From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pavel Emelyanov Subject: Re: [RFC 1/3] procfs: fdinfo -- Extend information about epoll target files Date: Wed, 22 Feb 2017 11:29:23 +0300 Message-ID: <58AD4BE3.2080803@virtuozzo.com> References: <20170221171254.954209904@openvz.org> <20170221191655.GC27653@uranus> <58AD4147.20801@virtuozzo.com> <20170222075438.GB22938@uranus> <58AD4733.5060304@virtuozzo.com> <20170222081833.GE22938@uranus> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20170222081833.GE22938@uranus> Sender: linux-fsdevel-owner@vger.kernel.org To: Cyrill Gorcunov Cc: Andy Lutomirski , Linux FS Devel , "linux-kernel@vger.kernel.org" , Linux API , Al Viro , Andrew Morton , Andrew Vagin , Michael Kerrisk , Kirill Kolyshkin , Jason Baron , Andrey Vagin List-Id: linux-api@vger.kernel.org On 02/22/2017 11:18 AM, Cyrill Gorcunov wrote: > On Wed, Feb 22, 2017 at 11:09:23AM +0300, Pavel Emelyanov wrote: >>>> >>>> Actually it shouldn't. If you extend the kcmp argument to accept the >>>> epollfd:epollslot pair, this would be effectively the same as if you >>>> had all your epoll-ed files injected into your fdtable with "strange" >>>> fd numbers. We already have two-level rbtree for this in criu, adding >>>> extended ("strange") fd to it should be OK. >>> >>> Nope. Pavel, I guess you forget how we handle file tree in criu currently. >>> We call for kcmp only if we have to -- when primary key for two entries >>> is the same. >> >> True, but the latter is an optimization to reduce the number of syscalls. > > Exactly. While syscalls are quite effective, they are still not coming > for free, so I'm trying to reduce their number as much as possible. > >> Look, in order to have a primary key you need to do some system call for the >> fd you check (read from proc or stat the descriptor). But for target files in >> e-polls you don't make this per-fd syscall to get primary key, just call the >> kcmp instead. > > I have to parse fdinfo anyway, because I need to fetch queued events and mask. > So I'll _have_ to make this per-fd syscall for parsing. And this opens > a way to optimize overall picture -- we can immediately read primary > key and reduce kcmp calls. You read fdinfo per-epoll, but kcmp-s we're talking here are about per-target-files. So having dev:ino pair would help to reduce the number of kcmps, but even w/o this extension we can work OK. Besides, in most of the cases fd number you'd read from epoll's fdinfo will actually be present in task's fdtable, so you can call a single kcmp, make sure the file is correct and that's it. The need to actually _search_ for the runaway file with the set of kcmp will (should) be quite rare case. -- Pavel