Messages in this thread | | | Date | Wed, 19 Apr 2017 08:37:03 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 |
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On Wed, Apr 19, 2017 at 03:15:53PM +0200, Peter Zijlstra wrote: > On Wed, Apr 19, 2017 at 06:02:45AM -0700, Paul E. McKenney wrote: > > On Wed, Apr 19, 2017 at 01:28:45PM +0200, Peter Zijlstra wrote: > > > > > > So the thing Maz complained about is because KVM assumes > > > synchronize_srcu() is 'free' when there is no srcu_read_lock() activity. > > > This series 'breaks' that. > > > > > > I've not looked hard enough at the new SRCU to see if its possible to > > > re-instate that feature. > > > > And with the fix I gave Maz, the parallelized version is near enough > > to being free as well. It was just a stupid bug on my part: I forgot > > to check for expedited when scheduling callbacks. > > Right, although for the old SRCU it was true for !expedited as well.
Which is all good fun until someone does a call_srcu() on each and every munmap() syscall. ;-)
> Just turns out the KVM memslots crud already uses > synchronize_srcu_expedited(). > > <rant>without a friggin' comment; hate @expedited</rant>
And I won't even try defend the old try_stop_cpus()-based expedited algorithm in today's context, even if it did seem to be a good idea at the time. That said, back at that time, the expectation was that expedited grace periods would only be used for very rare boot-time configuration changes, at which time who cares? But there are a lot more expedited use cases these days, so the implementation had to change.
But the current code is much better housebroken. ;-)
Thanx, Paul
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