[Qemu-devel] [PATCH] coroutine: add ./configure --disable-coroutine-pool

[Stefan Hajnoczi <stefanha@redhat.com>]

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>
---
 configure        | 24 ++++++++++++++++++++++++
 qemu-coroutine.c | 34 +++++++++++++++++++---------------
 2 files changed, 43 insertions(+), 15 deletions(-)

diff --git a/configure b/configure
index e989609..5cf6341 100755
--- a/configure
+++ b/configure
@@ -235,6 +235,7 @@ guest_agent=""
 want_tools="yes"
 libiscsi=""
 coroutine=""
+coroutine_pool=""
 seccomp=""
 glusterfs=""
 glusterfs_discard="no"
@@ -875,6 +876,10 @@ 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"
@@ -1161,6 +1166,8 @@ 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"
@@ -3279,6 +3286,17 @@ 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
 
@@ -3644,6 +3662,7 @@ echo "libiscsi support  $libiscsi"
 echo "build guest agent $guest_agent"
 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              $gcov_tool"
@@ -3999,6 +4018,11 @@ if test "$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
diff --git a/qemu-coroutine.c b/qemu-coroutine.c
index 423430d..4708521 100644
--- a/qemu-coroutine.c
+++ b/qemu-coroutine.c
@@ -30,15 +30,17 @@ static unsigned int pool_size;
 
 Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
 {
-    Coroutine *co;
-
-    qemu_mutex_lock(&pool_lock);
-    co = QSLIST_FIRST(&pool);
-    if (co) {
-        QSLIST_REMOVE_HEAD(&pool, pool_next);
-        pool_size--;
+    Coroutine *co = NULL;
+
+    if (CONFIG_COROUTINE_POOL) {
+        qemu_mutex_lock(&pool_lock);
+        co = QSLIST_FIRST(&pool);
+        if (co) {
+            QSLIST_REMOVE_HEAD(&pool, pool_next);
+            pool_size--;
+        }
+        qemu_mutex_unlock(&pool_lock);
     }
-    qemu_mutex_unlock(&pool_lock);
 
     if (!co) {
         co = qemu_coroutine_new();
@@ -51,15 +53,17 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
 
 static void coroutine_delete(Coroutine *co)
 {
-    qemu_mutex_lock(&pool_lock);
-    if (pool_size < POOL_MAX_SIZE) {
-        QSLIST_INSERT_HEAD(&pool, co, pool_next);
-        co->caller = NULL;
-        pool_size++;
+    if (CONFIG_COROUTINE_POOL) {
+        qemu_mutex_lock(&pool_lock);
+        if (pool_size < POOL_MAX_SIZE) {
+            QSLIST_INSERT_HEAD(&pool, co, pool_next);
+            co->caller = NULL;
+            pool_size++;
+            qemu_mutex_unlock(&pool_lock);
+            return;
+        }
         qemu_mutex_unlock(&pool_lock);
-        return;
     }
-    qemu_mutex_unlock(&pool_lock);
 
     qemu_coroutine_delete(co);
 }
-- 
1.8.3.1