Re: [BACKPORT v4.12-rc1] drm/i915: Do not sync RCU during shrinking

[Jani Nikula <jani.nikula@intel.com>]

On Thu, 18 May 2017, Joonas Lahtinen <joonas.lahtinen@linux.intel.com> wrote:
> Due to the complex dependencies between workqueues and RCU, which
> are not easily detected by lockdep, do not synchronize RCU during
> shrinking.
>
> On low-on-memory systems (mem=1G for example), the RCU sync leads
> to all system workqueus freezing and unrelated lockdep splats are
> displayed according to reports. GIT bisecting done by J. R.
> Okajima points to the commit where RCU syncing was extended.
>
> RCU sync gains us very little benefit in real life scenarios
> where the amount of memory used by object backing storage is
> dominant over the metadata under RCU, so drop it altogether.
>
>  " Yeeeaah, if core could just, go ahead and reclaim RCU
>    queues, that'd be great. "
>
>   - Chris Wilson, 2016 (0eafec6d3244)
>
> v2: More information to commit message.
> v3: Remove "grep _rcu_" escapee from i915_gem_shrink_all (Andrea)
>
> Fixes: c053b5a506d3 ("drm/i915: Don't call synchronize_rcu_expedited under struct_mutex")
> Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
> Reported-by: J. R. Okajima <hooanon05g@gmail.com>
> Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
> Tested-by: Hugh Dickins <hughd@google.com>
> Tested-by: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: J. R. Okajima <hooanon05g@gmail.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: <stable@vger.kernel.org> # v4.11+

Pushed to drm-intel-fixes, thanks for the patch.

BR,
Jani.

> ---
>  drivers/gpu/drm/i915/i915_gem_shrinker.c | 5 -----
>  1 file changed, 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_shrinker.c b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> index 129ed30..57d9f7f 100644
> --- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
> +++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> @@ -59,9 +59,6 @@ static void i915_gem_shrinker_unlock(struct drm_device *dev, bool unlock)
>  		return;
>  
>  	mutex_unlock(&dev->struct_mutex);
> -
> -	/* expedite the RCU grace period to free some request slabs */
> -	synchronize_rcu_expedited();
>  }
>  
>  static bool any_vma_pinned(struct drm_i915_gem_object *obj)
> @@ -274,8 +271,6 @@ unsigned long i915_gem_shrink_all(struct drm_i915_private *dev_priv)
>  				I915_SHRINK_ACTIVE);
>  	intel_runtime_pm_put(dev_priv);
>  
> -	synchronize_rcu(); /* wait for our earlier RCU delayed slab frees */
> -
>  	return freed;
>  }

-- 
Jani Nikula, Intel Open Source Technology Center