Re: Backport of commit a75bb4eb9e565b9f5115e2e8c07377ce32cbe69a

From: Nathan Chancellor <natechancellor@gmail.com>
To: Matthias Kaehlcke <mka@chromium.org>
Cc: Sasha Levin <sashal@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	stable@vger.kernel.org,
	Nick Desaulniers <ndesaulniers@google.com>,
	clang-built-linux@googlegroups.com
Subject: Re: Backport of commit a75bb4eb9e565b9f5115e2e8c07377ce32cbe69a
Date: Tue, 23 Apr 2019 13:03:55 -0700	[thread overview]
Message-ID: <20190423200355.GA17824@archlinux-i9> (raw)
In-Reply-To: <20190423195628.GD112750@google.com>

On Tue, Apr 23, 2019 at 12:56:28PM -0700, Matthias Kaehlcke wrote:
> On Tue, Apr 23, 2019 at 12:45:12PM -0700, Nathan Chancellor wrote:
> > On Tue, Apr 23, 2019 at 03:35:12PM -0400, Sasha Levin wrote:
> > > On Tue, Apr 23, 2019 at 12:04:21PM -0700, Nathan Chancellor wrote:
> > > > Hi Greg and Sasha,
> > > > 
> > > > Please apply this commit to 4.4 through 5.0 (patches are threaded in
> > > > reply to this one), which will prevent Clang from emitting references
> > > > to compiler runtime functions and doing some performance-killing
> > > > optimization when using CONFIG_CC_OPTIMIZE_FOR_SIZE.
> > > > 
> > > > Please let me know if I did something wrong or if there are any
> > > > objections.
> > > 
> > > This looks like a fix for a performance regression, which don't usually
> > > end up in stable unless they are severe enough.
> > > 
> > 
> > Yes and no. The original reason for the revert was because of the
> > compiler runtime functions:
> > 
> > https://lore.kernel.org/lkml/CAG5bF+S6OvBnsaR6UpMCqjDR9_hMo6qRMHGiW+iCaRYQW4C3YA@mail.gmail.com/
> > 
> > The Chromium guys hit this [1] and have applied the patch for their
> > kernels, I think it should be applied for everyone.
> 
> We picked it into our kernels because we needed a fix *now* to unbreak
> our builds, but I agree it should be fixed at least in relevant LTS
> versions.
> 

Of course, I wasn't implying that was a bad thing :) it's further
justification for this being a stable worthy patch as it fixes a real
world issue and could fix more.

> > > This patch looks simple enough, and seeing peterz on it suggests to me
> > > that this was significant enough for someone to notice. Is this really
> > > the case, or is this just one of those %0.01 performance regression
> > > things?
> > > 
> > 
> > I don't know of anyone else who has hit this but Qualcomm enabled
> > CONFIG_CC_OPTIMIZE_FOR_SIZE in their 4.9 kernels with clang [2] so it
> > would be nice to have this to avoid any further issues down the road.
> > 
> > [1]: https://chromium.googlesource.com/chromiumos/third_party/kernel/+/8bb957d781e2350515c7cffd7e7dc0dfc1777f3b
> > [2]: https://source.codeaurora.org/quic/la/kernel/msm-4.9/commit/?id=e828e50380a34533c37eff98345847fd74667ddd
> 
> I suspect the Qualcomm tree would hit the VDSO issue if they use a
> sufficiently recent version of clang.
> 
> Thanks Nathan for posting this to -stable, I should have cc-ed them in
> the first place.

Sure thing, thanks for the reply and input!

Nathan