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

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Message-ID: <k2eh4ysckrvh3hubkfh7u3x7mow4cnoo3tvxfygriyxha6zx3n@i4hdghgjvasx>
Date: Mon, 28 Oct 2024 14:22:08 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Akhil P Oommen <quic_akhilpo@...cinc.com>, 
	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>, David 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, 
	Arnd Bergmann <arnd@...nel.org>
Subject: Re: [PATCH] drm/msm/a6xx: Fix excessive stack usage

On Mon, Oct 28, 2024 at 12:31:50PM +0100, Konrad Dybcio wrote:
> On 28.10.2024 11:52 AM, Dmitry Baryshkov wrote:
> > On Mon, Oct 28, 2024 at 11:36:15AM +0100, Konrad Dybcio wrote:
> >> On 28.10.2024 11:27 AM, Dmitry Baryshkov wrote:
> >>> On Mon, 28 Oct 2024 at 12:08, Akhil P Oommen <quic_akhilpo@...cinc.com> wrote:
> >>>>
> >>>> On 10/28/2024 1:56 PM, Dmitry Baryshkov wrote:
> >>>>> On Sun, Oct 27, 2024 at 11:35:47PM +0530, 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>
> >>>>>>
> >>>>>> Signed-off-by: Akhil P Oommen <quic_akhilpo@...cinc.com>
> >>>>>> ---
> >>>>>>  drivers/gpu/drm/msm/adreno/a6xx_gmu.h |  1 +
> >>>>>>  drivers/gpu/drm/msm/adreno/a6xx_hfi.c | 34 ++++++++++++++++++++++------------
> >>>>>>  2 files changed, 23 insertions(+), 12 deletions(-)
> >>>>>>
> >>>>>> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gmu.h 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;
> >>>>>>  };
> >>>>>>
> >>>>>>  static inline u32 gmu_read(struct a6xx_gmu *gmu, u32 offset)
> >>>>>> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_hfi.c b/drivers/gpu/drm/msm/adreno/a6xx_hfi.c
> >>>>>> index cdb3f6e74d3e..55e51c81be1f 100644
> >>>>>> --- a/drivers/gpu/drm/msm/adreno/a6xx_hfi.c
> >>>>>> +++ b/drivers/gpu/drm/msm/adreno/a6xx_hfi.c
> >>>>>> @@ -630,32 +630,42 @@ static void a6xx_build_bw_table(struct a6xx_hfi_msg_bw_table *msg)
> >>>>>>
> >>>>>>  static int a6xx_hfi_send_bw_table(struct a6xx_gmu *gmu)
> >>>>>>  {
> >>>>>> -    struct a6xx_hfi_msg_bw_table msg = { 0 };
> >>>>>> +    struct a6xx_hfi_msg_bw_table *msg;
> >>>>>>      struct a6xx_gpu *a6xx_gpu = container_of(gmu, struct a6xx_gpu, gmu);
> >>>>>>      struct adreno_gpu *adreno_gpu = &a6xx_gpu->base;
> >>>>>>
> >>>>>> +    if (gmu->bw_table)
> >>>>>> +            goto send;
> >>>>>> +
> >>>>>> +    msg = devm_kzalloc(gmu->dev, sizeof(*msg), GFP_KERNEL);
> >>>>>
> >>>>> Is it necessary after being sent? Isn't it better to just kzalloc() it
> >>>>> and then kfree() it at the end of the function?
> >>>>
> >>>> Keeping it around will help to cut down unnecessary work during
> >>>> subsequent gpu wake ups.
> >>>
> >>> Then, I'd say, it is better to make it a part of the a6xx_gpu struct.
> >>
> >> I think a6xx_gmu makes more logical sense here.
> >>
> >> FWIW, the driver allocates both _gmu and _gpu for all GPUs regardless
> > 
> > Hmm, are we expected to handle / perform BW requests in case of GMU-less
> > devices?
> 
> opp-table does that for us
> 
> In case of no gmu ("gmu wrapper"), Linux is the only entity that controls
> things

Ack

-- 
With best wishes
Dmitry