linux-kernel - Re: [PATCH] driver core: Add branch prediction hints in really

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Message-ID: <CAPSr9jE8EbWue31LP18eF9+KysZHNRqHNrzif8H4NkCzHHtueQ@mail.gmail.com>
Date:   Tue, 6 Nov 2018 22:43:39 +0800
From:   Muchun Song <smuchun@...il.com>
To:     rafael@...nel.org
Cc:     gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] driver core: Add branch prediction hints in really_probe()

Hi Rafael,

If we want the driver core to test driver remove functions, we can
enable CONFIG_DEBUG_TEST_DRIVER_REMOVE. This option is
just for testing it. So, in most cases, the option is disabled and the if
condition is false. So I think we can add an unlikely() to it.

Rafael J. Wysocki <rafael@...nel.org> 于2018年11月6日周二 下午10:30写道：
>
> On Tue, Nov 6, 2018 at 2:47 PM Muchun Song <smuchun@...il.com> 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@...il.com>
>
> Have you measured the practical impact of this patch in any way?
>
> > ---
> >  drivers/base/dd.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > index 169412ee4ae8..8eba453c4e5d 100644
> > --- a/drivers/base/dd.c
> > +++ b/drivers/base/dd.c
> > @@ -450,7 +450,7 @@ static int really_probe(struct device *dev, struct device_driver *drv)
> >         bool test_remove = IS_ENABLED(CONFIG_DEBUG_TEST_DRIVER_REMOVE) &&
> >                            !drv->suppress_bind_attrs;
> >
> > -       if (defer_all_probes) {
> > +       if (unlikely(defer_all_probes)) {
> >                 /*
> >                  * Value of defer_all_probes can be set only by
> >                  * device_defer_all_probes_enable() which, in turn, will call
> > @@ -508,7 +508,7 @@ static int really_probe(struct device *dev, struct device_driver *drv)
> >                         goto probe_failed;
> >         }
> >
> > -       if (test_remove) {
> > +       if (unlikely(test_remove)) {
> >                 test_remove = false;
> >
> >                 if (dev->bus->remove)
> > --
> > 2.17.1
> >