Messages in this thread | | | Date | Tue, 23 Jun 2015 20:04:11 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH 12/13] stop_machine: Remove lglock |
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On Tue, Jun 23, 2015 at 10:30:38AM -0700, Paul E. McKenney wrote: > Good, you don't need this because you can check for dynticks later. > You will need to check for offline CPUs.
get_online_cpus() for_each_online_cpus() { ... }
is what the new code does.
> > - /* > > - * Each pass through the following loop attempts to force a > > - * context switch on each CPU. > > - */ > > - while (try_stop_cpus(cma ? cm : cpu_online_mask, > > - synchronize_sched_expedited_cpu_stop, > > - NULL) == -EAGAIN) { > > - put_online_cpus(); > > - atomic_long_inc(&rsp->expedited_tryfail); > > - > > - /* Check to see if someone else did our work for us. */ > > - s = atomic_long_read(&rsp->expedited_done); > > - if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) { > > - /* ensure test happens before caller kfree */ > > - smp_mb__before_atomic(); /* ^^^ */ > > - atomic_long_inc(&rsp->expedited_workdone1); > > - free_cpumask_var(cm); > > - return; > > Here you lose batching. Yeah, I know that synchronize_sched_expedited() > is -supposed- to be used sparingly, but it is not cool for the kernel > to melt down just because some creative user found a way to heat up a > code path. Need a mutex_trylock() with a counter and checking for > others having already done the needed work.
I really think you're making that expedited nonsense far too accessible.
But it was exactly that trylock I was trying to get rid of.
> And we still need to be able to drop back to synchronize_sched() > (AKA wait_rcu_gp(call_rcu_sched) in this case) in case we have both a > creative user and a long-running RCU-sched read-side critical section.
No, a long-running RCU-sched read-side is a bug and we should fix that, its called a preemption-latency, we don't like those.
> > + for_each_online_cpu(cpu) { > > + struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu); > > > > - /* Recheck to see if someone else did our work for us. */ > > - s = atomic_long_read(&rsp->expedited_done); > > - if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) { > > - /* ensure test happens before caller kfree */ > > - smp_mb__before_atomic(); /* ^^^ */ > > - atomic_long_inc(&rsp->expedited_workdone2); > > - free_cpumask_var(cm); > > - return; > > - } > > + /* Offline CPUs, idle CPUs, and any CPU we run on are quiescent. */ > > + if (!(atomic_add_return(0, &rdtp->dynticks) & 0x1)) > > + continue; > > Let's see... This does work for idle CPUs and for nohz_full CPUs running > in userspace. > > It does not work for the current CPU, so the check needs an additional > check against raw_smp_processor_id(), which is easy enough to add.
Right, realized after I send it out, but it _should_ work for the current cpu too. Just pointless doing it.
> There always has been a race window involving CPU hotplug.
There is no hotplug race, the entire thing has get_online_cpus() held across it.
> > + stop_one_cpu(cpu, synchronize_sched_expedited_cpu_stop, NULL); > > My thought was to use smp_call_function_single(), and to have the function > called recheck dyntick-idle state, avoiding doing a set_tsk_need_resched() > if so.
set_tsk_need_resched() is buggy and should not be used.
> This would result in a single pass through schedule() instead > of stop_one_cpu()'s double context switch. It would likely also require > some rework of rcu_note_context_switch(), which stop_one_cpu() avoids > the need for.
_IF_ you're going to touch rcu_note_context_switch(), you might as well use a completion, set it for the number of CPUs that need a resched, spray resched-IPI and have rcu_note_context_switch() do a complete().
But I would really like to avoid adding code to rcu_note_context_switch(), because we run that on _every_ single context switch.
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