RE: [PATCH 4/5] mm/hotplug: Avoid RCU stalls when removing large amounts of memory

["Alastair D'Silva" <alastair@d-silva.org>]

> -----Original Message-----
> From: Michal Hocko <mhocko@kernel.org>
> Sent: Monday, 17 June 2019 5:47 PM
> To: Alastair D'Silva <alastair@au1.ibm.com>
> Cc: alastair@d-silva.org; Arun KS <arunks@codeaurora.org>; Mukesh Ojha
> <mojha@codeaurora.org>; Logan Gunthorpe <logang@deltatee.com>; Wei
> Yang <richard.weiyang@gmail.com>; Peter Zijlstra <peterz@infradead.org>;
> Ingo Molnar <mingo@kernel.org>; linux-mm@kvack.org; Qian Cai
> <cai@lca.pw>; Thomas Gleixner <tglx@linutronix.de>; Andrew Morton
> <akpm@linux-foundation.org>; Mike Rapoport <rppt@linux.vnet.ibm.com>;
> Baoquan He <bhe@redhat.com>; David Hildenbrand <david@redhat.com>;
> Josh Poimboeuf <jpoimboe@redhat.com>; Pavel Tatashin
> <pasha.tatashin@soleen.com>; Juergen Gross <jgross@suse.com>; Oscar
> Salvador <osalvador@suse.com>; Jiri Kosina <jkosina@suse.cz>; linux-
> kernel@vger.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@d-silva.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@d-silva.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.


-- 
Alastair D'Silva           mob: 0423 762 819
skype: alastair_dsilva     msn: alastair@d-silva.org
blog: http://alastair.d-silva.org    Twitter: @EvilDeece