Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods
From: Paul E. McKenney
Date: Sat Feb 21 2015 - 20:43:57 EST
On Sat, Feb 21, 2015 at 05:08:52PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 20, 2015 at 09:45:39AM -0800, Arjan van de Ven wrote:
> > On 2/20/2015 9:43 AM, Peter Zijlstra wrote:
> > >On Fri, Feb 20, 2015 at 09:32:39AM -0800, Arjan van de Ven wrote:
> > >>there's a few others as well that I'm chasing down...
> > >>.. but the flip side, prior to running ring 3 code, why NOT do fast expedites?
> > >
> > >So my objections are twofold:
> > >
> > > - I object to fast expedites in principle; they spray IPIs across the
> > > system, so ideally we'd not have them at all, therefore also not at
> > > boot.
> > >
> > > Because as soon as the option exists, people will use it for other
> > > things too.
> > the option exists today in sysfs and kernel parameter...
> Yeah, Paul and me have been having this argument for a while now ;-)
Indeed we have. ;-)
And if expedited grace periods start causing latency issues in real-world
workloads, I will address those issues.
In the meantime, one of the nice things about NO_HZ_FULL is that
synchronize_sched_expedited() avoids IPIing CPUs having a single runnable
task that is running in nohz_full mode. ;-)
> > >And esp. in bootup code you can special case a lot of stuff; there's
> > >limited concurrency esp. because userspace it not there yet. So we might
> > >not actually need those sync calls.
> > yeah I am going down that angle as well absolutely.
> > but there are cases that may well be legit (or are 5 function calls deep into common code)
> Good ;-)
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