linux-kernel - Re: [PATCH 2/4] drm/msm: Avoid mutex in shrinker

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Message-ID: <CAD=FV=USXBm-ZLafNWbUK=Ny7_vwtyG164mQFs87SkXqim-Vpw@mail.gmail.com>
Date:   Wed, 31 Mar 2021 15:44:24 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <freedreno@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] drm/msm: Avoid mutex in shrinker_count()

Hi,

On Wed, Mar 31, 2021 at 3:14 PM Rob Clark <robdclark@...il.com> wrote:
>
> @@ -818,11 +820,19 @@ static void update_inactive(struct msm_gem_object *msm_obj)
>         mutex_lock(&priv->mm_lock);
>         WARN_ON(msm_obj->active_count != 0);
>
> +       if (msm_obj->dontneed)
> +               mark_unpurgable(msm_obj);
> +
>         list_del_init(&msm_obj->mm_list);
> -       if (msm_obj->madv == MSM_MADV_WILLNEED)
> +       if (msm_obj->madv == MSM_MADV_WILLNEED) {
>                 list_add_tail(&msm_obj->mm_list, &priv->inactive_willneed);
> -       else
> +       } else if (msm_obj->madv == MSM_MADV_DONTNEED) {
>                 list_add_tail(&msm_obj->mm_list, &priv->inactive_dontneed);
> +               mark_purgable(msm_obj);
> +       } else {
> +               WARN_ON(msm_obj->madv != __MSM_MADV_PURGED);
> +               list_add_tail(&msm_obj->mm_list, &priv->inactive_purged);

I'm probably being dense, but what's the point of adding it to the
"inactive_purged" list here? You never look at that list, right? You
already did a list_del_init() on this object's list pointer
("mm_list"). I don't see how adding it to a bogus list helps with
anything.


> @@ -198,6 +203,33 @@ static inline bool is_vunmapable(struct msm_gem_object *msm_obj)
>         return (msm_obj->vmap_count == 0) && msm_obj->vaddr;
>  }
>
> +static inline void mark_purgable(struct msm_gem_object *msm_obj)
> +{
> +       struct msm_drm_private *priv = msm_obj->base.dev->dev_private;
> +
> +       WARN_ON(!mutex_is_locked(&priv->mm_lock));
> +
> +       if (WARN_ON(msm_obj->dontneed))
> +               return;

The is_purgeable() function also checks other things besides just
"MSM_MADV_DONTNEED". Do we need to check those too? Specifically:

 msm_obj->sgt && !msm_obj->base.dma_buf && !msm_obj->base.import_attach

...or is it just being paranoid?

I guess I'm just worried that if any of those might be important then
we'll consistently report back that we have a count of things that can
be purged but then scan() won't find anything to do. That wouldn't be
great.


> +       priv->shrinkable_count += msm_obj->base.size >> PAGE_SHIFT;
> +       msm_obj->dontneed = true;
> +}
> +
> +static inline void mark_unpurgable(struct msm_gem_object *msm_obj)
> +{
> +       struct msm_drm_private *priv = msm_obj->base.dev->dev_private;
> +
> +       WARN_ON(!mutex_is_locked(&priv->mm_lock));
> +
> +       if (WARN_ON(!msm_obj->dontneed))
> +               return;
> +
> +       priv->shrinkable_count -= msm_obj->base.size >> PAGE_SHIFT;
> +       WARN_ON(priv->shrinkable_count < 0);

If you changed the order maybe you could make shrinkable_count
"unsigned long" to match the shrinker API?

 new_shrinkable = msm_obj->base.size >> PAGE_SHIFT;
 WARN_ON(new_shrinkable > priv->shrinkable_count);
 priv->shrinkable_count -= new_shrinkable


-Doug