linux-kernel - Re: [PATCH v2 04/21] drm/i915/gt: Only invalidate TLBs exposed to user manipulation

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Date:   Mon, 18 Jul 2022 14:39:17 +0100
From:   Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
To:     Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     Chris Wilson <chris.p.wilson@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Airlie <airlied@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Fei Yang <fei.yang@...el.com>,
        Andi Shyti <andi.shyti@...ux.intel.com>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>
Subject: Re: [PATCH v2 04/21] drm/i915/gt: Only invalidate TLBs exposed to
 user manipulation


On 14/07/2022 13:06, Mauro Carvalho Chehab wrote:
> From: Chris Wilson <chris.p.wilson@...el.com>
> 
> Don't flush TLBs when the buffer is only used in the GGTT under full
> control of the kernel, as there's no risk of concurrent access
> and stale access from prefetch.
> 
> We only need to invalidate the TLB if they are accessible by the user.
> That helps to reduce the performance regression introduced by TLB
> invalidate logic.
> 
> Cc: stable@...r.kernel.org
> Fixes: 7938d61591d3 ("drm/i915: Flush TLBs before releasing backing store")

Do we really need or want stable and fixes on this one?

What do we think the performance improvement is, given there's very 
little in GGTT, which is not mapped via PPGTT as well?

I think it is safe, but part of me would ideally not even want to think 
about whether it is safe, if the performance improvement is 
non-existent. Which I can't imagine how there would be?

Regards,

Tvrtko

> Signed-off-by: Chris Wilson <chris.p.wilson@...el.com>
> Cc: Fei Yang <fei.yang@...el.com>
> Cc: Andi Shyti <andi.shyti@...ux.intel.com>
> Acked-by: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
> Signed-off-by: Mauro Carvalho Chehab <mchehab@...nel.org>
> ---
> 
> To avoid mailbombing on a large number of people, only mailing lists were C/C on the cover.
> See [PATCH v2 00/21] at: https://lore.kernel.org/all/cover.1657800199.git.mchehab@kernel.org/
> 
>   drivers/gpu/drm/i915/i915_vma.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> index ef3b04c7e153..646f419b2035 100644
> --- a/drivers/gpu/drm/i915/i915_vma.c
> +++ b/drivers/gpu/drm/i915/i915_vma.c
> @@ -538,7 +538,8 @@ int i915_vma_bind(struct i915_vma *vma,
>   				   bind_flags);
>   	}
>   
> -	set_bit(I915_BO_WAS_BOUND_BIT, &vma->obj->flags);
> +	if (bind_flags & I915_VMA_LOCAL_BIND)
> +		set_bit(I915_BO_WAS_BOUND_BIT, &vma->obj->flags);
>   
>   	atomic_or(bind_flags, &vma->flags);
>   	return 0;