Messages in this thread | | | Date | Wed, 1 Jul 2015 07:00:31 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging normal ones |
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On Wed, Jul 01, 2015 at 12:55:11PM +0200, Peter Zijlstra wrote: > On Wed, Jul 01, 2015 at 12:09:39PM +0200, Peter Zijlstra wrote: > > On Tue, Jun 30, 2015 at 04:46:33PM -0700, josh@joshtriplett.org wrote: > > > Consider it a fairly weak concern against. Increasing performance seems > > > like a good thing in general; I just don't relish the future "feels less > > > responsive" bug reports that take a long time to track down and turn out > > > to be "this completely unrelated driver was loaded and started using > > > expedited grace periods". > > > > random drivers, or for that matter, new-code of any sort. Should _NOT_ > > be using expedited grace periods. > > > > They're a horrid hack only suitable for unfixable ABI. > > Let me repeat, just in case I've not been clear. Expedited grace periods > are _BAD_ and should be avoided at all costs.
That is a bit extreme, Peter.
There should not be a problem using them for operations that are not done while the system is running the main workload. Which is why I am OK with synchronize_net() using synchronize_rcu_expedited() when RTNL is held. The operations that do that are setup/configuration operations that you won't normally be doing while your HPC or realtime workload is running.
I believe that many of the other uses are similar.
> They perturb the _entire_ machine.
The portion of the machine that is non-idle and not executing in nohz_full userspace, that is. So nohz_full usermode execution is unperturbed by expedited grace periods.
In addition, synchronize_srcu_expedited() does -not- perturb anything other than the task actually executing synchronize_srcu_expedited(). So those read-side memory barriers are gaining you something, and there should not be much need to push back on synchronize_srcu_expedited().
> Yes we can polish the turd, but in the end its still a turd.
And I intend to polish it even more. ;-)
> Sadly people seem to have taken a liking to them, ooh a make RCU go > faster button. And there's not been much if any pushback on people using > it.
There aren't all that many uses, so I don't believe that people are abusing it that much. There are only four non-RCU uses of synchronize_rcu_expedited() and only two non-RCU uses of synchronize_sched_expedited(). In contrast, there are a couple hundred uses of synchronize_rcu() and about 40 uses of synchronize_sched().
So I am not seeing much evidence of wanton use of either synchronize_srcu() or synchronize_sched().
Are a huge pile of them coming in this merge window or something? What raised your concerns on this issue?
Thanx, Paul
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