netdev - Re: [Patch net] net_sched: replace yield() with cond

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Message-ID: <CAM_iQpUa-w9k4EFafQCEpbmR0PANwkLieYMxGwi59373h8jHUg@mail.gmail.com>
Date:   Wed, 5 Apr 2017 16:42:18 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Mike Galbraith <efault@....de>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [Patch net] net_sched: replace yield() with cond_resched()

On Tue, Apr 4, 2017 at 10:56 PM, Mike Galbraith <efault@....de> wrote:
> On Tue, 2017-04-04 at 22:19 -0700, Cong Wang wrote:
>> On Tue, Apr 4, 2017 at 8:55 PM, Mike Galbraith <efault@....de> wrote:
>
>> > That won't help, cond_resched() has the same impact upon a lone
>> > SCHED_FIFO task as yield() does.. none.
>>
>> Hmm? In the comment you quote:
>>
>>  * If you want to use yield() to wait for something, use wait_event().
>>  * If you want to use yield() to be 'nice' for others, use cond_resched().
>>
>> So if cond_resched() doesn't help, why this misleading comment?
>
> This is not an oh let's be nice guys thing, it's a perfect match of...
>
> <copy/paste>
>  * while (!event)
>  *      yield();
> (/copy/paste>
>
> ..get off the CPU until this happens thing.  With nobody to yield the C
> PU to, some_qdisc_is_busy() will remain true forever more.


This is exactly the misleading part, a while-loop waiting for an event
can always be a be-nice-for-others thing, because if not we can just
spin as a spinlock. You probably want to improve that comment to
explain when cond_resched() is a right solution to replace yield(),
so that I could know when it is not.

Thanks.