linux-kernel - Re: [PATCH] driver core: Define dev_err_probe() as _

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Date:   Sat, 6 Aug 2022 09:12:04 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     tglx@...utronix.de, jgg@...pe.ca, ira.weiny@...el.com,
        dan.j.williams@...el.com, andriy.shevchenko@...ux.intel.com,
        wonchung@...gle.com, list@...l.com, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] driver core: Define dev_err_probe() as __cold

On Sat, Aug 06, 2022 at 08:49:23AM +0200, Christophe JAILLET wrote:
> Give a hint to the compiler that dev_err_probe() is used for error
> handling. So calling paths are unlikely.
> 
> >From gcc documentation:
> 	The paths leading to calls of cold functions within code are marked
> 	as unlikely by the branch prediction mechanism.
> 
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> ---
>  include/linux/device.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 424b55df0272..4ac16bde9bf7 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -1093,7 +1093,7 @@ void device_links_supplier_sync_state_pause(void);
>  void device_links_supplier_sync_state_resume(void);
>  
>  extern __printf(3, 4)
> -int dev_err_probe(const struct device *dev, int err, const char *fmt, ...);
> +int __cold dev_err_probe(const struct device *dev, int err, const char *fmt, ...);

As the probe() path is by default "slow", does this actually help
anything?  I never recommend using any sort of manual likely/unlikely
hints unless the results can be seen, otherwise the compiler and CPU
almost always do a better job over time.

thanks,

greg k-h