drm: Use a little stash on the stack to avoid kmalloc in most DRM ioctls.

The kmalloc was taking up about 1.5% of the CPU on an ioctl-heavy workload
(x11perf -aa10text on 965).  Initial results look like they have a
corresponding improvement in performance for aa10text, but more numbers might
not hurt.

Thanks to ajax for pointing out this performance regression I'd introduced
back in 2007.

[airlied: well I introduced it sneakily inside Eric's patch]

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index c26ee08..c4ada8b 100644 (file)
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -421,6 +421,7 @@ int drm_ioctl(struct inode *inode, struct file *filp,
        drm_ioctl_t *func;
        unsigned int nr = DRM_IOCTL_NR(cmd);
        int retcode = -EINVAL;
+       char stack_kdata[128];
        char *kdata = NULL;
 
        atomic_inc(&dev->ioctl_count);
@@ -459,10 +460,14 @@ int drm_ioctl(struct inode *inode, struct file *filp,
                retcode = -EACCES;
        } else {
                if (cmd & (IOC_IN | IOC_OUT)) {
-                       kdata = kmalloc(_IOC_SIZE(cmd), GFP_KERNEL);
-                       if (!kdata) {
-                               retcode = -ENOMEM;
-                               goto err_i1;
+                       if (_IOC_SIZE(cmd) <= sizeof(stack_kdata)) {
+                               kdata = stack_kdata;
+                       } else {
+                               kdata = kmalloc(_IOC_SIZE(cmd), GFP_KERNEL);
+                               if (!kdata) {
+                                       retcode = -ENOMEM;
+                                       goto err_i1;
+                               }
                        }
                }
 
@@ -483,7 +488,7 @@ int drm_ioctl(struct inode *inode, struct file *filp,
        }
 
       err_i1:
-       if (kdata)
+       if (kdata != stack_kdata)
                kfree(kdata);
        atomic_dec(&dev->ioctl_count);
        if (retcode)