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 09:47:15 +0200
From:   Michal Hocko <mhocko@...nel.org>
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?

>  	}
>  
>  	set_zone_contiguous(zone);
> -- 
> 2.21.0
> 

-- 
Michal Hocko
SUSE Labs