Messages in this thread | | | Date | Fri, 19 Oct 2018 05:31:06 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu: Benefit from expedited grace period in __wait_rcu_gp |
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On Fri, Oct 19, 2018 at 02:49:05AM +0200, KarimAllah Ahmed wrote: > When expedited grace-period is set, both synchronize_sched > synchronize_rcu_bh can be optimized to have a significantly lower latency. > > Improve wait_rcu_gp handling to also account for expedited grace-period. > The downside is that wait_rcu_gp will not wait anymore for all RCU variants > concurrently when an expedited grace-period is set, however, given the > improved latency it does not really matter. > > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Cc: Josh Triplett <josh@joshtriplett.org> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > Cc: Lai Jiangshan <jiangshanlai@gmail.com> > Cc: linux-kernel@vger.kernel.org > Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
Cute!
Unfortunately, there are a few problems with this patch:
1. I will be eliminating synchronize_rcu_mult() due to the fact that the upcoming RCU flavor consolidation eliminates its sole caller. See 5fc9d4e000b1 ("rcu: Eliminate synchronize_rcu_mult()") in my -rcu tree. This would of course also eliminate the effects of this patch.
2. The real-time guys' users are not going to be at all happy with the IPIs resulting from the _expedited() API members. Yes, they can boot with rcupdate.rcu_normal=1, but they don't always need that big a hammer, and use of this kernel parameter can slow down boot, hibernation, suspend, network configuration, and much else besides. We therefore don't want them to have to use rcupdate.rcu_normal=1 unless absolutely necessary.
3. If the real-time guys' users were to have booted with rcupdate.rcu_normal=1, then synchronize_sched_expedited() would invoke _synchronize_rcu_expedited, which would invoke wait_rcu_gp(), which would invoke _wait_rcu_gp() which would invoke __wait_rcu_gp(), which, given your patch, would in turn invoke synchronize_sched_expedited(). This situation could well prevent their systems from meeting their response-time requirements.
So I cannot accept this patch nor for that matter any similar patch.
But what were you really trying to get done here? If you were thinking of adding another synchronize_rcu_mult(), the flavor consolidation will make that unnecessary in most cases. If you are trying to speed up CPU-hotplug operations, I suggest using the rcu_expedited sysctl variable when taking a CPU offline. If something else, please let me know what it is so that we can work out how the problem might best be solved.
Thanx, Paul
> --- > kernel/rcu/update.c | 34 ++++++++++++++++++++++++++++------ > 1 file changed, 28 insertions(+), 6 deletions(-) > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > index 68fa19a..44b8817 100644 > --- a/kernel/rcu/update.c > +++ b/kernel/rcu/update.c > @@ -392,13 +392,27 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array, > might_sleep(); > continue; > } > - init_rcu_head_on_stack(&rs_array[i].head); > - init_completion(&rs_array[i].completion); > + > for (j = 0; j < i; j++) > if (crcu_array[j] == crcu_array[i]) > break; > - if (j == i) > - (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu); > + if (j != i) > + continue; > + > + if ((crcu_array[i] == call_rcu_sched || > + crcu_array[i] == call_rcu_bh) > + && rcu_gp_is_expedited()) { > + if (crcu_array[i] == call_rcu_sched) > + synchronize_sched_expedited(); > + else > + synchronize_rcu_bh_expedited(); > + > + continue; > + } > + > + init_rcu_head_on_stack(&rs_array[i].head); > + init_completion(&rs_array[i].completion); > + (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu); > } > > /* Wait for all callbacks to be invoked. */ > @@ -407,11 +421,19 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array, > (crcu_array[i] == call_rcu || > crcu_array[i] == call_rcu_bh)) > continue; > + > + if ((crcu_array[i] == call_rcu_sched || > + crcu_array[i] == call_rcu_bh) > + && rcu_gp_is_expedited()) > + continue; > + > for (j = 0; j < i; j++) > if (crcu_array[j] == crcu_array[i]) > break; > - if (j == i) > - wait_for_completion(&rs_array[i].completion); > + if (j != i) > + continue; > + > + wait_for_completion(&rs_array[i].completion); > destroy_rcu_head_on_stack(&rs_array[i].head); > } > } > -- > 2.7.4 >
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