linux-kernel - Re: Sleeping BUG in khugepaged for i586

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Date:   Wed, 14 Jun 2017 17:28:16 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Larry Finger <Larry.Finger@...inger.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: Sleeping BUG in khugepaged for i586

On Mon, 12 Jun 2017, Michal Hocko wrote:

> > These are not soft lockups, these are need_resched warnings.  We monitor 
> > how long need_resched has been set and when a thread takes an excessive 
> > amount of time to reschedule after it has been set.  A loop of 512 pages 
> > with ptl contention and doing {clear,copy}_user_highpage() shows that 
> > need_resched can sit without scheduling for an excessive amount of time.
> 
> How much is excessive here?

We monitor anything that holds the cpu for more than 1/20th of a second, 
but this specific occurrence has been observed for ~1/8th.  The majority 
of mm/ is quite good in this regard.