Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 29 Jul 2019 10:35:15 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Minchan Kim <minchan@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Miguel de Dios <migueldedios@google.com>,
        Wei Wang <wvw@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [PATCH] mm: release the spinlock on zap_pte_range
Message-ID: <20190729083515.GD9330@dhcp22.suse.cz>
References: <20190729071037.241581-1-minchan@kernel.org>
 <20190729074523.GC9330@dhcp22.suse.cz>
 <20190729082052.GA258885@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190729082052.GA258885@google.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon 29-07-19 17:20:52, Minchan Kim wrote:
> On Mon, Jul 29, 2019 at 09:45:23AM +0200, Michal Hocko wrote:
> > On Mon 29-07-19 16:10:37, Minchan Kim wrote:
> > > In our testing(carmera recording), Miguel and Wei found unmap_page_range
> > > takes above 6ms with preemption disabled easily. When I see that, the
> > > reason is it holds page table spinlock during entire 512 page operation
> > > in a PMD. 6.2ms is never trivial for user experince if RT task couldn't
> > > run in the time because it could make frame drop or glitch audio problem.
> > 
> > Where is the time spent during the tear down? 512 pages doesn't sound
> > like a lot to tear down. Is it the TLB flushing?
> 
> Miguel confirmed there is no such big latency without mark_page_accessed
> in zap_pte_range so I guess it's the contention of LRU lock as well as
> heavy activate_page overhead which is not trivial, either.

Please give us more details ideally with some numbers.
-- 
Michal Hocko
SUSE Labs