[PATCH] sched/fair: Clean up comment in nohz_idle_balance()

[PATCH] sched/fair: Clean up comment in nohz_idle_balance()

[Andrea Parri]

Concerning the comment associated to the atomic_fetch_andnot() in
nohz_idle_balance(), Vincent explains [1]:

  "[...] the comment is useless and can be removed [...]  it was
   referring to a line code above the comment that was present in
   a previous iteration of the patchset. This line disappeared in
   final version but the comment has stayed."

So remove the comment.

Vincent also points out that the full ordering associated to the
atomic_fetch_andnot() primitive could be relaxed, but this patch
insists on the current more conservative/fully ordered solution:

"Performance" isn't a concern, stay away from "correctness"/subtle
relaxed (re)ordering if possible..., just make sure not to confuse
the next reader with misleading/out-of-date comments.

[1] http://lkml.kernel.org/r/CAKfTPtBjA-oCBRkO6__npQwL3+HLjzk7riCcPU1R7YdO-EpuZg@mail.gmail.com

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ac855b2f47746..db514993565b2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9533,9 +9533,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 		return false;
 	}
 
-	/*
-	 * barrier, pairs with nohz_balance_enter_idle(), ensures ...
-	 */
+	/* could be _relaxed() */
 	flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
 	if (!(flags & NOHZ_KICK_MASK))
 		return false;
-- 
2.17.1

Re: [PATCH] sched/fair: Clean up comment in nohz_idle_balance()

[Vincent Guittot]

On Tue, 27 Nov 2018 at 12:01, Andrea Parri
<andrea.parri@amarulasolutions.com> wrote:
>
> Concerning the comment associated to the atomic_fetch_andnot() in
> nohz_idle_balance(), Vincent explains [1]:
>
>   "[...] the comment is useless and can be removed [...]  it was
>    referring to a line code above the comment that was present in
>    a previous iteration of the patchset. This line disappeared in
>    final version but the comment has stayed."
>
> So remove the comment.
>
> Vincent also points out that the full ordering associated to the
> atomic_fetch_andnot() primitive could be relaxed, but this patch
> insists on the current more conservative/fully ordered solution:
>
> "Performance" isn't a concern, stay away from "correctness"/subtle
> relaxed (re)ordering if possible..., just make sure not to confuse
> the next reader with misleading/out-of-date comments.
>
> [1] http://lkml.kernel.org/r/CAKfTPtBjA-oCBRkO6__npQwL3+HLjzk7riCcPU1R7YdO-EpuZg@mail.gmail.com
>
> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>

Looks good to me

> ---
>  kernel/sched/fair.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ac855b2f47746..db514993565b2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9533,9 +9533,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>                 return false;
>         }
>
> -       /*
> -        * barrier, pairs with nohz_balance_enter_idle(), ensures ...
> -        */
> +       /* could be _relaxed() */
>         flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
>         if (!(flags & NOHZ_KICK_MASK))
>                 return false;
> --
> 2.17.1
>

[tip:locking/core] sched/fair: Clean up comment in nohz_idle_balance()

[tip-bot for Andrea Parri]

Commit-ID:  80eb865768703c0f85a0603762742ae1dedf21f0
Gitweb:     https://git.kernel.org/tip/80eb865768703c0f85a0603762742ae1dedf21f0
Author:     Andrea Parri <andrea.parri@amarulasolutions.com>
AuthorDate: Tue, 27 Nov 2018 12:01:10 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 11 Dec 2018 14:54:57 +0100

sched/fair: Clean up comment in nohz_idle_balance()

Concerning the comment associated to the atomic_fetch_andnot() in
nohz_idle_balance(), Vincent explains [1]:

  "[...] the comment is useless and can be removed [...]  it was
   referring to a line code above the comment that was present in
   a previous iteration of the patchset. This line disappeared in
   final version but the comment has stayed."

So remove the comment.

Vincent also points out that the full ordering associated to the
atomic_fetch_andnot() primitive could be relaxed, but this patch
insists on the current more conservative/fully ordered solution:

"Performance" isn't a concern, stay away from "correctness"/subtle
relaxed (re)ordering if possible..., just make sure not to confuse
the next reader with misleading/out-of-date comments.

[1] http://lkml.kernel.org/r/CAKfTPtBjA-oCBRkO6__npQwL3+HLjzk7riCcPU1R7YdO-EpuZg@mail.gmail.com

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20181127110110.5533-1-andrea.parri@amarulasolutions.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ac855b2f4774..db514993565b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9533,9 +9533,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 		return false;
 	}
 
-	/*
-	 * barrier, pairs with nohz_balance_enter_idle(), ensures ...
-	 */
+	/* could be _relaxed() */
 	flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
 	if (!(flags & NOHZ_KICK_MASK))
 		return false;