Messages in this thread | | | Subject | Re: Udpated sys_membarrier() speedup patch, FYI | From | Avi Kivity <> | Date | Mon, 31 Jul 2017 09:03:09 +0300 |
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On 07/28/2017 12:02 AM, Mathieu Desnoyers wrote: > ----- On Jul 27, 2017, at 4:58 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote: > >> ----- On Jul 27, 2017, at 4:37 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com >> wrote: >> >>> On Thu, Jul 27, 2017 at 11:04:13PM +0300, Avi Kivity wrote: > [...] >>>>> + >>>>> + this_cpu = raw_smp_processor_id(); >>>>> + for_each_online_cpu(cpu) { >>>>> + struct task_struct *p; >>>>> + >>>>> + if (cpu == this_cpu) >>>>> + continue; >>>>> + rcu_read_lock(); >>>>> + p = task_rcu_dereference(&cpu_rq(cpu)->curr); >>>>> + if (p && p->mm == current->mm) >>>>> + __cpumask_set_cpu(cpu, tmpmask); >>>> This gets you some false positives, if the CPU idled then mm will >>>> not have changed. >>> Good point! The battery-powered embedded guys would probably prefer >>> we not needlessly IPI idle CPUs. We cannot rely on RCU's dyntick-idle >>> state in nohz_full cases. Not sure if is_idle_task() can be used >>> safely, given things like play_idle(). >> Would changing the check in this loop to: >> >> if (p && !is_idle_task(p) && p->mm == current->mm) { >> >> work for you ? > Avi, is there an optimization that allows current->mm to be non-null > when the idle task is scheduled that I am missing ? > > I would have expected current->mm to be always NULL for idle > tasks. > >
I remembered that current->mm does not change when switching to a kernel task, but my Kernlish is very rusty, or maybe it has changed.
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