Messages in this thread | | | Date | Sun, 11 Nov 2018 17:47:36 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched() with synchronize_rcu() |
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On Mon, Nov 12, 2018 at 01:53:29AM +0100, Peter Zijlstra wrote: > On Sun, Nov 11, 2018 at 04:45:28PM -0800, Paul E. McKenney wrote: > > On Mon, Nov 12, 2018 at 01:12:33AM +0100, Peter Zijlstra wrote: > > > On Sun, Nov 11, 2018 at 11:43:52AM -0800, Paul E. McKenney wrote: > > > > Now that synchronize_rcu() waits for preempt-disable regions of code > > > > as well as RCU read-side critical sections, synchronize_sched() can be > > > > replaced by synchronize_rcu(). This commit therefore makes this change. > > > > > > Yes, but it also waits for an actual RCU quiestent state, which makes > > > synchoinize_rcu() potentially much more expensive than an actual > > > synchronize_sched(). > > > > None of the readers have changed. > > > > For the updaters, if CONFIG_PREEMPT=n, synchronize_rcu() and > > synchronize_sched() always were one and the same. When CONFIG_PREEMPT=y, > > synchronize_rcu() and synchronize_sched() are now one and the same. > > The Changelog does not state this; and does the commit that makes that > happen state the regression potential?
The Changelog says this:
Now that synchronize_rcu() waits for preempt-disable regions of code as well as RCU read-side critical sections, synchronize_sched() can be replaced by synchronize_rcu(). This commit therefore makes this change.
The "synchronize_rcu() waits for preempt-disable regions of code as well as RCU read-side critical sections" seems pretty unambiguous to me. Exactly what more are you wanting said there?
There were quite a few commits involved in making this happen. Perhaps the most pertinent are these:
3e3100989869 ("rcu: Defer reporting RCU-preempt quiescent states when disabled") 45975c7d21a1 ("rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds")
Normal grace periods are almost always quite long compared to typical read-side critical sections, preempt-disable regions of code, and so on. So in the common case this should be OK. Or are you instead worried about synchronize_sched_expedited()?
> > > So why are we doing this? > > > > Given that synchronize_rcu() and synchronize_sched() are now always one > > and the same, this is a distinction without a difference. > > The Changelog did not state a reason for the patch. Therefore it is a > bad patch.
??? Here is the current definition of synchronize_sched() in mainline:
static inline void synchronize_sched(void) { synchronize_rcu(); }
Thanx, Paul
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