Messages in this thread | | | Date | Tue, 15 Nov 2016 09:16:55 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 6/7] rcu: Make expedited grace periods recheck dyntick idle state |
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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.
> 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.
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