From mboxrd@z Thu Jan 1 00:00:00 1970 From: Minchan Kim Subject: Re: [PATCH v4 3/5] [RFC] arm64: Add support for idle bit in swap PTE Date: Tue, 6 Aug 2019 23:47:47 +0900 Message-ID: <20190806144747.GA72938@google.com> References: <20190805170451.26009-1-joel@joelfernandes.org> <20190805170451.26009-3-joel@joelfernandes.org> <20190806084203.GJ11812@dhcp22.suse.cz> <20190806103627.GA218260@google.com> <20190806104755.GR11812@dhcp22.suse.cz> <20190806111446.GA117316@google.com> <20190806115703.GY11812@dhcp22.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190806115703.GY11812@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org To: Michal Hocko Cc: Joel Fernandes , linux-kernel@vger.kernel.org, Robin Murphy , Alexey Dobriyan , Andrew Morton , Borislav Petkov , Brendan Gregg , Catalin Marinas , Christian Hansen , dancol@google.com, fmayer@google.com, "H. Peter Anvin" , Ingo Molnar , Jonathan Corbet , Kees Cook , kernel-team@android.com, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Mike Rapoport , namhyung@google.com, paulmck@linux.ibm.com, Roman Gushchin , Stephen List-Id: linux-api@vger.kernel.org On Tue, Aug 06, 2019 at 01:57:03PM +0200, Michal Hocko wrote: > On Tue 06-08-19 07:14:46, Joel Fernandes wrote: > > On Tue, Aug 06, 2019 at 12:47:55PM +0200, Michal Hocko wrote: > > > On Tue 06-08-19 06:36:27, Joel Fernandes wrote: > > > > On Tue, Aug 06, 2019 at 10:42:03AM +0200, Michal Hocko wrote: > > > > > On Mon 05-08-19 13:04:49, Joel Fernandes (Google) wrote: > > > > > > This bit will be used by idle page tracking code to correctly identify > > > > > > if a page that was swapped out was idle before it got swapped out. > > > > > > Without this PTE bit, we lose information about if a page is idle or not > > > > > > since the page frame gets unmapped. > > > > > > > > > > And why do we need that? Why cannot we simply assume all swapped out > > > > > pages to be idle? They were certainly idle enough to be reclaimed, > > > > > right? Or what does idle actualy mean here? > > > > > > > > Yes, but other than swapping, in Android a page can be forced to be swapped > > > > out as well using the new hints that Minchan is adding? > > > > > > Yes and that is effectivelly making them idle, no? > > > > That depends on how you think of it. > > I would much prefer to have it documented so that I do not have to guess ;) > > > If you are thinking of a monitoring > > process like a heap profiler, then from the heap profiler's (that only cares > > about the process it is monitoring) perspective it will look extremely odd if > > pages that are recently accessed by the process appear to be idle which would > > falsely look like those processes are leaking memory. The reality being, > > Android forced those pages into swap because of other reasons. I would like > > for the swapping mechanism, whether forced swapping or memory reclaim, not to > > interfere with the idle detection. > > Hmm, but how are you going to handle situation when the page is unmapped > and refaulted again (e.g. a normal reclaim of a pagecache)? You are > losing that information same was as in the swapout case, no? Or am I > missing something? If page is unmapped, it's not a idle memory any longer because it's free memory. We could detect the pte is not present. If page is refaulted, it's not a idle memory any longer because it's accessed again. We could detect it because the newly allocated page doesn't have a PG_idle page flag. Both case, idle page tracking couldn't report them as IDLE so it's okay.