linux-kernel - Re: [Intel-gfx] [PATCH v2 05/21] drm/i915/gt: Skip TLB invalidations once wedged

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Date:   Mon, 18 Jul 2022 18:06:30 +0200
From:   Mauro Carvalho Chehab <mauro.chehab@...ux.intel.com>
To:     Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        linux-kernel@...r.kernel.org,
        Chris Wilson <chris.p.wilson@...el.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Dave Airlie <airlied@...hat.com>, stable@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH v2 05/21] drm/i915/gt: Skip TLB
 invalidations once wedged

On Mon, 18 Jul 2022 14:45:22 +0100
Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com> wrote:

> On 14/07/2022 13:06, Mauro Carvalho Chehab wrote:
> > From: Chris Wilson <chris.p.wilson@...el.com>
> > 
> > Skip all further TLB invalidations once the device is wedged and
> > had been reset, as, on such cases, it can no longer process instructions
> > on the GPU and the user no longer has access to the TLB's in each engine.
> > 
> > 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")  
> 
> Is the claim of a performance regression this solved based on a wedged 
> GPU which does not work any more to the extend where mmio tlb 
> invalidation requests keep timing out? If so please clarify in the 
> commit text and then it looks good to me. Even if it is IMO a very 
> borderline situation to declare something a fix.

Indeed this helps on a borderline situation: if GT is wedged, TLB 
invalidation will timeout, so it makes sense to keep the patch with a
comment like:

    drm/i915/gt: Skip TLB invalidations once wedged
    
    Skip all further TLB invalidations once the device is wedged and
    had been reset, as, on such cases, it can no longer process instructions
    on the GPU and the user no longer has access to the TLB's in each engine.
    
    So, an attempt to do a TLB cache invalidation will produce a timeout.
    
    That helps to reduce the performance regression introduced by TLB
    invalidate logic.

Regards,
Mauro