* Re: [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y
[not found] <tip-35a773a07926a22bf19d77ee00024522279c4e68@git.kernel.org>
@ 2016-09-22 18:25 ` Mikulas Patocka
2016-09-22 18:30 ` Paul E. McKenney
2016-09-22 18:42 ` Peter Zijlstra
0 siblings, 2 replies; 6+ messages in thread
From: Mikulas Patocka @ 2016-09-22 18:25 UTC (permalink / raw)
To: hpa, paulmck, josh, rostedt, mathieu.desnoyers, jiangshanlai,
torvalds, mingo, tglx, peterz, linux-kernel, oleg
Cc: linux-tip-commits
I've found a document that says that cond_resched() is needed on
preemptible kernels to mark RCU quiescent states:
https://lwn.net/Articles/603252/
Paul, is cond_resched() still needed on current RCU implementation? Or not?
Mikulas
On Thu, 22 Sep 2016, tip-bot for Peter Zijlstra wrote:
> Commit-ID: 35a773a07926a22bf19d77ee00024522279c4e68
> Gitweb: http://git.kernel.org/tip/35a773a07926a22bf19d77ee00024522279c4e68
> Author: Peter Zijlstra <peterz@infradead.org>
> AuthorDate: Mon, 19 Sep 2016 12:57:53 +0200
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Thu, 22 Sep 2016 14:53:46 +0200
>
> sched/core: Avoid _cond_resched() for PREEMPT=y
>
> On fully preemptible kernels _cond_resched() is pointless, so avoid
> emitting any code for it.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Mikulas Patocka <mpatocka@redhat.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
> include/linux/sched.h | 4 ++++
> kernel/sched/core.c | 2 ++
> 2 files changed, 6 insertions(+)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f00ee8e..b99fcd1 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -3209,7 +3209,11 @@ static inline int signal_pending_state(long state, struct task_struct *p)
> * cond_resched_lock() will drop the spinlock before scheduling,
> * cond_resched_softirq() will enable bhs before scheduling.
> */
> +#ifndef CONFIG_PREEMPT
> extern int _cond_resched(void);
> +#else
> +static inline int _cond_resched(void) { return 0; }
> +#endif
>
> #define cond_resched() ({ \
> ___might_sleep(__FILE__, __LINE__, 0); \
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b2ec53c..d7babcc 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4883,6 +4883,7 @@ SYSCALL_DEFINE0(sched_yield)
> return 0;
> }
>
> +#ifndef CONFIG_PREEMPT
> int __sched _cond_resched(void)
> {
> if (should_resched(0)) {
> @@ -4892,6 +4893,7 @@ int __sched _cond_resched(void)
> return 0;
> }
> EXPORT_SYMBOL(_cond_resched);
> +#endif
>
> /*
> * __cond_resched_lock() - if a reschedule is pending, drop the given lock,
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y
2016-09-22 18:25 ` [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y Mikulas Patocka
@ 2016-09-22 18:30 ` Paul E. McKenney
2016-09-22 18:42 ` Peter Zijlstra
1 sibling, 0 replies; 6+ messages in thread
From: Paul E. McKenney @ 2016-09-22 18:30 UTC (permalink / raw)
To: Mikulas Patocka
Cc: hpa, josh, rostedt, mathieu.desnoyers, jiangshanlai, torvalds,
mingo, tglx, peterz, linux-kernel, oleg, linux-tip-commits
On Thu, Sep 22, 2016 at 02:25:01PM -0400, Mikulas Patocka wrote:
> I've found a document that says that cond_resched() is needed on
> preemptible kernels to mark RCU quiescent states:
>
> https://lwn.net/Articles/603252/
>
> Paul, is cond_resched() still needed on current RCU implementation? Or not?
It can be if you loop in the kernel for a very long time.
But cond_resched_rcu_qs() is instead needed in cases where there is only
one runnable non-idle non-nohz task on a given CPU.
Thanx, Paul
> Mikulas
>
>
>
> On Thu, 22 Sep 2016, tip-bot for Peter Zijlstra wrote:
>
> > Commit-ID: 35a773a07926a22bf19d77ee00024522279c4e68
> > Gitweb: http://git.kernel.org/tip/35a773a07926a22bf19d77ee00024522279c4e68
> > Author: Peter Zijlstra <peterz@infradead.org>
> > AuthorDate: Mon, 19 Sep 2016 12:57:53 +0200
> > Committer: Ingo Molnar <mingo@kernel.org>
> > CommitDate: Thu, 22 Sep 2016 14:53:46 +0200
> >
> > sched/core: Avoid _cond_resched() for PREEMPT=y
> >
> > On fully preemptible kernels _cond_resched() is pointless, so avoid
> > emitting any code for it.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Mikulas Patocka <mpatocka@redhat.com>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > ---
> > include/linux/sched.h | 4 ++++
> > kernel/sched/core.c | 2 ++
> > 2 files changed, 6 insertions(+)
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index f00ee8e..b99fcd1 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -3209,7 +3209,11 @@ static inline int signal_pending_state(long state, struct task_struct *p)
> > * cond_resched_lock() will drop the spinlock before scheduling,
> > * cond_resched_softirq() will enable bhs before scheduling.
> > */
> > +#ifndef CONFIG_PREEMPT
> > extern int _cond_resched(void);
> > +#else
> > +static inline int _cond_resched(void) { return 0; }
> > +#endif
> >
> > #define cond_resched() ({ \
> > ___might_sleep(__FILE__, __LINE__, 0); \
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index b2ec53c..d7babcc 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4883,6 +4883,7 @@ SYSCALL_DEFINE0(sched_yield)
> > return 0;
> > }
> >
> > +#ifndef CONFIG_PREEMPT
> > int __sched _cond_resched(void)
> > {
> > if (should_resched(0)) {
> > @@ -4892,6 +4893,7 @@ int __sched _cond_resched(void)
> > return 0;
> > }
> > EXPORT_SYMBOL(_cond_resched);
> > +#endif
> >
> > /*
> > * __cond_resched_lock() - if a reschedule is pending, drop the given lock,
> >
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y
2016-09-22 18:25 ` [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y Mikulas Patocka
2016-09-22 18:30 ` Paul E. McKenney
@ 2016-09-22 18:42 ` Peter Zijlstra
2016-09-22 20:28 ` Paul E. McKenney
1 sibling, 1 reply; 6+ messages in thread
From: Peter Zijlstra @ 2016-09-22 18:42 UTC (permalink / raw)
To: Mikulas Patocka
Cc: hpa, paulmck, josh, rostedt, mathieu.desnoyers, jiangshanlai,
torvalds, mingo, tglx, linux-kernel, oleg, linux-tip-commits
On Thu, Sep 22, 2016 at 02:25:01PM -0400, Mikulas Patocka wrote:
> I've found a document that says that cond_resched() is needed on
> preemptible kernels to mark RCU quiescent states:
>
> https://lwn.net/Articles/603252/
>
> Paul, is cond_resched() still needed on current RCU implementation? Or not?
No, we ripped that out. See 4a81e8328d37 ("rcu: Reduce overhead of
cond_resched() checks for RCU")
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y
2016-09-22 18:42 ` Peter Zijlstra
@ 2016-09-22 20:28 ` Paul E. McKenney
2016-09-23 8:44 ` Peter Zijlstra
0 siblings, 1 reply; 6+ messages in thread
From: Paul E. McKenney @ 2016-09-22 20:28 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Mikulas Patocka, hpa, josh, rostedt, mathieu.desnoyers,
jiangshanlai, torvalds, mingo, tglx, linux-kernel, oleg,
linux-tip-commits
On Thu, Sep 22, 2016 at 08:42:28PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 22, 2016 at 02:25:01PM -0400, Mikulas Patocka wrote:
> > I've found a document that says that cond_resched() is needed on
> > preemptible kernels to mark RCU quiescent states:
> >
> > https://lwn.net/Articles/603252/
> >
> > Paul, is cond_resched() still needed on current RCU implementation? Or not?
>
> No, we ripped that out. See 4a81e8328d37 ("rcu: Reduce overhead of
> cond_resched() checks for RCU")
I would instead say that we changed the algorithm to reduce the overhead
in the common case, but yes, this commit did change things quite a bit.
Thanx, Paul
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y
2016-09-22 20:28 ` Paul E. McKenney
@ 2016-09-23 8:44 ` Peter Zijlstra
2016-09-23 14:47 ` Paul E. McKenney
0 siblings, 1 reply; 6+ messages in thread
From: Peter Zijlstra @ 2016-09-23 8:44 UTC (permalink / raw)
To: Paul E. McKenney
Cc: Mikulas Patocka, hpa, josh, rostedt, mathieu.desnoyers,
jiangshanlai, torvalds, mingo, tglx, linux-kernel, oleg,
linux-tip-commits
On Thu, Sep 22, 2016 at 01:28:17PM -0700, Paul E. McKenney wrote:
> On Thu, Sep 22, 2016 at 08:42:28PM +0200, Peter Zijlstra wrote:
> > On Thu, Sep 22, 2016 at 02:25:01PM -0400, Mikulas Patocka wrote:
> > > I've found a document that says that cond_resched() is needed on
> > > preemptible kernels to mark RCU quiescent states:
> > >
> > > https://lwn.net/Articles/603252/
> > >
> > > Paul, is cond_resched() still needed on current RCU implementation? Or not?
> >
> > No, we ripped that out. See 4a81e8328d37 ("rcu: Reduce overhead of
> > cond_resched() checks for RCU")
>
> I would instead say that we changed the algorithm to reduce the overhead
> in the common case, but yes, this commit did change things quite a bit.
Sure, but critical point (for this discussion) is that cond_resched() no
longer implies anything RCU, it really only is a voluntary reschedule
point.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y
2016-09-23 8:44 ` Peter Zijlstra
@ 2016-09-23 14:47 ` Paul E. McKenney
0 siblings, 0 replies; 6+ messages in thread
From: Paul E. McKenney @ 2016-09-23 14:47 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Mikulas Patocka, hpa, josh, rostedt, mathieu.desnoyers,
jiangshanlai, torvalds, mingo, tglx, linux-kernel, oleg,
linux-tip-commits
On Fri, Sep 23, 2016 at 10:44:42AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 22, 2016 at 01:28:17PM -0700, Paul E. McKenney wrote:
> > On Thu, Sep 22, 2016 at 08:42:28PM +0200, Peter Zijlstra wrote:
> > > On Thu, Sep 22, 2016 at 02:25:01PM -0400, Mikulas Patocka wrote:
> > > > I've found a document that says that cond_resched() is needed on
> > > > preemptible kernels to mark RCU quiescent states:
> > > >
> > > > https://lwn.net/Articles/603252/
> > > >
> > > > Paul, is cond_resched() still needed on current RCU implementation? Or not?
> > >
> > > No, we ripped that out. See 4a81e8328d37 ("rcu: Reduce overhead of
> > > cond_resched() checks for RCU")
> >
> > I would instead say that we changed the algorithm to reduce the overhead
> > in the common case, but yes, this commit did change things quite a bit.
>
> Sure, but critical point (for this discussion) is that cond_resched() no
> longer implies anything RCU, it really only is a voluntary reschedule
> point.
Agreed, cond_resched_rcu_qs() is what you use to do both a cond_resched()
and an RCU quiescent state.
Thanx, Paul
^ permalink raw reply [flat|nested] 6+ messages in thread
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2016-09-22 18:25 ` [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y Mikulas Patocka
2016-09-22 18:30 ` Paul E. McKenney
2016-09-22 18:42 ` Peter Zijlstra
2016-09-22 20:28 ` Paul E. McKenney
2016-09-23 8:44 ` Peter Zijlstra
2016-09-23 14:47 ` Paul E. McKenney
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