linux-kernel - Re: Q. drm/i915 shrinker, synchronize_rcu

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Date:   Wed, 10 May 2017 12:54:26 +0300
From:   Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     "J. R. Okajima" <hooanon05g@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        chris@...is-wilson.co.uk, daniel.vetter@...ll.ch,
        jani.nikula@...el.com, linux-kernel@...r.kernel.org
Subject: Re: Q. drm/i915 shrinker, synchronize_rcu_expedited() from handlers

On ti, 2017-05-09 at 20:04 -0700, Hugh Dickins wrote:
> On Mon, 8 May 2017, Joonas Lahtinen wrote:
> > On pe, 2017-05-05 at 14:57 -0700, Hugh Dickins wrote:
> > > On Fri, 5 May 2017, Joonas Lahtinen wrote:
> > > > On ma, 2017-05-01 at 11:05 +0900, J. R. Okajima wrote:
> > > > > Thanx for the reply.
> > > > > 
> > > > > Andrea Arcangeli:
> > > > > > 
> > > > > > Yes I already reported this, my original fix was way more efficient
> > > > > > (and also safer considering the above) than what landed upstream. My
> > > > > > feedback was ignored though.
> > > > > > 
> > > > > > https://lists.freedesktop.org/archives/intel-gfx/2017-April/125414.html
> > > > > 
> > > > > I see.
> > > > > Actually on my test system for v4.11-rc8, kthreadd, kworker, kswapd and
> > > > > others all stopped working due to the synchronize_rcu_expedited call
> > > > > from i915_gem_shrinker_count. It is definitly a show stopper for me as
> > > > > an i915 user.
> > > > 
> > > > Filing a bug in freedesktop.org with all the details is the fastest way
> > > > of getting help. Without the bug (and with such little information as
> > > > the previous e-mail) it's hard to estimate the extent and nature of the
> > > > bug.
> > > > 
> > > > I've anyway gone and prepared a patch to drop the RCU sync completely
> > > > from shrinker phase, as discussed originally with Chris.
> > > 
> > > Is that a patch that will be suitable for 4.11-stable?  Please do post
> > > it here.  I had not experienced this i915-induced hang at all when
> > > Andrea first mentioned it, nor even on 4.11-rc8; but now with 4.11
> > > final I can get it fairly easily (I haven't tried Andrea's fix yet).
> > 
> > Please try:
> > 
> > https://patchwork.freedesktop.org/patch/154713/
> > 
> > If it works, a Tested-by: would be appreciated.
> 
> Yes, that works for me, thank you.
> 
> Tested-by: Hugh Dickins <hughd@...gle.com>
> 
> But the linked patch seems to be lacking a Reported-by (not me) tag,
> a Fixes tag, a Cc stable tag, and any indication in the Subject or
> commit message that this patch is something needed to fix hangs
> observed by several people - it just sounds like a minor cleanup.

It is a patch that was agreed to be pushed anyway, so if it wouldn't
have resolved the problem, I'd have pushed it as is.

I'll add J. R. Okajima as Reported-by and refer to the bisected commit,
even though so far Freedesktop Bugzilla or intel-gfx mailing list has
no other reports.

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation