[PATCH] mm,oom: Reduce needless dereference.

[PATCH] mm,oom: Reduce needless dereference.

[Tetsuo Handa]

Since we assigned mm = victim->mm before pr_err(),
we don't need to dereference victim->mm again at pr_err().
This saves a few instructions.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 mm/oom_kill.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index c84e784..1808db32 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -756,10 +756,10 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
 	mark_oom_victim(victim);
 	pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
-		task_pid_nr(victim), victim->comm, K(victim->mm->total_vm),
-		K(get_mm_counter(victim->mm, MM_ANONPAGES)),
-		K(get_mm_counter(victim->mm, MM_FILEPAGES)),
-		K(get_mm_counter(victim->mm, MM_SHMEMPAGES)));
+	       task_pid_nr(victim), victim->comm, K(mm->total_vm),
+	       K(get_mm_counter(mm, MM_ANONPAGES)),
+	       K(get_mm_counter(mm, MM_FILEPAGES)),
+	       K(get_mm_counter(mm, MM_SHMEMPAGES)));
 	task_unlock(victim);
 
 	/*
-- 
1.8.3.1

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Re: [PATCH] mm,oom: Reduce needless dereference.

[Vlastimil Babka]

On 03/08/2016 12:02 PM, Tetsuo Handa wrote:
> Since we assigned mm = victim->mm before pr_err(),
> we don't need to dereference victim->mm again at pr_err().
> This saves a few instructions.

That sounds obvious, right. Yet once in a while I try to test these for
fun, and there can be indeed surprises :)

./scripts/bloat-o-meter says:
add/remove: 0/0 grow/shrink: 1/0 up/down: 1/0 (1)
function                                     old     new   delta
oom_kill_process                            1085    1086      +1

a naive asmdiff is too complicated to follow  but it seems from the
number of lines that indeed there are less instructions in your case,
but still the code is a bit larger.

Just a reminder that compilers can be quite counter-intuitive. Anyway, a
liquid-helium-path like this probably doesn't need such
microoptimisations as an extra patch, given the ongoing churn in the oom
area?

> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  mm/oom_kill.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index c84e784..1808db32 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -756,10 +756,10 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
>  	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
>  	mark_oom_victim(victim);
>  	pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
> -		task_pid_nr(victim), victim->comm, K(victim->mm->total_vm),
> -		K(get_mm_counter(victim->mm, MM_ANONPAGES)),
> -		K(get_mm_counter(victim->mm, MM_FILEPAGES)),
> -		K(get_mm_counter(victim->mm, MM_SHMEMPAGES)));
> +	       task_pid_nr(victim), victim->comm, K(mm->total_vm),
> +	       K(get_mm_counter(mm, MM_ANONPAGES)),
> +	       K(get_mm_counter(mm, MM_FILEPAGES)),
> +	       K(get_mm_counter(mm, MM_SHMEMPAGES)));
>  	task_unlock(victim);
>  
>  	/*
> 

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Re: [PATCH] mm,oom: Reduce needless dereference.

[Johannes Weiner]

On Tue, Mar 08, 2016 at 08:02:31PM +0900, Tetsuo Handa wrote:
> Since we assigned mm = victim->mm before pr_err(),
> we don't need to dereference victim->mm again at pr_err().
> This saves a few instructions.
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

Yes. Once we introduce a local variable for something, we should use
it consistently to refer to that thing. Anything else is confusing.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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Re: [PATCH] mm,oom: Reduce needless dereference.

[Michal Hocko]

On Tue 08-03-16 13:30:32, Johannes Weiner wrote:
> On Tue, Mar 08, 2016 at 08:02:31PM +0900, Tetsuo Handa wrote:
> > Since we assigned mm = victim->mm before pr_err(),
> > we don't need to dereference victim->mm again at pr_err().
> > This saves a few instructions.
> > 
> > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> 
> Yes. Once we introduce a local variable for something, we should use
> it consistently to refer to that thing. Anything else is confusing.

The victim->mm association is stable here because of the task_lock but I
agree that this might be not obvious and reusing the local variable is
easier to read and understand. I doubt we care about the change in the
generated code as an argument though. So just for the sake of clean up

Acked-by: Michal Hocko <mhocko@suse.com>

> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> 
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-- 
Michal Hocko
SUSE Labs

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