linux-kernel - Re: [PATCH R4 4/7] xen/balloon: Protect against CPU exhaust by event/x process

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Date:	Tue, 15 Mar 2011 16:17:48 +0100
From:	Daniel Kiper <dkiper@...-space.pl>
To:	Ian Campbell <Ian.Campbell@...citrix.com>
Cc:	Daniel Kiper <dkiper@...-space.pl>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"andi.kleen@...el.com" <andi.kleen@...el.com>,
	"haicheng.li@...ux.intel.com" <haicheng.li@...ux.intel.com>,
	"fengguang.wu@...el.com" <fengguang.wu@...el.com>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	"v.tolstov@...fip.ru" <v.tolstov@...fip.ru>,
	"pasik@....fi" <pasik@....fi>,
	"dave@...ux.vnet.ibm.com" <dave@...ux.vnet.ibm.com>,
	"wdauchy@...il.com" <wdauchy@...il.com>,
	"rientjes@...gle.com" <rientjes@...gle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH R4 4/7] xen/balloon: Protect against CPU exhaust by event/x process

On Mon, Mar 14, 2011 at 03:04:49PM +0000, Ian Campbell wrote:
> On Tue, 2011-03-08 at 21:48 +0000, Daniel Kiper wrote:
> > Protect against CPU exhaust by event/x process during
> > errors by adding some delays in scheduling next event
> > and retry count limit.
>
> The addition of a default retry count limit reverses the change made in
> bc2c0303226ec716854d3c208c7f84fe7aa35cd7. That change was made to allow
> system wide ballooning daemons to work as expected and I don't think a
> strong argument has been made for undoing it here.

It is possible to restore original balloon driver behavior by setting
balloon_stats.max_retry_count = 0 and balloon_stats.max_schedule_delay = 1
using sysfs.

> We are talking about polling at most once a second (backing off pretty
> quickly to once every 32s with this patch) -- is that really enough to
> "exhaust" the CPU running event/x?

OK, it is not precise. I will change that to:

xen/balloon: Reduce CPU utilization by event/x process

> Also this patch seems to make the driver quite chatty:
>
> > +	pr_info("xen_balloon: Retry count: %lu/%lu\n", balloon_stats.retry_count,
> > +			balloon_stats.max_retry_count);
>
> Not needed. The balloon driver is a best effort background thing, it
> doesn't need to be spamming the system logs each time something doesn't
> go quite right first time, it should just continue on silently in the
> background. It should only be logging if something goes catastrophically
> wrong (in which case pr_info isn't really sufficient).

Here http://lists.xensource.com/archives/html/xen-devel/2011-02/msg00649.html
Kondrad suggested to add some printk() to inform user what is going on.
I agree with him. However, If balloon driver is controlled by external
process it could pollute logs to some extent. I think that issue could
be easliy resolved by adding quiet flag.

Additionally, I think that errors which are sent to logs by balloon
driver are not critical one. That is why I decided to use pr_info(),
however, I cosidered using pr_warn(). If you think that pr_warn()
is better I could change that part of code.

Daniel
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