[POWERPC] spu sched: forced preemption at execution

If we start a spu context with realtime priority we want it to run
immediately and not wait until some other lower priority thread has
finished.  Try to find a suitable victim and use it's spu in this
case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>

3 files changed, 76 insertions, 0 deletions

diff --git a/arch/powerpc/platforms/cell/spufs/context.c b/arch/powerpc/platforms/cell/spufs/context.c
index 056a8ad02385..d581f4ec99ba 100644
--- a/arch/powerpc/platforms/cell/spufs/context.c
+++ b/arch/powerpc/platforms/cell/spufs/context.c
@@ -53,6 +53,7 @@ struct spu_context *alloc_spu_context(struct spu_gang *gang)
         ctx->owner = get_task_mm(current);
         if (gang)
                 spu_gang_add_ctx(gang, ctx);
+        ctx->rt_priority = current->rt_priority;
         ctx->prio = current->prio;
         goto out;
 out_free:
diff --git a/arch/powerpc/platforms/cell/spufs/sched.c b/arch/powerpc/platforms/cell/spufs/sched.c
index eb06a030ca09..814f65e025f1 100644
--- a/arch/powerpc/platforms/cell/spufs/sched.c
+++ b/arch/powerpc/platforms/cell/spufs/sched.c
@@ -282,6 +282,74 @@ static struct spu *spu_get_idle(struct spu_context *ctx)
 }
 
 /**
+ * find_victim - find a lower priority context to preempt
+ * @ctx:        canidate context for running
+ *
+ * Returns the freed physical spu to run the new context on.
+ */
+static struct spu *find_victim(struct spu_context *ctx)
+{
+        struct spu_context *victim = NULL;
+        struct spu *spu;
+        int node, n;
+
+        /*
+         * Look for a possible preemption candidate on the local node first.
+         * If there is no candidate look at the other nodes.  This isn't
+         * exactly fair, but so far the whole spu schedule tries to keep
+         * a strong node affinity.  We might want to fine-tune this in
+         * the future.
+         */
+ restart:
+        node = cpu_to_node(raw_smp_processor_id());
+        for (n = 0; n < MAX_NUMNODES; n++, node++) {
+                node = (node < MAX_NUMNODES) ? node : 0;
+                if (!node_allowed(node))
+                        continue;
+
+                mutex_lock(&spu_prio->active_mutex[node]);
+                list_for_each_entry(spu, &spu_prio->active_list[node], list) {
+                        struct spu_context *tmp = spu->ctx;
+
+                        if (tmp->rt_priority < ctx->rt_priority &&
+                            (!victim || tmp->rt_priority < victim->rt_priority))
+                                victim = spu->ctx;
+                }
+                mutex_unlock(&spu_prio->active_mutex[node]);
+
+                if (victim) {
+                        /*
+                         * This nests ctx->state_mutex, but we always lock
+                         * higher priority contexts before lower priority
+                         * ones, so this is safe until we introduce
+                         * priority inheritance schemes.
+                         */
+                        if (!mutex_trylock(&victim->state_mutex)) {
+                                victim = NULL;
+                                goto restart;
+                        }
+
+                        spu = victim->spu;
+                        if (!spu) {
+                                /*
+                                 * This race can happen because we've dropped
+                                 * the active list mutex.  No a problem, just
+                                 * restart the search.
+                                 */
+                                mutex_unlock(&victim->state_mutex);
+                                victim = NULL;
+                                goto restart;
+                        }
+                        spu_unbind_context(spu, victim);
+                        mutex_unlock(&victim->state_mutex);
+                        return spu;
+                }
+        }
+
+        return NULL;
+}
+
+/**
  * spu_activate - find a free spu for a context and execute it
  * @ctx:        spu context to schedule
  * @flags:      flags (currently ignored)
@@ -300,6 +368,12 @@ int spu_activate(struct spu_context *ctx, unsigned long flags)
                 struct spu *spu;
 
                 spu = spu_get_idle(ctx);
+                /*
+                 * If this is a realtime thread we try to get it running by
+                 * preempting a lower priority thread.
+                 */
+                if (!spu && ctx->rt_priority)
+                        spu = find_victim(ctx);
                 if (spu) {
                         spu_bind_context(spu, ctx);
                         return 0;
diff --git a/arch/powerpc/platforms/cell/spufs/spufs.h b/arch/powerpc/platforms/cell/spufs/spufs.h
index 421f59167c55..85b182d16464 100644
--- a/arch/powerpc/platforms/cell/spufs/spufs.h
+++ b/arch/powerpc/platforms/cell/spufs/spufs.h
@@ -83,6 +83,7 @@ struct spu_context {
         /* scheduler fields */
         struct list_head rq;
         unsigned long sched_flags;
+        unsigned long rt_priority;
         int prio;
 };