linux-kernel - Re: [rfc 19/45] cpu alloc: NFS statistics

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Message-ID: <20071120215048.GA14522@Krystal>
Date:	Tue, 20 Nov 2007 16:50:48 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Christoph Lameter <clameter@....com>, ak@...e.de,
	akpm@...ux-foundation.org, travis@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc 19/45] cpu alloc: NFS statistics

* Trond Myklebust (trond.myklebust@....uio.no) wrote:
> 
> On Tue, 2007-11-20 at 16:28 -0500, Mathieu Desnoyers wrote:
> > * Trond Myklebust (trond.myklebust@....uio.no) wrote:
> > > 
> > > On Tue, 2007-11-20 at 12:49 -0800, Christoph Lameter wrote:
> > > > On Tue, 20 Nov 2007, Mathieu Desnoyers wrote:
> > > > 
> > > > > > Index: linux-2.6/fs/nfs/iostat.h
> > > > > > ===================================================================
> > > > > > --- linux-2.6.orig/fs/nfs/iostat.h	2007-11-15 21:17:24.391404458 -0800
> > > > > > +++ linux-2.6/fs/nfs/iostat.h	2007-11-15 21:25:33.167654066 -0800
> > > > > > @@ -123,7 +123,7 @@ static inline void nfs_inc_server_stats(
> > > > > >  	int cpu;
> > > > > >  
> > > > > >  	cpu = get_cpu();
> > > > > > -	iostats = per_cpu_ptr(server->io_stats, cpu);
> > > > > > +	iostats = CPU_PTR(server->io_stats, cpu);
> > > > > >  	iostats->events[stat] ++;
> > > > > 
> > > > > Is there a way to change this into a CPU_ADD ?
> > > > 
> > > > Yes I must have missed that.
> > > > 
> > > > Could be
> > > > 
> > > > CPU_INC(server->io_stats->events[stat]);
> > > > 
> > > > > 
> > > > > >  	put_cpu_no_resched();
> > > > > 
> > > > > Why put_cpu_no_resched here ?
> > > > 
> > > > We do not want to reschedule here? We may have already disabled interrupts 
> > > > or some such thing.
> > > 
> > > Some of these statistics are updated from inside a spinlocked
> > > environment, hence the put_no_resched().
> > > 
> > 
> > The we could use __get_cpu_var instead ? (since we know preemption is
> > _always_ disabled when we are called) ?
> 
> We don't know that. I said that _some_ of these statistics are updated
> in that way.
> 

Then my original point is valid : put_no_resched() will cause unwanted
scheduler latencies. It's designed only to be used from within the
scheduler code itself. The correct approach would be a standard
put_cpu().

Or am I missing something ?

Mathieu

> Cheers
>   Trond
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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