linux-kernel - RE: [Intel-gfx] [PATCH v2 01/21] drm/i915/gt: Ignore TLB invalidations on idle engines

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Date:   Mon, 18 Jul 2022 15:50:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mauro Carvalho Chehab' <mauro.chehab@...ux.intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
CC:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Chris Wilson <chris.p.wilson@...el.com>,
        Matthew Auld <matthew.auld@...el.com>,
        Dave Airlie <airlied@...hat.com>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [Intel-gfx] [PATCH v2 01/21] drm/i915/gt: Ignore TLB
 invalidations on idle engines

From: Mauro Carvalho Chehab
> Sent: 18 July 2022 15:54
> 
> On Mon, 18 Jul 2022 14:16:10 +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>
> > >
> > > Check if the device is powered down prior to any engine activity,
> > > as, on such cases, all the TLBs were already invalidated, so an
> > > explicit TLB invalidation is not needed, thus reducing the
> > > performance regression impact due to it.
> > >
> > > This becomes more significant with GuC, as it can only do so when
> > > the connection to the GuC is awake.
> > >
> > > Cc: stable@...r.kernel.org
> > > Fixes: 7938d61591d3 ("drm/i915: Flush TLBs before releasing backing store")
> >
> > Patch itself looks fine but I don't think we closed on the issue of
> > stable/fixes on this patch?
> 
> No, because TLB cache invalidation takes time and causes time outs, which
> in turn affects applications and produce Kernel warnings.

It's not only the TLB flushes that cause grief.

There is a loop that forces a write-back of all the frame buffer pages.
With a large display and some cpu (like my Ivy bridge one) that
takes long enough with pre-emption disabled that wakeup of RT processes
(and any pinned to the cpu) takes far longer than one might have
wished for.

Since some X servers request a flush every few seconds this makes
the system unusable for some workloads.

	David

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