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Date:   Fri, 16 Aug 2019 09:12:43 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Daniel Vetter <daniel@ffwll.ch>
Cc:     Michal Hocko <mhocko@kernel.org>, Feng Tang <feng.tang@intel.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        Jann Horn <jannh@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux MM <linux-mm@kvack.org>,
        =?utf-8?B?SsOpcsO0bWU=?= Glisse <jglisse@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Rientjes <rientjes@google.com>,
        Wei Wang <wvw@google.com>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Christian =?utf-8?B?S8O2bmln?= <christian.koenig@amd.com>
Subject: Re: [Intel-gfx] [PATCH 2/5] kernel.h: Add non_block_start/end()
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On Fri, Aug 16, 2019 at 08:20:55AM +0200, Daniel Vetter wrote:
> On Fri, Aug 16, 2019 at 3:00 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > On Thu, Aug 15, 2019 at 10:49:31PM +0200, Daniel Vetter wrote:
> > > On Thu, Aug 15, 2019 at 10:27 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > > On Thu, Aug 15, 2019 at 10:16:43PM +0200, Daniel Vetter wrote:
> > > > > So if someone can explain to me how that works with lockdep I can of
> > > > > course implement it. But afaics that doesn't exist (I tried to explain
> > > > > that somewhere else already), and I'm no really looking forward to
> > > > > hacking also on lockdep for this little series.
> > > >
> > > > Hmm, kind of looks like it is done by calling preempt_disable()
> > >
> > > Yup. That was v1, then came the suggestion that disabling preemption
> > > is maybe not the best thing (the oom reaper could still run for a long
> > > time comparatively, if it's cleaning out gigabytes of process memory
> > > or what not, hence this dedicated debug infrastructure).
> >
> > Oh, I'm coming in late, sorry
> >
> > Anyhow, I was thinking since we agreed this can trigger on some
> > CONFIG_DEBUG flag, something like
> >
> >     /* This is a sleepable region, but use preempt_disable to get debugging
> >      * for calls that are not allowed to block for OOM [.. insert
> >      * Michal's explanation.. ] */
> >     if (IS_ENABLED(CONFIG_DEBUG_ATOMIC_SLEEP) && !mmu_notifier_range_blockable(range))
> >         preempt_disable();
> >     ops->invalidate_range_start();
> 
> I think we also discussed that, and some expressed concerns it would
> change behaviour/timing too much for testing. Since this does does
> disable preemption for real, not just for might_sleep.

I don't follow, this is a debug kernel, it will have widly different
timing. 

Further the point of this debugging on atomic_sleep is to be as
timing-independent as possible since functions with rare sleeps should
be guarded by might_sleep() in their common paths.

I guess I don't get the push to have some low overhead debugging for
this? Is there something special you are looking for?

Jason