Re: [GIT PULL] Re: REGRESSION: Performance regressions fromswitching anon_vma->lock to mutex

[Ingo Molnar]

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> On Fri, Jun 17, 2011 at 12:58:03AM +0200, Ingo Molnar wrote:
> > 
> > * Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> > 
> > > > There's a crazy solution for that: the idle thread could process 
> > > > RCU callbacks carefully, as if it was running user-space code.
> > > 
> > > In Ben's kernel NFS server case the system may not be idle.
> > 
> > An always-100%-busy NFS server is very unlikely, but even in the 
> > hypothetical case a kernel NFS server is really performing system 
> > calls from a kernel thread in essence. If it doesn't do it explicitly 
> > then its main loop can easily include a "check RCU callbacks" call.
> 
> As long as they make sure to call it in a clean environment: no 
> locks held and so on.  But I am a bit worried about the possibility 
> of someone forgetting to put one of these where it is needed -- it 
> would work just fine for most workloads, but could fail only for 
> rare workloads.

Yeah, some sort of worst-case-tick mechanism would guarantee that we 
wont remain without RCU GC.

> That said, invoking RCU core/callback processing from the scheduler 
> context certainly sounds like an interesting way to speed up grace 
> periods.

It also moves whatever priority logic is needed closer to the 
scheduler that has to touch those data structures anyway.

RCU, at least partially, is a scheduler driven garbage collector even 
today: beyond context switch quiescent states the main practical role 
of the per CPU timer tick itself is scheduling. So having it close to 
when we do context-switches anyway looks pretty natural - worth 
trying.

It might not work out in practice, but at first sight it would 
simplify a few things i think.

Thanks,

	Ingo
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