linux-kernel - Re: [PATCH RESEND v4] fs/epoll: Remove unnecessary wakeups of nested epoll that in ET mode

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Date:   Sat, 28 Sep 2019 21:07:26 +0200
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     hev <r@....cc>, linux-fsdevel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Davide Libenzi <davidel@...ilserver.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Eric Wong <e@...24.org>, Jason Baron <jbaron@...mai.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v4] fs/epoll: Remove unnecessary wakeups of nested
 epoll that in ET mode

On 2019-09-28 04:29, Andrew Morton wrote:
> On Wed, 25 Sep 2019 09:56:03 +0800 hev <r@....cc> wrote:
> 
>> From: Heiher <r@....cc>
>> 
>> Take the case where we have:
>> 
>>         t0
>>          | (ew)
>>         e0
>>          | (et)
>>         e1
>>          | (lt)
>>         s0
>> 
>> t0: thread 0
>> e0: epoll fd 0
>> e1: epoll fd 1
>> s0: socket fd 0
>> ew: epoll_wait
>> et: edge-trigger
>> lt: level-trigger
>> 
>> We only need to wakeup nested epoll fds if something has been queued 
>> to the
>> overflow list, since the ep_poll() traverses the rdllist during 
>> recursive poll
>> and thus events on the overflow list may not be visible yet.
>> 
>> Test code:
> 
> Look sane to me.  Do you have any performance testing results which
> show a benefit?
> 
> epoll maintainership isn't exactly a hive of activity nowadays :(
> Roman, would you please have time to review this?

Yes, I can revisit this once more next week.

Heiher, mind to prepare a patchset with your test suit and make it a 
part
of kselftest?  I hope nobody has any objections.

--
Roman