* 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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[not found] <tip-35a773a07926a22bf19d77ee00024522279c4e68@git.kernel.org>
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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