Re: [PATCH 8/9] drm/i915: Flush GPU rendering with a lockless wait during a pagefault

Damien Lespiau
2014-02-10T15:52:03Z

On Fri, Feb 07, 2014 at 06:37:06PM -0200, Rodrigo Vivi wrote:
> From: Chris Wilson <chris@chris-wilson.co.uk>
>
> Arjan van de Ven reported that on his test machine that he was seeing
> stalls of greater than 1 frame greatly impacting the user experience. He
> tracked this down to being the locked flush during a pagefault as being
> the culprit hogging the struct_mutex and so blocking any other user from
> proceeding. Stalling on a pagefault is bad behaviour on userspace's
> part, for one it means that they are ignoring the coherency rules on
> pointer access through the GTT, but fortunately we can apply the same
> trick as the set-to-domain ioctl to do a lightweight, nonblocking flush
> of outstanding rendering first.
>
> "Prior to the patch it looks like this
> (this one testrun does not show the 20ms+ I've seen occasionally)
>
> 4.99 ms 2.36 ms 31360 __wait_seqno i915_wait_seqno i915_gem_object_wait_rendering i915_gem_object_set_to_gtt_domain i915_gem_fault __do_fault handle_
> +pte_fault handle_mm_fault __do_page_fault do_page_fault page_fault
> 4.99 ms 2.75 ms 107751 __wait_seqno i915_gem_wait_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 4.99 ms 1.63 ms 1666 i915_mutex_lock_interruptible i915_gem_fault __do_fault handle_pte_fault handle_mm_fault __do_page_fault do_page_fault page_fa
> +ult
> 4.93 ms 2.45 ms 980 i915_mutex_lock_interruptible intel_crtc_page_flip drm_mode_page_flip_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_
> +sysret
> 4.89 ms 2.20 ms 3283 i915_mutex_lock_interruptible i915_gem_wait_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 4.34 ms 1.66 ms 1715 i915_mutex_lock_interruptible i915_gem_pwrite_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 3.73 ms 3.73 ms 49 i915_mutex_lock_interruptible i915_gem_set_domain_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 3.17 ms 0.33 ms 931 i915_mutex_lock_interruptible i915_gem_madvise_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 2.97 ms 0.43 ms 1029 i915_mutex_lock_interruptible i915_gem_busy_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 2.55 ms 0.51 ms 735 i915_gem_get_tiling drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
>
> After the patch it looks like this:
>
> 4.99 ms 2.14 ms 22212 __wait_seqno i915_gem_wait_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 4.86 ms 0.99 ms 14170 __wait_seqno i915_gem_object_wait_rendering__nonblocking i915_gem_fault __do_fault handle_pte_fault handle_mm_fault __do_page_
> +fault do_page_fault page_fault
> 3.59 ms 1.31 ms 325 i915_gem_get_tiling drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 3.37 ms 3.37 ms 65 i915_mutex_lock_interruptible i915_gem_wait_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 2.58 ms 2.58 ms 65 i915_mutex_lock_interruptible i915_gem_do_execbuffer.isra.23 i915_gem_execbuffer2 drm_ioctl i915_compat_ioctl compat_sys_ioctl
> +ia32_sysret
> 2.19 ms 2.19 ms 65 i915_mutex_lock_interruptible intel_crtc_page_flip drm_mode_page_flip_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_
> +sysret
> 2.18 ms 2.18 ms 65 i915_mutex_lock_interruptible i915_gem_busy_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
> 1.66 ms 1.66 ms 65 i915_gem_set_tiling drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
>
> It may not look like it, but this is quite a large difference, and I've
> been unable to reproduce > 5 msec delays at all, while before they do
> happen (just not in the trace above)."
>
> gem_gtt_hog on an old Pineview (GMA3150),
> before: 4969.119ms
> after: 4122.749ms
>
> Reported-by: Arjan van de Ven <arjan.van.de.ven@intel.com>
> Testcase: igt/gem_gtt_hog
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>

--
Damien

> ---
> drivers/gpu/drm/i915/i915_gem.c | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index a8a069f..6008d88 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1184,7 +1184,7 @@ i915_gem_object_wait_rendering(struct drm_i915_gem_object *obj,
> */
> static __must_check int
> i915_gem_object_wait_rendering__nonblocking(struct drm_i915_gem_object *obj,
> - struct drm_file *file,
> + struct drm_i915_file_private *file_priv,
> bool readonly)
> {
> struct drm_device *dev = obj->base.dev;
> @@ -1211,7 +1211,7 @@ i915_gem_object_wait_rendering__nonblocking(struct drm_i915_gem_object *obj,
>
> reset_counter = atomic_read(&dev_priv->gpu_error.reset_counter);
> mutex_unlock(&dev->struct_mutex);
> - ret = __wait_seqno(ring, seqno, reset_counter, true, NULL, file->driver_priv);
> + ret = __wait_seqno(ring, seqno, reset_counter, true, NULL, file_priv);
> mutex_lock(&dev->struct_mutex);
> if (ret)
> return ret;
> @@ -1260,7 +1260,9 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
> * We will repeat the flush holding the lock in the normal manner
> * to catch cases where we are gazumped.
> */
> - ret = i915_gem_object_wait_rendering__nonblocking(obj, file, !write_domain);
> + ret = i915_gem_object_wait_rendering__nonblocking(obj,
> + file->driver_priv,
> + !write_domain);
> if (ret)
> goto unref;
>
> @@ -1392,6 +1394,15 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
>
> trace_i915_gem_object_fault(obj, page_offset, true, write);
>
> + /* Try to flush the object off the GPU first without holding the lock.
> + * Upon reacquiring the lock, we will perform our sanity checks and then
> + * repeat the flush holding the lock in the normal manner to catch cases
> + * where we are gazumped.
> + */
> + ret = i915_gem_object_wait_rendering__nonblocking(obj, NULL, !write);
> + if (ret)
> + goto unlock;
> +
> /* Access to snoopable pages through the GTT is incoherent. */
> if (obj->cache_level != I915_CACHE_NONE && !HAS_LLC(dev)) {
> ret = -EINVAL;
> --
> 1.8.3.1
>
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