[PATCH] sched/fair: remove the spin_lock operations

[PATCH] sched/fair: remove the spin_lock operations

[Hui Su]

Since 'ab93a4bc955b ("sched/fair: Remove
distribute_running fromCFS bandwidth")',there is
nothing to protect between raw_spin_lock_irqsave/store()
in do_sched_cfs_slack_timer().

So remove it.

Signed-off-by: Hui Su <sh_def@163.com>
---
 kernel/sched/fair.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 290f9e38378c..5ecbf5e63198 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5105,9 +5105,6 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
 		return;
 
 	distribute_cfs_runtime(cfs_b);
-
-	raw_spin_lock_irqsave(&cfs_b->lock, flags);
-	raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
 }
 
 /*
-- 
2.29.0

Re: [PATCH] sched/fair: remove the spin_lock operations

[Phil Auld]

On Fri, Oct 30, 2020 at 10:46:21PM +0800 Hui Su wrote:
> Since 'ab93a4bc955b ("sched/fair: Remove
> distribute_running fromCFS bandwidth")',there is
> nothing to protect between raw_spin_lock_irqsave/store()
> in do_sched_cfs_slack_timer().
> 
> So remove it.
> 
> Signed-off-by: Hui Su <sh_def@163.com>
> ---
>  kernel/sched/fair.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 290f9e38378c..5ecbf5e63198 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5105,9 +5105,6 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>  		return;
>  
>  	distribute_cfs_runtime(cfs_b);
> -
> -	raw_spin_lock_irqsave(&cfs_b->lock, flags);
> -	raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  }
>  
>  /*
> -- 
> 2.29.0
> 
> 

Nice :)

Reviewed-by: Phil Auld <pauld@redhat.com>
-- 

Re: [PATCH] sched/fair: remove the spin_lock operations

[Benjamin Segall]

Hui Su <sh_def@163.com> writes:

> Since 'ab93a4bc955b ("sched/fair: Remove
> distribute_running fromCFS bandwidth")',there is
> nothing to protect between raw_spin_lock_irqsave/store()
> in do_sched_cfs_slack_timer().
>
> So remove it.

Reviewed-by: Ben Segall <bsegall@google.com>

(I might nitpick the subject to be clear that it should be trivial
because the lock area is empty, or call them dead or something, but it's
not all that important)

>
> Signed-off-by: Hui Su <sh_def@163.com>
> ---
>  kernel/sched/fair.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 290f9e38378c..5ecbf5e63198 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5105,9 +5105,6 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>  		return;
>  
>  	distribute_cfs_runtime(cfs_b);
> -
> -	raw_spin_lock_irqsave(&cfs_b->lock, flags);
> -	raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  }
>  
>  /*

RE: [PATCH] sched/fair: remove the spin_lock operations

[David Laight]

From: Benjamin Segall
> Sent: 30 October 2020 18:48
> 
> Hui Su <sh_def@163.com> writes:
> 
> > Since 'ab93a4bc955b ("sched/fair: Remove
> > distribute_running fromCFS bandwidth")',there is
> > nothing to protect between raw_spin_lock_irqsave/store()
> > in do_sched_cfs_slack_timer().
> >
> > So remove it.
> 
> Reviewed-by: Ben Segall <bsegall@google.com>
> 
> (I might nitpick the subject to be clear that it should be trivial
> because the lock area is empty, or call them dead or something, but it's
> not all that important)

I don't know about this case, but a lock+unlock can be used
to ensure that nothing else holds the lock when acquiring
the lock requires another lock be held.

So if the normal sequence is:
	lock(table)
	# lookup item
	lock(item)
	unlock(table)
	....
	unlock(item)

Then it can make sense to do:
	lock(table)
	lock(item)
	unlock(item)
	....
	unlock(table)

although that ought to deserve a comment.

	avid

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH] sched/fair: remove the spin_lock operations

[Phil Auld]

On Fri, Oct 30, 2020 at 10:16:29PM +0000 David Laight wrote:
> From: Benjamin Segall
> > Sent: 30 October 2020 18:48
> > 
> > Hui Su <sh_def@163.com> writes:
> > 
> > > Since 'ab93a4bc955b ("sched/fair: Remove
> > > distribute_running fromCFS bandwidth")',there is
> > > nothing to protect between raw_spin_lock_irqsave/store()
> > > in do_sched_cfs_slack_timer().
> > >
> > > So remove it.
> > 
> > Reviewed-by: Ben Segall <bsegall@google.com>
> > 
> > (I might nitpick the subject to be clear that it should be trivial
> > because the lock area is empty, or call them dead or something, but it's
> > not all that important)
> 
> I don't know about this case, but a lock+unlock can be used
> to ensure that nothing else holds the lock when acquiring
> the lock requires another lock be held.
> 
> So if the normal sequence is:
> 	lock(table)
> 	# lookup item
> 	lock(item)
> 	unlock(table)
> 	....
> 	unlock(item)
> 
> Then it can make sense to do:
> 	lock(table)
> 	lock(item)
> 	unlock(item)
> 	....
> 	unlock(table)
> 
> although that ought to deserve a comment.
>

Nah, this one used to be like this :

        raw_spin_lock_irqsave(&cfs_b->lock, flags);
        lsub_positive(&cfs_b->runtime, runtime);
        cfs_b->distribute_running = 0;
        raw_spin_unlock_irqrestore(&cfs_b->lock, flags);

It's just a leftover. I agree that if it was there for some other
purpose that it would really need a comment. In this case, it's an
artifact of patch-based development I think.


Cheers,
Phil


> 	avid
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

-- 

Re: [PATCH] sched/fair: remove the spin_lock operations

[Hui Su]

On Mon, Nov 02, 2020 at 08:53:41AM -0500, Phil Auld wrote:
> On Fri, Oct 30, 2020 at 10:16:29PM +0000 David Laight wrote:
> > From: Benjamin Segall
> > > Sent: 30 October 2020 18:48
> > > 
> > > Hui Su <sh_def@163.com> writes:
> > > 
> > > > Since 'ab93a4bc955b ("sched/fair: Remove
> > > > distribute_running fromCFS bandwidth")',there is
> > > > nothing to protect between raw_spin_lock_irqsave/store()
> > > > in do_sched_cfs_slack_timer().
> > > >
> > > > So remove it.
> > > 
> > > Reviewed-by: Ben Segall <bsegall@google.com>
> > > 
> > > (I might nitpick the subject to be clear that it should be trivial
> > > because the lock area is empty, or call them dead or something, but it's
> > > not all that important)
> > 
> > I don't know about this case, but a lock+unlock can be used
> > to ensure that nothing else holds the lock when acquiring
> > the lock requires another lock be held.
> > 
> > So if the normal sequence is:
> > 	lock(table)
> > 	# lookup item
> > 	lock(item)
> > 	unlock(table)
> > 	....
> > 	unlock(item)
> > 
> > Then it can make sense to do:
> > 	lock(table)
> > 	lock(item)
> > 	unlock(item)
> > 	....
> > 	unlock(table)
> > 
> > although that ought to deserve a comment.
> >
> 
> Nah, this one used to be like this :
> 
>         raw_spin_lock_irqsave(&cfs_b->lock, flags);
>         lsub_positive(&cfs_b->runtime, runtime);
>         cfs_b->distribute_running = 0;
>         raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
> 
> It's just a leftover. I agree that if it was there for some other
> purpose that it would really need a comment. In this case, it's an
> artifact of patch-based development I think.
> 
> 
> Cheers,
> Phil
> 

Yeah, thanks for your explanation, Phil.

It is just a leftover.

[tip: sched/core] sched/fair: Remove superfluous lock section in do_sched_cfs_slack_timer()

[tip-bot2 for Hui Su]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     cdb310474dece99985e4cdd2b96b1324e39c1c9d
Gitweb:        https://git.kernel.org/tip/cdb310474dece99985e4cdd2b96b1324e39c1c9d
Author:        Hui Su <sh_def@163.com>
AuthorDate:    Fri, 30 Oct 2020 22:46:21 +08:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 10 Nov 2020 18:39:05 +01:00

sched/fair: Remove superfluous lock section in do_sched_cfs_slack_timer()

Since ab93a4bc955b ("sched/fair: Remove distribute_running fromCFS
bandwidth"), there is nothing to protect between
raw_spin_lock_irqsave/store() in do_sched_cfs_slack_timer().

Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Phil Auld <pauld@redhat.com>
Reviewed-by: Ben Segall <bsegall@google.com>
Link: https://lkml.kernel.org/r/20201030144621.GA96974@rlk
---
 kernel/sched/fair.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2755a7e..3e5d98f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5126,9 +5126,6 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
 		return;
 
 	distribute_cfs_runtime(cfs_b);
-
-	raw_spin_lock_irqsave(&cfs_b->lock, flags);
-	raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
 }
 
 /*