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

From: Paul E. McKenney
Date: Thu Jun 16 2011 - 20:45:49 EST

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.

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

Thanx, Paul
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