On Sun, Mar 25, 2018 at 12:23:28PM -0700, Paul E. McKenney wrote: > On Sun, Mar 25, 2018 at 08:50:04PM +0300, Yury Norov wrote: > > kick_all_cpus_sync() forces all CPUs to sync caches by sending broadcast > > IPI. > > If CPU is in extended quiescent state (idle task or nohz_full userspace), > > this > > work may be done at the exit of this state. Delaying synchronization helps > > to > > save power if CPU is in idle state and decrease latency for real-time tasks. > > > > This patch introduces kick_active_cpus_sync() and uses it in mm/slab and > > arm64 > > code to delay syncronization. > > > > For task isolation (https://lkml.org/lkml/2017/11/3/589), IPI to the CPU > > running > > isolated task would be fatal, as it breaks isolation. The approach with > > delaying > > of synchronization work helps to maintain isolated state. > > > > I've tested it with test from task isolation series on ThunderX2 for more > > than > > 10 hours (10k giga-ticks) without breaking isolation. > > > > Signed-off-by: Yury Norov <yno...@caviumnetworks.com> > > --- > > arch/arm64/kernel/insn.c | 2 +- > > include/linux/smp.h | 2 ++ > > kernel/smp.c | 24 ++++++++++++++++++++++++ > > mm/slab.c | 2 +- > > 4 files changed, 28 insertions(+), 2 deletions(-) > > > > diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c > > index 2718a77da165..9d7c492e920e 100644 > > --- a/arch/arm64/kernel/insn.c > > +++ b/arch/arm64/kernel/insn.c > > @@ -291,7 +291,7 @@ int __kprobes aarch64_insn_patch_text(void *addrs[], > > u32 insns[], int cnt) > > * synchronization. > > */ > > ret = aarch64_insn_patch_text_nosync(addrs[0], > > insns[0]); > > - kick_all_cpus_sync(); > > + kick_active_cpus_sync(); > > return ret; > > } > > } > > diff --git a/include/linux/smp.h b/include/linux/smp.h > > index 9fb239e12b82..27215e22240d 100644 > > --- a/include/linux/smp.h > > +++ b/include/linux/smp.h > > @@ -105,6 +105,7 @@ int smp_call_function_any(const struct cpumask *mask, > > smp_call_func_t func, void *info, int wait); > > > > void kick_all_cpus_sync(void); > > +void kick_active_cpus_sync(void); > > void wake_up_all_idle_cpus(void); > > > > /* > > @@ -161,6 +162,7 @@ smp_call_function_any(const struct cpumask *mask, > > smp_call_func_t func, > > } > > > > static inline void kick_all_cpus_sync(void) { } > > +static inline void kick_active_cpus_sync(void) { } > > static inline void wake_up_all_idle_cpus(void) { } > > > > #ifdef CONFIG_UP_LATE_INIT > > diff --git a/kernel/smp.c b/kernel/smp.c > > index 084c8b3a2681..0358d6673850 100644 > > --- a/kernel/smp.c > > +++ b/kernel/smp.c > > @@ -724,6 +724,30 @@ void kick_all_cpus_sync(void) > > } > > EXPORT_SYMBOL_GPL(kick_all_cpus_sync); > > > > +/** > > + * kick_active_cpus_sync - Force CPUs that are not in extended > > + * quiescent state (idle or nohz_full userspace) sync by sending > > + * IPI. Extended quiescent state CPUs will sync at the exit of > > + * that state. > > + */ > > +void kick_active_cpus_sync(void) > > +{ > > + int cpu; > > + struct cpumask kernel_cpus; > > + > > + smp_mb(); > > + > > + cpumask_clear(&kernel_cpus); > > + preempt_disable(); > > + for_each_online_cpu(cpu) { > > + if (!rcu_eqs_special_set(cpu)) > > If we get here, the CPU is not in a quiescent state, so we therefore > must IPI it, correct? > > But don't you also need to define rcu_eqs_special_exit() so that RCU > can invoke it when it next leaves its quiescent state? Or are you able > to ignore the CPU in that case? (If you are able to ignore the CPU in > that case, I could give you a lower-cost function to get your job done.) > > Thanx, Paul
What's actually needed for synchronization is issuing memory barrier on target CPUs before we start executing kernel code. smp_mb() is implicitly called in smp_call_function*() path for it. In rcu_eqs_special_set() -> rcu_dynticks_eqs_exit() path, smp_mb__after_atomic() is called just before rcu_eqs_special_exit(). So I think, rcu_eqs_special_exit() may be left untouched. Empty rcu_eqs_special_exit() in new RCU path corresponds empty do_nothing() in old IPI path. Or my understanding of smp_mb__after_atomic() is wrong? By default, smp_mb__after_atomic() is just alias to smp_mb(). But some architectures define it differently. x86, for example, aliases it to just barrier() with a comment: "Atomic operations are already serializing on x86". I was initially thinking that it's also fine to leave rcu_eqs_special_exit() empty in this case, but now I'm not sure... Anyway, answering to your question, we shouldn't ignore quiescent CPUs, and rcu_eqs_special_set() path is really needed as it issues memory barrier on them. Yury > > + cpumask_set_cpu(cpu, &kernel_cpus); > > + } > > + smp_call_function_many(&kernel_cpus, do_nothing, NULL, 1); > > + preempt_enable(); > > +} > > +EXPORT_SYMBOL_GPL(kick_active_cpus_sync); > > + > > /** > > * wake_up_all_idle_cpus - break all cpus out of idle > > * wake_up_all_idle_cpus try to break all cpus which is in idle state even > > diff --git a/mm/slab.c b/mm/slab.c > > index 324446621b3e..678d5dbd6f46 100644 > > --- a/mm/slab.c > > +++ b/mm/slab.c > > @@ -3856,7 +3856,7 @@ static int __do_tune_cpucache(struct kmem_cache > > *cachep, int limit, > > * cpus, so skip the IPIs. > > */ > > if (prev) > > - kick_all_cpus_sync(); > > + kick_active_cpus_sync(); > > > > check_irq_on(); > > cachep->batchcount = batchcount; > > -- > > 2.14.1 > >
