linux-kernel - Re: [PATCH] drm/msm/a6xx: Fix excessive stack usage

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Message-ID: <pipbl5fxssydcw3ehqsq3oea56nc37un7ok3oersvrqrlssyo6@5ksxbtcixfbo>
Date: Mon, 28 Oct 2024 12:50:25 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Akhil P Oommen <quic_akhilpo@...cinc.com>, 
	Arnd Bergmann <arnd@...nel.org>, Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>, 
	Konrad Dybcio <konradybcio@...nel.org>, Abhinav Kumar <quic_abhinavk@...cinc.com>, 
	Marijn Suijten <marijn.suijten@...ainline.org>, Dave Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] drm/msm/a6xx: Fix excessive stack usage

On Mon, Oct 28, 2024 at 11:39:16AM +0100, Konrad Dybcio wrote:
> On 28.10.2024 10:52 AM, Akhil P Oommen wrote:
> > On 10/28/2024 12:13 AM, Arnd Bergmann wrote:
> >> On Sun, Oct 27, 2024, at 18:05, Akhil P Oommen wrote:
> >>> Clang-19 and above sometimes end up with multiple copies of the large
> >>> a6xx_hfi_msg_bw_table structure on the stack. The problem is that
> >>> a6xx_hfi_send_bw_table() calls a number of device specific functions to
> >>> fill the structure, but these create another copy of the structure on
> >>> the stack which gets copied to the first.
> >>>
> >>> If the functions get inlined, that busts the warning limit:
> >>>
> >>> drivers/gpu/drm/msm/adreno/a6xx_hfi.c:631:12: error: stack frame size 
> >>> (1032) exceeds limit (1024) in 'a6xx_hfi_send_bw_table' 
> >>> [-Werror,-Wframe-larger-than]
> >>>
> >>> Fix this by kmalloc-ating struct a6xx_hfi_msg_bw_table instead of using
> >>> the stack. Also, use this opportunity to skip re-initializing this table
> >>> to optimize gpu wake up latency.
> >>>
> >>> Cc: Arnd Bergmann <arnd@...nel.org>
> >>
> >> Please change this to "Reported-by:"
> > 
> > Sure.
> > 
> >>
> >> The patch looks correct to me, just one idea for improvement.
> >>
> >>> b/drivers/gpu/drm/msm/adreno/a6xx_gmu.h
> >>> index 94b6c5cab6f4..b4a79f88ccf4 100644
> >>> --- a/drivers/gpu/drm/msm/adreno/a6xx_gmu.h
> >>> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gmu.h
> >>> @@ -99,6 +99,7 @@ struct a6xx_gmu {
> >>>  	struct completion pd_gate;
> >>>
> >>>  	struct qmp *qmp;
> >>> +	struct a6xx_hfi_msg_bw_table *bw_table;
> >>>  };
> >>
> >> I think the bw_table is better just embedded
> >> in here rather than referenced as a pointer:
> >>
> > There are some low tier chipsets with relatively lower RAM size that
> > doesn't require this table. So, dynamically allocating this here helps
> > to save 640 bytes (minus the overhead of tracking).
> 
> I'd second this, said chipsets often ship with 1-2 GiB of RAM (which
> is still a lot in comparison, but you know.. every little bit counts)

Okay from my side. Yeah, poor Gnome runnning on top of 1 GiB device is
very sad.

-- 
With best wishes
Dmitry