Re: [RFC][PATCH] fs: optimize inotify/fsnotify code for unwatched files

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Jun 22, 2015 at 08:52:29PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 22, 2015 at 08:11:21AM -0700, Paul E. McKenney wrote:
> > That depends on how slow the resulting slow global state would be.
> > We have some use cases (definitely KVM, perhaps also some of the VFS
> > code) that need the current speed, as opposed to the profound slowness
> > that three trips through synchronize_sched() would provide.
> 
> So what we have with that percpu-rwsem code that I send out earlier
> today is a conditional smp_mb(), and I think we can do the same for
> SRCU.
> 
> I'm just not sure !GP is common enough for all SRCU cases to be worth
> doing.

Especially given that we don't want the readers to have to acquire a
lock in order to get a consistent view of whether or not a grace period
is in progress.

> Those that rely on sync_srcu() and who do it rarely would definitely
> benefit. The same with those that rarely do call_srcu().
> 
> But those that heavily use call_srcu() would be better off with the
> prolonged GP with 3 sync_sched() calls in.

Those are indeed two likely possibilities.  Other possibilities include
cases where synchronize_srcu() is invoked rarely, but where its latency
is visible to userspace, and those where there really is a need to
wait synchronously for a grace period, so that call_srcu() doesn't buy
you anything.

							Thanx, Paul

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