linux-kernel - Re: [PATCH v3 06/13] epoll: introduce helpers for adding/removing events to uring

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Message-ID: <904d7aea51552c9be9afb3b19bfac66b@suse.de>
Date:   Mon, 03 Jun 2019 12:02:22 +0200
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     azat@...event.org, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 06/13] epoll: introduce helpers for adding/removing
 events to uring

On 2019-06-03 11:09, Peter Zijlstra wrote:
> On Fri, May 31, 2019 at 08:58:19PM +0200, Roman Penyaev wrote:
>> On 2019-05-31 18:51, Peter Zijlstra wrote:
> 
>> > But like you show, it can be done. It also makes the thing wait-free, as
>> > opposed to merely lockless.
>> 
>> You think it's better?  I did not like this variant from the very
>> beginning because of the unnecessary complexity.  But maybe you're
>> right.  No busy loops on user side makes it wait-free.  And also
>> I can avoid c11 in kernel using cmpxchg along with atomic_t.
> 
> Imagine the (v)CPU going for an extended nap right between publishing 
> the
> new tail and writing the !0 entry. Then your userspace is stuck burning
> cycles without getting anything useful done.

Yes, that is absolutely not nice.  That also worries me.  I wanted
to minimize number of atomic ops on hot path, and of course in that
respect this busy-loop is more attractive.

I will polish and switch back to the wait-free variant and resend the
whole patchset.  Could you please take a look?  Will add you to CC.

--
Roman