Messages in this thread | | | Date | Tue, 15 Nov 2016 06:36:06 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 6/7] rcu: Make expedited grace periods recheck dyntick idle state |
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On Tue, Nov 15, 2016 at 09:16:55AM +0100, Peter Zijlstra wrote: > On Mon, Nov 14, 2016 at 10:12:37AM -0800, Paul E. McKenney wrote: > > On Mon, Nov 14, 2016 at 06:37:33PM +0100, Peter Zijlstra wrote: > > > On Mon, Nov 14, 2016 at 09:25:12AM -0800, Josh Triplett wrote: > > > > On Mon, Nov 14, 2016 at 08:57:12AM -0800, Paul E. McKenney wrote: > > > > > Expedited grace periods check dyntick-idle state, and avoid sending > > > > > IPIs to idle CPUs, including those running guest OSes, and, on NOHZ_FULL > > > > > kernels, nohz_full CPUs. However, the kernel has been observed checking > > > > > a CPU while it was non-idle, but sending the IPI after it has gone > > > > > idle. This commit therefore rechecks idle state immediately before > > > > > sending the IPI, refraining from IPIing CPUs that have since gone idle. > > > > > > > > > > Reported-by: Rik van Riel <riel@redhat.com> > > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > > > > > > > atomic_add_return(0, ...) seems odd. Do you actually want that, rather > > > > than atomic_read(...)? If so, can you please document exactly why? > > > > > > Yes that is weird. The only effective difference is that it would do a > > > load-exclusive instead of a regular load. > > > > It is weird, and checking to see if it is safe to convert it and its > > friends to something with less overhead is on my list. This starts > > with a patch series I will post soon that consolidates all these > > atomic_add_return() calls into a single function, which will ease testing > > and other verification. > > > > All that aside, please keep in mind that much is required from this load. > > It is part of a network of ordered operations that guarantee that any > > operation from any CPU preceding a given grace period is seen to precede > > any other operation from any CPU following that same grace period. > > And each and every CPU must agree on the order of those two operations, > > otherwise, RCU is broken. > > OK, so something similar to: > > smp_mb(); > atomic_read(); > > then? That would order, with global transitivity, against prior > operations.
Maybe. The consolidation in the later patch series is a first step towards potential weakening.
> > In addition, please note also that these operations are nowhere near > > any fastpaths. > > My concern is mostly that it reads very weird. I appreciate this not > being fast path code, but confusing code is bad in any form.
It is the long-standing code that has been checking dyntick-idle counters for quite some time. Just applying that same code to a new use case in within the expedited grace periods, as you can see by looking a bit earlier in that same function.
Thanx, Paul
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