linux-kernel - Re: [PATCH] drm: use trace_printk rather than printk in drm

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Date:   Wed, 31 Jul 2019 14:26:07 +0800
From:   Fuqian Huang <huangfq.daxian@...il.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm: use trace_printk rather than printk in drm_dbg.

Joe Perches <joe@...ches.com> 於 2019年7月31日週三 下午2:06寫道：
>
> On Wed, 2019-07-31 at 10:45 +0800, Fuqian Huang wrote:
> > In drivers/gpu/drm/amd/amdgpu/amdgpu_ih.c,
> > amdgpu_ih_process calls DRM_DEBUG which calls drm_dbg and
> > finally calls printk.
> > As amdgpu_ih_process is called from an interrupt handler,
> > and interrupt handler should be short as possible.
> >
> > As printk may lead to bogging down the system or can even
> > create a live lock. printk should not be used in IRQ context.
> > Instead, trace_printk is recommended.
> > Link: https://lwn.net/Articles/365835
> []
> > diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
> []
> > @@ -236,7 +236,7 @@ void drm_dbg(unsigned int category, const char *format, ...)
> >       vaf.fmt = format;
> >       vaf.va = &args;
> >
> > -     printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
> > +     trace_printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
> >              __builtin_return_address(0), &vaf);
> >
> >       va_end(args);
>
> This makes all 4000+ drm_dbg/DRM_DEBUG uses emit
> a trace_printk.
>
> I suggest instead you make only the interrupt uses
> use a different function and not drm_dbg.
>
> Or maybe add an in_interrupt() check like
>
>         if (in_interrupt())
>                 printk(KERN_DEBUG etc...)
>         else
>                 trace_printk(etc...)

I will send a v2 patch to fix this.