coroutine: add ./configure --disable-coroutine-pool

The 'gthread' coroutine backend was written before the freelist (aka
pool) existed in qemu-coroutine.c.

This means that every thread is expected to exit when its coroutine
terminates.  It is not possible to reuse threads from a pool.

This patch automatically disables the pool when 'gthread' is used.  This
allows the 'gthread' backend to work again (for example,
tests/test-coroutine completes successfully instead of hanging).

I considered implementing thread reuse but I don't want quirks like CPU
affinity differences due to coroutine threads being recycled.  The
'gthread' backend is a reference backend and it's therefore okay to skip
the pool optimization.

Note this patch also makes it easy to toggle the pool for benchmarking
purposes:

  ./configure --with-coroutine-backend=ucontext \
              --disable-coroutine-pool

Reported-by: Gabriel Kerneis <gabriel@kerneis.info>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Gabriel Kerneis <gabriel@kerneis.info>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

configure

@@ -238,6 +238,7 @@ win_sdk="no"
 want_tools="yes"
 libiscsi=""
 coroutine=""
+coroutine_pool=""
 seccomp=""
 glusterfs=""
 glusterfs_discard="no"
@@ -888,6 +889,10 @@ for opt do
   ;;
   --with-coroutine=*) coroutine="$optarg"
   ;;
+  --disable-coroutine-pool) coroutine_pool="no"
+  ;;
+  --enable-coroutine-pool) coroutine_pool="yes"
+  ;;
   --disable-docs) docs="no"
   ;;
   --enable-docs) docs="yes"
@@ -1189,6 +1194,8 @@ echo "  --disable-seccomp        disable seccomp support"
 echo "  --enable-seccomp         enables seccomp support"
 echo "  --with-coroutine=BACKEND coroutine backend. Supported options:"
 echo "                           gthread, ucontext, sigaltstack, windows"
+echo "  --disable-coroutine-pool disable coroutine freelist (worse performance)"
+echo "  --enable-coroutine-pool  enable coroutine freelist (better performance)"
 echo "  --enable-glusterfs       enable GlusterFS backend"
 echo "  --disable-glusterfs      disable GlusterFS backend"
 echo "  --enable-gcov            enable test coverage analysis with gcov"
@@ -3362,6 +3369,17 @@ else
   esac
 fi
 
+if test "$coroutine_pool" = ""; then
+  if test "$coroutine" = "gthread"; then
+    coroutine_pool=no
+  else
+    coroutine_pool=yes
+  fi
+fi
+if test "$coroutine" = "gthread" -a "$coroutine_pool" = "yes"; then
+  error_exit "'gthread' coroutine backend does not support pool (use --disable-coroutine-pool)"
+fi
+
 ##########################################
 # check if we have open_by_handle_at
 
@@ -3733,6 +3751,7 @@ echo "build guest agent $guest_agent"
 echo "QGA VSS support   $guest_agent_with_vss"
 echo "seccomp support   $seccomp"
 echo "coroutine backend $coroutine"
+echo "coroutine pool    $coroutine_pool"
 echo "GlusterFS support $glusterfs"
 echo "virtio-blk-data-plane $virtio_blk_data_plane"
 echo "gcov              $gcov_tool"
@@ -4092,6 +4111,11 @@ if test "$rbd" = "yes" ; then
 fi
 
 echo "CONFIG_COROUTINE_BACKEND=$coroutine" >> $config_host_mak
+if test "$coroutine_pool" = "yes" ; then
+  echo "CONFIG_COROUTINE_POOL=1" >> $config_host_mak
+else
+  echo "CONFIG_COROUTINE_POOL=0" >> $config_host_mak
+fi
 
 if test "$open_by_handle_at" = "yes" ; then
   echo "CONFIG_OPEN_BY_HANDLE=y" >> $config_host_mak

qemu-coroutine.c

@@ -30,8 +30,9 @@ static unsigned int pool_size;
 
 Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
 {
-    Coroutine *co;
+    Coroutine *co = NULL;
 
+    if (CONFIG_COROUTINE_POOL) {
         qemu_mutex_lock(&pool_lock);
         co = QSLIST_FIRST(&pool);
         if (co) {
@@ -39,6 +40,7 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
             pool_size--;
         }
         qemu_mutex_unlock(&pool_lock);
+    }
 
     if (!co) {
         co = qemu_coroutine_new();
@@ -51,6 +53,7 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
 
 static void coroutine_delete(Coroutine *co)
 {
+    if (CONFIG_COROUTINE_POOL) {
         qemu_mutex_lock(&pool_lock);
         if (pool_size < POOL_MAX_SIZE) {
             QSLIST_INSERT_HEAD(&pool, co, pool_next);
@@ -60,6 +63,7 @@ static void coroutine_delete(Coroutine *co)
             return;
         }
         qemu_mutex_unlock(&pool_lock);
+    }
 
     qemu_coroutine_delete(co);
 }