linux-kernel - Re: [RFC 1/3] procfs: fdinfo -- Extend information about epoll target files

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Date:   Wed, 22 Feb 2017 11:29:23 +0300
From:   Pavel Emelyanov <xemul@...tuozzo.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>
CC:     Andy Lutomirski <luto@...capital.net>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...uxfoundation.org>,
        Andrew Vagin <avagin@...tuozzo.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Kirill Kolyshkin <kir@...nvz.org>,
        Jason Baron <jbaron@...mai.com>,
        Andrey Vagin <avagin@...nvz.org>
Subject: Re: [RFC 1/3] procfs: fdinfo -- Extend information about epoll target
 files

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