Messages in this thread | | | Date | Mon, 12 Nov 2018 03:07:10 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched() with synchronize_rcu() |
| |
On Sun, Nov 11, 2018 at 05:47:36PM -0800, Paul E. McKenney wrote: > 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?
The quoted bit only states that synchronize_rcu() is sufficient; it does not say it is equivalent and the patch is a nop. It also doesn't say that the purpose is to get rid of the synchronize_sched() function.
> 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")
The latter; it does not mention that this will possible make synchronize_sched() quite a bit more expensive on PREEMPT=y 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()?
No, I still feel expedited should not exist at all ;-)
But for PREEMPT=y synchronize_sched() can be quite a bit shorter than synchronize_rcu(), since we don't have to wait for preempted read side stuff.
> > > > 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(); > }
Again, the patch didn't say that.
If the Changelog would've read something like:
"Since synchronize_sched() is now equivalent to synchronize_rcu(), replace the synchronize_sched() usage such that we can eventually remove the interface."
It would've been clear that the patch is a nop and what the purpose was.
| |