linux-kernel - Re: [PATCH 4/5] mm/hotplug: Avoid RCU stalls when removing large amounts of memory

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Date:   Mon, 17 Jun 2019 10:21:56 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Alastair D'Silva <alastair@...ilva.org>
Cc:     'Alastair D'Silva' <alastair@....ibm.com>,
        'Arun KS' <arunks@...eaurora.org>,
        'Mukesh Ojha' <mojha@...eaurora.org>,
        'Logan Gunthorpe' <logang@...tatee.com>,
        'Wei Yang' <richard.weiyang@...il.com>,
        'Peter Zijlstra' <peterz@...radead.org>,
        'Ingo Molnar' <mingo@...nel.org>, linux-mm@...ck.org,
        'Qian Cai' <cai@....pw>,
        'Thomas Gleixner' <tglx@...utronix.de>,
        'Andrew Morton' <akpm@...ux-foundation.org>,
        'Mike Rapoport' <rppt@...ux.vnet.ibm.com>,
        'Baoquan He' <bhe@...hat.com>,
        'David Hildenbrand' <david@...hat.com>,
        'Josh Poimboeuf' <jpoimboe@...hat.com>,
        'Pavel Tatashin' <pasha.tatashin@...een.com>,
        'Juergen Gross' <jgross@...e.com>,
        'Oscar Salvador' <osalvador@...e.com>,
        'Jiri Kosina' <jkosina@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] mm/hotplug: Avoid RCU stalls when removing large
 amounts of memory

On Mon 17-06-19 17:57:16, Alastair D'Silva wrote:
> > -----Original Message-----
> > From: Michal Hocko <mhocko@...nel.org>
> > Sent: Monday, 17 June 2019 5:47 PM
> > To: Alastair D'Silva <alastair@....ibm.com>
> > Cc: alastair@...ilva.org; Arun KS <arunks@...eaurora.org>; Mukesh Ojha
> > <mojha@...eaurora.org>; Logan Gunthorpe <logang@...tatee.com>; Wei
> > Yang <richard.weiyang@...il.com>; Peter Zijlstra <peterz@...radead.org>;
> > Ingo Molnar <mingo@...nel.org>; linux-mm@...ck.org; Qian Cai
> > <cai@....pw>; Thomas Gleixner <tglx@...utronix.de>; Andrew Morton
> > <akpm@...ux-foundation.org>; Mike Rapoport <rppt@...ux.vnet.ibm.com>;
> > Baoquan He <bhe@...hat.com>; David Hildenbrand <david@...hat.com>;
> > Josh Poimboeuf <jpoimboe@...hat.com>; Pavel Tatashin
> > <pasha.tatashin@...een.com>; Juergen Gross <jgross@...e.com>; Oscar
> > Salvador <osalvador@...e.com>; Jiri Kosina <jkosina@...e.cz>; linux-
> > kernel@...r.kernel.org
> > Subject: Re: [PATCH 4/5] mm/hotplug: Avoid RCU stalls when removing large
> > amounts of memory
> > 
> > On Mon 17-06-19 14:36:30,  Alastair D'Silva  wrote:
> > > From: Alastair D'Silva <alastair@...ilva.org>
> > >
> > > When removing sufficiently large amounts of memory, we trigger RCU
> > > stall detection. By periodically calling cond_resched(), we avoid
> > > bogus stall warnings.
> > >
> > > Signed-off-by: Alastair D'Silva <alastair@...ilva.org>
> > > ---
> > >  mm/memory_hotplug.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index
> > > e096c987d261..382b3a0c9333 100644
> > > --- a/mm/memory_hotplug.c
> > > +++ b/mm/memory_hotplug.c
> > > @@ -578,6 +578,9 @@ void __remove_pages(struct zone *zone, unsigned
> > long phys_start_pfn,
> > >  		__remove_section(zone, __pfn_to_section(pfn),
> > map_offset,
> > >  				 altmap);
> > >  		map_offset = 0;
> > > +
> > > +		if (!(i & 0x0FFF))
> > > +			cond_resched();
> > 
> > We already do have cond_resched before __remove_section. Why is an
> > additional needed?
> 
> I was getting stalls when removing ~1TB of memory.

Have debugged what is the source of the stall? We do cond_resched once a
memory section which should be a constant unit of work regardless of the
total amount of memory to be removed.
-- 
Michal Hocko
SUSE Labs