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From: Greg KH <gregkh@linuxfoundation.org>
To: Muchun Song <smuchun@gmail.com>
Cc: rafael@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] driver core: Add branch prediction hints in really_probe()
Date: Tue, 6 Nov 2018 21:26:58 +0100	[thread overview]
Message-ID: <20181106202658.GA26208@kroah.com> (raw)
In-Reply-To: <20181106134630.29591-1-smuchun@gmail.com>

On Tue, Nov 06, 2018 at 09:46:30PM +0800, Muchun Song wrote:
> If condition is false in most cases. So, add an unlikely() to the if
> condition, so that the optimizer assumes that the condition is false.
> 
> Signed-off-by: Muchun Song <smuchun@gmail.com>

As Rafael said, don't do stuff like this unless you can actually measure
the difference.  Last time we tested the kernel for this a few years
ago, about 75% of the unlikely/likely additions were incorrect, removing
them made the kernel faster.  Turns out the compiler and cpu know better
than developers do :)

Also, the probe() path is never a hot-path to worry about stuff like
this.

thanks,

greg k-h

      parent reply	other threads:[~2018-11-06 20:27 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-06 13:46 [PATCH] driver core: Add branch prediction hints in really_probe() Muchun Song
2018-11-06 14:30 ` Rafael J. Wysocki
2018-11-06 14:43   ` Muchun Song
2018-11-06 14:59     ` Rafael J. Wysocki
2018-11-06 15:12       ` Muchun Song
2018-11-06 20:26 ` Greg KH [this message]

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