linux-kernel - Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable again and shut down on idle)

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Date:	Tue, 26 Jan 2016 19:31:18 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Christoph Lameter <cl@...ux.com>, Michal Hocko <mhocko@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable
 again and shut down on idle)

On Tue, 2016-01-26 at 10:25 -0600, Christoph Lameter wrote:
> On Mon, 25 Jan 2016, Michal Hocko wrote:
> 
> > > Why would the deferring cause this overhead?
> > 
> > I guess the profile speaks for itself, doesn't it?
> 
> But the system is going idle? Why would this impact performance?

We enter/exit idle a lot.

Your reluctance to move it seem to suggest that 99.99% of CPUs on the
planet chewing up cycles (measured) doing what for most is useless work
on every micro-idle is a perfectly fine price to pay to ensure that
.01% (or whatever tiny minority) get what they want.

I disagree.  You're burning electrons for no benefit at all to me on my
box.  You want to do high speed trading, that's fine, but I expect my
box to be able to pop in and out of idle without having to pay a toll
to the high speed trading bandits of the world, thank you very much.

This specialty thing does not belong in the generic fast path.

	-Mike