Re: [Intel-gfx] [PATCH 2/5] kernel.h: Add non_block_start/end()

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>,
	"Jérôme 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 König" <christian.koenig@amd.com>
Subject: Re: [Intel-gfx] [PATCH 2/5] kernel.h: Add non_block_start/end()
Date: Fri, 16 Aug 2019 09:12:43 -0300	[thread overview]
Message-ID: <20190816121243.GB5398@ziepe.ca> (raw)
In-Reply-To: <CAKMK7uH0oa10LoCiEbj1NqAfWitbdOa-jQm9hM=iNL-=8gH9nw@mail.gmail.com>

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