+restart:
   if (wait_timeout == 0 || nsock == 0)
     rc = 0;
   else
@@ -427,13 +431,44 @@ rpl_select (int nfds, fd_set *rfds, fd_set *wfds, fd_set 
*xfds,
   if (rc == 0 && nsock > 0)
     rc = select (0, &handle_rfds, &handle_wfds, &handle_xfds, &tv0);
 
+  if (nhandles > 1)
+    {
+      /* Count results that are not counted in the return value of select.  */
+      nhandles = 1;
+      for (i = 0; i < nfds; i++)
+        {
+          if ((anyfds_in[i / CHAR_BIT] & (1 << (i & (CHAR_BIT - 1)))) == 0)
+            continue;
+
+          h = (HANDLE) _get_osfhandle (i);
+          if (h == handle_array[nhandles])
+            {
+              /* Not a socket.  */
+              nhandles++;
+              windows_poll_handle (h, i, &rbits, &wbits, &xbits);
+              if (rbits.out[i / CHAR_BIT] & (1 << (i & (CHAR_BIT - 1)))
+                  || wbits.out[i / CHAR_BIT] & (1 << (i & (CHAR_BIT - 1)))
+                  || xbits.out[i / CHAR_BIT] & (1 << (i & (CHAR_BIT - 1))))
+                rc++;
+            }
+        }
+
+      if (rc == 0 && wait_timeout == INFINITE)
+        {
+          /* Sleep 1 millisecond to avoid busy wait and retry with the
+             original fd_sets.  */
+          memcpy (&handle_rfds, rfds, sizeof (fd_set));
+          memcpy (&handle_wfds, wfds, sizeof (fd_set));
+          memcpy (&handle_xfds, xfds, sizeof (fd_set));
+          SleepEx (1, TRUE);
+          goto restart;
+        }
+    }
+
   /* Now fill in the results.  */
   FD_ZERO (rfds);
   FD_ZERO (wfds);
   FD_ZERO (xfds);
-
-  /* Place a sentinel at the end of the array.  */
-  handle_array[nhandles] = NULL;
   nhandles = 1;
   for (i = 0; i < nfds; i++)
     {
@@ -443,8 +478,7 @@ rpl_select (int nfds, fd_set *rfds, fd_set *wfds, fd_set 
*xfds,
       h = (HANDLE) _get_osfhandle (i);
       if (h != handle_array[nhandles])
         {
-          /* Perform handle->descriptor mapping.  Don't update rc, as these
-             results are counted in the return value of Winsock's select.  */
+          /* Perform handle->descriptor mapping.  */
           WSAEventSelect ((SOCKET) h, NULL, 0);
           if (FD_ISSET (h, &handle_rfds))
             FD_SET (i, rfds);
@@ -457,22 +491,12 @@ rpl_select (int nfds, fd_set *rfds, fd_set *wfds, fd_set 
*xfds,
         {
           /* Not a socket.  */
           nhandles++;
-          windows_poll_handle (h, i, &rbits, &wbits, &xbits);
           if (rbits.out[i / CHAR_BIT] & (1 << (i & (CHAR_BIT - 1))))
-            {
-              rc++;
-              FD_SET (i, rfds);
-            }
+            FD_SET (i, rfds);
           if (wbits.out[i / CHAR_BIT] & (1 << (i & (CHAR_BIT - 1))))
-            {
-              rc++;
-              FD_SET (i, wfds);
-            }
+            FD_SET (i, wfds);
           if (xbits.out[i / CHAR_BIT] & (1 << (i & (CHAR_BIT - 1))))
-            {
-              rc++;
-              FD_SET (i, xfds);
-            }
+            FD_SET (i, xfds);
         }
     }
 
-- 
1.7.10.1