linux-kernel - Re: Hackbench pipes regression bisected to PSI

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Message-ID: <20181126174402.GR23260@techsingularity.net>
Date:   Mon, 26 Nov 2018 17:44:02 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Tejun Heo <tj@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: Hackbench pipes regression bisected to PSI

On Mon, Nov 26, 2018 at 12:32:18PM -0500, Johannes Weiner wrote:
> On Mon, Nov 26, 2018 at 04:54:47PM +0000, Mel Gorman wrote:
> > On Mon, Nov 26, 2018 at 11:07:24AM -0500, Johannes Weiner wrote:
> > > @@ -509,6 +509,15 @@ config PSI
> > >  
> > >  	  Say N if unsure.
> > >  
> > > +config PSI_DEFAULT_DISABLED
> > > +	bool "Require boot parameter to enable pressure stall information tracking"
> > > +	default n
> > > +	depends on PSI
> > > +	help
> > > +	  If set, pressure stall information tracking will be disabled
> > > +	  per default but can be enabled through passing psi_enable=1
> > > +	  on the kernel commandline during boot.
> > > +
> > >  endmenu # "CPU/Task time and stats accounting"
> > >  
> > 
> > Should this default y on the basis that someone only wants the feature if
> > they are aware of it? This is not that important as CONFIG_PSI is disabled
> > by default and it's up to distribution maintainers to use their brain.
> 
> I went with the NUMA balancing example again here, which defaults to
> enabling the feature at boot time. IMO that makes sense, as somebody
> would presumably first read through the PSI help text, then decide y
> on that before being asked the second question. A "yes, but
> <stipulations>" for vendor kernels seems more appropriate than
> requiring a double yes for other users to simply get the feature.
> 

That's fair enough. The original NUMA balancing thinking was that it
should be enabled because there is a reasonable expectation that it
would improve performance regardless of user awareness. PSI is not
necessarily the same as it requires a consumer but I accept that a
distro maintainer should read the Kconfig text and figure it out.

I'll make sure the updated version gets tested, thanks.

-- 
Mel Gorman
SUSE Labs